research paper on human environment

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Article contents

The environment in health and well-being.

• George Morris George Morris European Centre for Environment and Human Health, University of Exeter Medical School, Truro, United Kingdom
•  and  Patrick Saunders Patrick Saunders University of Staffordshire, University of Birmingham, and WHO Collaborating Centre
• https://doi.org/10.1093/acrefore/9780199389414.013.101
• Published online: 29 March 2017

Most people today readily accept that their health and disease are products of personal characteristics such as their age, gender, and genetic inheritance; the choices they make; and, of course, a complex array of factors operating at the level of society. Individuals frequently have little or no control over the cultural, economic, and social influences that shape their lives and their health and well-being. The environment that forms the physical context for their lives is one such influence and comprises the places where people live, learn work, play, and socialize, the air they breathe, and the food and water they consume. Interest in the physical environment as a component of human health goes back many thousands of years and when, around two and a half millennia ago, humans started to write down ideas about health, disease, and their determinants, many of these ideas centered on the physical environment.

The modern public health movement came into existence in the 19th century as a response to the dreadful unsanitary conditions endured by the urban poor of the Industrial Revolution. These conditions nurtured disease, dramatically shortening life. Thus, a public health movement that was ultimately to change the health and prosperity of millions of people across the world was launched on an “environmental conceptualization” of health. Yet, although the physical environment, especially in towns and cities, has changed dramatically in the 200 years since the Industrial Revolution, so too has our understanding of the relationship between the environment and human health and the importance we attach to it.

The decades immediately following World War II were distinguished by declining influence for public health as a discipline. Health and disease were increasingly “individualized”—a trend that served to further diminish interest in the environment, which was no longer seen as an important component in the health concerns of the day. Yet, as the 20th century wore on, a range of factors emerged to r-establish a belief in the environment as a key issue in the health of Western society. These included new toxic and infectious threats acting at the population level but also the renaissance of a “socioecological model” of public health that demanded a much richer and often more subtle understanding of how local surroundings might act to both improve and damage human health and well-being.

Yet, just as society has begun to shape a much more sophisticated response to reunite health with place and, with this, shape new policies to address complex contemporary challenges, such as obesity, diminished mental health, and well-being and inequities, a new challenge has emerged. In its simplest terms, human activity now seriously threatens the planetary processes and systems on which humankind depends for health and well-being and, ultimately, survival. Ecological public health—the need to build health and well-being, henceforth on ecological principles—may be seen as the society’s greatest 21st-century imperative. Success will involve nothing less than a fundamental rethink of the interplay between society, the economy, and the environment. Importantly, it will demand an environmental conceptualization of the public health as no less radical than the environmental conceptualization that launched modern public health in the 19th century, only now the challenge presents on a vastly extended temporal and spatial scale.

• environmental and human health
• environment
• environmental epidemiology
• environmental health inequalities
• ecological public health

Introduction

This article traces the development of ideas about the environment in human health and well-being over time. Our primary focus is the period since the early 19th century , sometimes termed the “modern public health era.” This has been not only a time of unprecedented scientific, technological, and societal transition but also a time during which perspectives on the relationship of humans to their environment, and its implications for their health and well-being, have undergone significant change.

Curiosity about the environment as a factor in human health and well-being, and indeed health-motivated interventions to manage the physical context for life, substantially predate the modern public health era. The archaeological record provides evidence of sewer lines, primitive toilets, and water-supply arrangements in settlements in Asia, the Middle East, South America, and Southern Europe, dating back many thousands of years (Rosen, 1993 ). Some religious traditions also imply recognition of the importance of environmental factors in health. For example, restrictions on the consumption of certain foods probably derive from a belief that these foods carried risks to health; a passage in the book of Leviticus conveys the existence of a belief in the relationship between the internal state of a house and the health of its occupants (Leviticus [14:33–45], quoted in Frumkin, 2005 ).

The sixty-two books of the “Hippocratic Corpus” dating from 430–330 bc are the accepted bedrock of Western medicine (Lloyd, 1983 ), not least because they departed from the purely supernatural explanations for health and disease which hitherto held sway. For the first time, ideas about medicine, diseases, and their causes were being written down. Among these were ideas about the environment and its relationship to mental and physical health (Lloyd, 1983 ; Rosen, 1993 ; Kessel, 2006 ). While scarcely a template for how societies would come to think about environment and health in the modern era, one Hippocratic text in particular, On Airs, Waters and Places , introduces several ideas that do retain currency. For example, the simple message that good health is unlikely to be achieved and maintained in poor environmental conditions is enduring. Also, through specific reference to the health relevance of changes in water, soil, vegetation, sunlight, winds, climate, and seasonality, On Airs, Waters and Places conceives an environment made up of distinct compartments and spatial scales from local to global, recognizing that perturbations in these compartments, and on these scales, may result in disease. Such thinking remains conceptually and operationally relevant today. Hazardous agents are still frequently addressed in “environmental compartments” such as water, soil, air, and food or by developing and applying environmental standards for the different categories of place where people work, live, learn, and socialize. In parts, the Hippocratic Corpus also presages the ecological perspectives now coloring 21st-century public health thinking. These include an understanding of the potential for human activity to impact negatively on the natural world and the importance of viewing the body within its environment as a composite whole.

Environment and Health in the Modern Public Health Era

Epidemiology is the basic science of public health and is concerned with the distribution of health and disease in populations across time and spaces, together with the determinants of that distribution. Environmental epidemiology is a subspecialty dealing with the effects of environmental exposures on health and disease, again, in populations. Since the early 19th century , the outputs of epidemiology have been key components of a “mixed economy of evidence” that has shaped and reshaped priorities and informed the decisions society takes to protect and improve population health (Petticrew et al., 2004 ; Baker & Nieuwenhuijsen, 2008 ).

In a classic paper from the 1990s, the respected epidemiologists, Mervyn and Ezra Susser, helpfully described different “epidemiological eras” in modern public health, each driven by a dominant paradigm concerning the causes of disease and supported by a particular analytical approach (Susser & Susser, 1996 ). This differentiation offers a useful framework within which to consider changing perspectives on the role of environment in health since the early 1900s.

The Environment in an “Era of Sanitary Statistics”

The Industrial Revolution came first to 19th-century Britain driven by technological innovation, abundant coal supplies, and supportive political/economic conditions. Also influential was a post-Reformation philosophy that extolled the work ethic and self-sufficiency. The events were to resonate throughout the world, bringing great prosperity to some, but others, especially the urban poor, endured poor housing, severe overcrowding, and an absence of wholesome water or sanitation. The growing industrial cities became crucibles of squalor, disease, and severely reduced life expectancy as their citizens suffered the ravages of typhus, tuberculosis, and successive cholera epidemics. Unhealthy working conditions and grossly polluted air also damaged health and compounded the misery of urban life at this time. Such challenges were common to all locations touched by the Industrial Revolution and became the catalyst for a new public health movement across Europe and North America (Rayner & Lang, 2012 ; Rosen, 1993 ).

Using the new science of medical statistics, investigators quickly established the locations with the poorest living conditions to be also those where disease and early death were most prevalent (Chadwick, 1842 ), fueling an ultimately transformational societal response—a “sanitary revolution” (Rosen, 1993 ). Such was the impact of this mix of slum clearance with the introduction of waterborne sewerage and piped water supplies that readers of the British Medical Journal , voting almost two centuries later, still chose it, from a shortlist of 15, as the most important medical milestone since the Journal was first published in 1840 . The 11,300 readers who voted even placed it above the discovery of antibiotics and the development of anaesthesia (Ferriman, 2007 ).

Despite its impact, the “sanitary revolution” was famously initiated and sustained on a biologically flawed paradigm regarding the mechanistic causes of disease. Yet “miasma” (the transmission of disease through noxious vapors), because it served as a metaphor for squalid insanitary conditions, still drove effective intervention (Morris et al., 2006 ; Nash, 2006 ). During this time, however, the emergence of epidemiology as the primary mode of inquiry of public health was also pivotal to success. Endorsing this view, Susser and Susser labeled the first half of the 19th century an “Era of Sanitary Statistics,” citing the frequent use of district-level data to link disease to, for example: filthy and degraded urban environments; overcrowding and poor housing and working conditions; and social factors like infant care (Susser & Susser, 1996 )).

Thus, recognition that the environment (physical and social) mattered for health and notions of a “permeable” human body in close connection with other organisms and the abiotic environment were embedded at the launch of the 19th-century public health movement. It is notable that the perspective of the reformers was quite properly “proximal,” that is, rooted in an acceptance of the importance of the local environment, physical and social. While the term “ecology” would not be coined until 1866 (Haekel, 1866 ) and “social ecology” much later still (Bookchin, 1990 ), the public health pioneers embraced what, in today’s terms, we would understand as a broadly socioecological perspective and discerned no conflict in this with their efforts to understand the immediate causes of disease and intervene in a focused way to prevent it (Nash, 2006 ).

Especially through the efforts to stop cholera, the sanitarians affirmed the pathogenic potential of unsanitary conditions and pioneered the epidemiological approach, initially as “environmental epidemiology” (Baker & Nieuwenhuijsen, 2008 ). Other legacies of the Era of Sanitary Statistics have been less enduring. Despite recent advocacy of a “precautionary principle” (see, e.g., Martuzzi, 2007 ; European Environment Agency, 2013 ), the willingness to act on the basis of strong suspicion of a societal-level environmental threat to population health has diminished, perhaps an inevitable casualty of increasing sophistication and “evidence-based” approaches in medicine and policy (Kessel, 2006 ; Brownson et al., 2009 ). Many of public health’s greatest triumphs have flowed from interventions that would have struggled to satisfy today’s evidential criteria. Also, despite a recent reconnection with such arguments, the inherent logic of seeing and tackling disease in its social and environmental context, so obvious to the pioneers of public health, has periodically been less visible in the rhetoric and actions of their successors.

It is appropriate at this point to emphasize the international character of the 19th-century public health movement. This movement can all too easily be presented as a British phenomenon, with seminal contributions from John Snow ( 1813–1858 ) on the investigation of cholera (Vinten-Johansen et al., 2003 ); William Farr ( 1807–1883 ), also on cholera but more widely on medical statistics (Susser & Adelstein,, 1975 ); Edward Jenner ( 1749–1823 ) on vaccination (Baxby, 2004 ), and Edwin Chadwick ( 1800–1890 ) on the assembly of data relating disease to the filth and squalor that came with poverty (Chadwick, 1842 ). In reality, public health, then as now, advanced through the contribution of many individuals in many nations. For example, the German pioneer of cellular biology, Rudolf Virchow ( 1821–1902 ), and his fellow countryman, the hygienist Johan Peter Frank ( 1745–1821 ), were hugely important (Rather, 1985 ). In France, Louis-Rene Vilerme ( 1782–1863 ), the doctor and pioneer of social epidemiology, highlighted links between poverty and death rates (Rosen, 1993 ) and, in the United States, the meticulous work of Lemuel Shattuck ( 1793–1859 ) bears direct comparison with that of Chadwick (Rayner & Lang, 2012 ).

It might be supposed that the consolidated outputs of European laboratories, especially in the decades between 1830 and 1870 , would have quickly expunged the miasmic paradigm from 19th-century medicine and public health. Yet, the concept of miasma was so inculcated in Western thought that, for many, it retained significant explanatory power. Thus, for much of the 19th century there was not a single settled view on disease contagion (e.g., see Kokayeff, 2013 ). Indeed, as late as 1869 some distinguished Medical Officers of Health in England still attributed diseases such as typhoid to “the insidious miasma of sewer gases” and dismissed germs as “pure nonsense.”

The Environment in an “Era of Infectious Disease Epidemiology”

Increasingly contested, the miasmic theory of disease was effectively supplanted in the 1880s by broad acceptance of the germ theory, ushering a new “Era of Infectious Disease Epidemiology” (Susser & Susser, 1996 ). In 1882 , Louis Pasteur’s techniques for growing organisms made it possible for Robert Koch ( 1843–1910 ) to demonstrate that a mycobacterium was the cause of tuberculosis and, shortly thereafter, to provide scientific proof that cholera was waterborne (Foster, 1970 ; Collard, 1976 ; Brock, 1999 ). In so doing, Koch established, what had been hypothesized by his teacher, Jacob Henle ( 1809–1885 ), some 40 years earlier that disease was microbial. Henle, Snow, Koch, and the biologist Ferdinand Cohn ( 1828–1898 ) are rightly seen as fathers of the science of medical microbiology that for a time would come to dominate thinking in medicine and public health (Rayner & Lang, 2012 ).

Initially at least, the germ theory did little to diminish interest in the environment as a determinant of health. Indeed, by revealing causal linkages between organisms isolated from their environmental carriers and specific diseases, it conferred scientific coherence on the established sanitary model and vindicated efforts to secure hygienic water, food, and housing. As Lesley Nash has observed, the germ theorists were initially content to meld the insights of bacteriology with longstanding environmental beliefs. Notions of a body in constant interaction with, and closely dependent on, its local social and physical context (in today’s terms a socioecological perspective) did not conflict with the narrower perspectives of laboratory science (Nash, 2006 ).

While relative contributions may be debated, over a short timeframe medical microbiology, isolation, immunization, and improving social/environmental conditions combined to sharply reduce the burden of infectious disease for Western society. Yet, by the early years of the 20th century , the capacity to examine disease at the microscopic level, which was the engine of diagnostics and therapeutics, was beginning to act on the very foundations that support public health. Medical science gradually made its focus the pathogenic agents of disease, moving attention away from the environment and eroding socioecological perspectives. Doctors seemed quite content to express health as an absence of disease, and medical science to project its role as the maintenance and reinforcement of “self-contained” human bodies (Nash, 2006 ). Through a growing tendency to see health, disease, and their determinants as attributes of individuals rather than characteristics of communities, wider society seemed almost complicit in an ‘individualization’ of health status. One implication of this blunting of a social/environmental thrust of public health was to divorce health from place, a development that would have profound implications in the very different epidemiological context that emerged following World War II.

The Environment in an Era of Chronic Disease Epidemiology

The dramatic reduction in infectious disease was certainly one reason why the epidemiological climate in Western society changed substantially in the mid- 20th century . But just as important was the emergence of a quite disparate set of pathologies believed to be of noncommunicable etiology. Coronary heart disease, cancers, and peptic ulcers, which became the targets in a new “Era of Chronic Disease Epidemiology” (Susser & Susser, 1996 ), were thought rather unlikely to have origins in exposure to what was an increasingly regulated and ostensibly improving physical environment. While the outputs of much postwar epidemiology seemed to endorse this view, it is useful, with hindsight, to recognize the influence of what might be seen as “fashions” in epidemiological inquiry. These fashions would influence how medical science and the wider society would come to regard diseases and their causes for a generation.

The response of the public health community to the new and alarming “noncommunicable” threats was, logically, to deploy descriptive epidemiology to reveal those most likely to be affected. Perhaps surprisingly, those who traditionally were most vulnerable to disease (the young, the old, the immunocompromised, etc.) did not appear to be at increased risk. Rather, the new epidemics disproportionately affected men in their middle years (Nabel & Braunwald, 2012 ). Supported by enhanced computing power and methodological advance (Susser & Susser, 1996 ), researchers began to converge on specific risk factors that correlated with diseases of greatest concern. Many, it seemed, were aspects of individual lifestyle and behaviors, ostensibly freely chosen. A particular attraction for the proponents of what was to become known as “risk factor epidemiology” was its capacity to represent, mathematically, the “relative risk” of contracting a disease between people exposed to a putative risk and those who were not. Some have dubbed this epidemiological approach to noncommunicable or chronic disease “black box epidemiology” because it can relate exposure to outcomes “without any necessary obligation to interpolate either intervening factors or even pathogenesis” (Susser & Susser, 1996 ). Another unfortunate characteristic of this approach to epidemiology is that, despite its laudable intent to understand and address disease in populations , its focus is on individuals within those populations. As a result, it fails to elucidate the societal forces whose influence and interplay shape the health and health-relevant choices of those individuals. When viewed through a policy lens, this mitigates in favor of simplistic solutions that target individuals divorced from context and that lack the traction to produce meaningful change.

In summary, the desire to create a mathematical measure of relative risk for a specific factor is understandable. However, risk factor epidemiology uses an approach that is much more flexible than material reality. In the real world, many different factors coexist and interact to create and destroy health. This is not, however, to deny risk factor epidemiology’s capacity, particularly in synergy with laboratory-based research, to break new ground. Notably, these methodologically driven approaches were key to elucidating links between smoking and lung cancer, heart disease and serum cholesterol, and between levels of prenatal folic acid intake and neural tube defects (Susser & Susser, 1996 ; Kessel, 2006 ; Perry, 1997 ).

The same basic criticism is voiced where similar “black box” epidemiological approaches are used to explore the contribution of a specific environmental agent, as in the case of much recent air pollution epidemiology (see below) (Kessel, 2006 ). Any specific pollutant under epidemiological investigation inevitably coexists with other pollutants and in a specific exposure context (e.g., prevailing climatic conditions). These coexisting factors may be critical in determining the health outcomes from exposure to the pollutant under investigation. Because the outputs of black box epidemiology are abstractions, the relative risk calculation represents an abstraction that can be limited in its capacity to inform policy.

The decades following World War II were a time of declining influence for public health and population perspectives, largely for reasons we have outlined. Yet, in its rhetoric and activities, the discipline of public health seemed at times almost complicit. Even its defining science of epidemiology seemed for a time more concerned to reinforce the insights of clinical medicine than to play the exploratory role on which its reputation had been founded (Susser & Susser, 1996 ). On the face of it, academic public health and the wider public health discipline had little to say about environment, no longer presenting it as an active component in the then current health challenges for Western society. As Nash has observed, physical environments were “recast as homogenous spaces which were traversed by pathogenic agents.” Nevertheless, divorced from the prevailing rhetoric, in many locations there was a parallel narrative depicting a workforce that continued to work at a local level, within established legal and administrative frameworks, to protect and maintain health-relevant environmental quality standards. However, the environmental health function was often set in the narrow, hazard-focused, and compartmentalized terms framed for it by laboratory science. The task was largely confined to identifying, monitoring, and controlling a limited set of toxic or infectious threats in their environmental carriers. Only when pathogenic organisms or toxic agents demonstrably escaped their industrial, agricultural, or marine confines to damage health and reinforce the porosity of the human body did environment briefly assume a higher profile.

Against this backdrop, it was not necessarily predictable or inevitable that environment would regain a central place in public health. Yet, by the end of the 20th century , a much richer understanding of the environmental contribution to human health and well-being had indeed emerged. This change cannot be attributed to a single factor in isolation. Some point to the key influence of Rachel Carson’s Silent Spring in 1962 (Carson, 1962 ), which expressed grave concern for the ecosystem effects of DDT, the linkage to potential human health effects, and the implications of a growing disconnect between humankind and nature. We do not deny the status of Carson’s work as a seminal text of a modern “environmentalism” that would rapidly gather pace and influence (Nash, 2006 ). However, we submit that it is only now, in the 21st century , when the reality of unprecedented anthropogenic damage to global processes and systems and its health implications is self-evident, that the health sector has fully made common cause with the environmentalist movement (e.g., see Butler et al., 2005 ; Butler & Harley, 2010 ) (We discuss this development later in this article under Ecological Public Health.

However, for reasons that are distinct from a mounting concern over anthropogenic threats to global environmental systems and processes, we argue that the closing decades of the 20th century and the early years of this century did see a rekindling of public health and societal interest in the local or proximal environment. This interest has continued into the 21st century . Developing interest in well-being as a concept, the belief that it is important and that it might be enhanced through the organized efforts of society, continues to engage the attention of academics and policymakers. Although well-being demonstrably impacts health and vice versa, well-being is about much more than health. Rather, it is a measure of what matters to people in every sphere of their lives. Despite its importance, well-being has proved a challenging target for policy. Some of its components are beyond the reach of policy. However, others, including aspects of the built and natural environment and people’s connection to it, are amenable to manipulation. Accordingly, research has been especially concerned to identify the qualities of their environment that are important for different people’s well-being, quality of life, and health at various life stages (Royal College of Physicians, 2016 ). Also, on a practical level, integrating the various well-being frameworks and indices that continue to emerge is an ongoing challenge. However, it is sufficient at this point simply to recognize that elevated concern for well-being and its connection to environment can only broaden and deepen concern for the environment in public health. It will continue to drive renewed interest in matters such as landscape, natural beauty and scenery; crime free, clean places; green, blue, and natural environments; and so on.

Reconnecting Health with Place

Five issues/developments merit particular mention for their role in reestablishing the local environment as a mainstream consideration in health in the developed world in the late 20th century . While recognizing that there is an interrelationship among some of the factors discussed, for simplicity, we discuss them separately here.

Air Pollution

In citing air pollution as a key factor in a late- 20th-century resurgence of interest in the environment, we recognize its much longer history as a contributor to ill health (Evelyn, 1661 ; Lloyd, 1983 ). We acknowledge, too, that accounts of the modern public health era since its inception have been suffused with references to air pollution events, their health implications, and the political and professional campaigns that have sought to mitigate risk (Kessel, 2006 ). However, despite a compelling case for action, the need for urgent intervention was only fully accepted after a number of high-profile air pollution episodes in the 20th century . In 1930 , a severe smog incident in Belgium’s Meuse Valley resulted in the death of sixty people. Prophetically, investigators were quick to highlight the potential for many more deaths, were such an incident to be repeated in a more highly populated area (Bell & Samet, 2005 ). In 1948 , a further twenty people were to die and many more suffer injury after an industrial pollution incident in Donora, Pennsylvania (Hamil, 2008 ), but the tipping point came four years later, with the London Smog of 1952 .

Between December 5 and December 9, a dense fog descended on London where it mixed with air, polluted by domestic and industrial emissions. The resulting thick smog was familiar to many urban dwellers, but in this case, a combination of cold weather and stagnant atmospheric conditions caused sulfur dioxide and smoke concentrations to reach and maintain extremely high levels for a sustained period. The smog had a paralyzing effect on the city’s transport system, and many other aspects of daily life were severely disrupted. But the most dramatic effects were on health. Death rates were to reach three times the normal level for the time of year, and demand for hospital beds far exceeded supply (Baker & Nieuwenhuijsen, 2008 ). While the smog dissipated after a few days, deaths rates remained high for several months thereafter. Subsequent analysis has revealed that, rather than the 3,000–4,000 deaths linked to the episode in at the time, a figure of 10,000–12,000 deaths is more probable (Bell et al., 2004 ).

The London smog is historically important, obviously because of the distressing toll in morbidity and mortality and because it catalyzed long-overdue legislative intervention in the UK in the form of the Clean Air Act of 1956 and the U.S. Clean Air Act 1963 . Critically, however, it reminded the public and politicians of the reality that, given the right conditions, population-level environmental exposures were still entirely capable of producing significant morbidity and mortality.

In combination with other factors, the clean air legislation that emerged in the wake of the smog reduced domestic and industrial fossil fuel emissions, and helped to secure significant reductions in background concentrations of smoke and sulfur dioxide (Royal College of Physicians, 2016 ). However, by the late 1980s, a new, more insidious, urban air pollution threat had begun to emerge. This pollution had its origins not in fixed-point emissions, but in the rapidly increasing numbers of motor vehicles and other fossil fuel-driven forms of transport in towns and cities. The pollutants of concern here, which lacked the visibility of the earlier sulfurous smogs, were fine particles, oxides of nitrogen, and ozone. So-called time-series analyses, using data on the temporal variation in environmental exposure and in health, aggregated over the same time period, were now applied to explore the issue of urban air pollution and health (e.g., see Pope et al., 1995 ; Dockery & Pope, 1996 ; Kessel, 2006 ). The studies revealed the cardiopulmonary effects of long-term exposure to much lower levels of ambient air pollution and, later, following further investigation, the absence of a threshold level for causing health effects. Recent outputs of ‘life-course’ epidemiology have also shown that air pollution affects health, not only through the exacerbation of symptoms in the elderly, but through various processes that have impacts from the womb, through childhood to adolescence, early adulthood, and on into middle and older age (Royal College of Physicians, 2016 ). Also, appreciation that air pollutants can be resident in the air for days or even weeks makes air pollution not simply a local problem, but one that demands source control at city, regional, and international levels. In the UK, for example, the equivalent of around 40,000 deaths every year can be attributed to fine particulates and NO 2 exposure from outdoor air (Royal College of Physicians, 2016 ).

Air pollution is probably the most thoroughly investigated of all environmental threats to health and well-being. Revelations about the true extent of its impact on health keep the issue in the headlines and emphasize the centrality of the physical environment within the public health project. Despite being a focus for academic interest and research fundings, the problem of urban air pollution is a very long way from resolution and is one factor that demands a fundamental reappraisal of how, as a species, we live, consume, and travel. (We discuss a wider, global dimension of the air pollution challenge later in this article.)

Everything Matters: The Environment as an Ingredient in Social Complexity

Another important and often overlooked reason for the late- 20th-century rekindling of interest in the environment and human health can be traced to developments within the wider discipline of public health. Ironically, the thinking behind what, by the 1990s, was being termed the “new public health” had its origins in much older ideas that gave prominence to the social structures in which health is created and destroyed (Baum, 1998 ; Awefeso, 2004 ). If we accept that health, disease, and social patterning in these matters are products of a complex interaction of influences at the level of society with the characteristics of individuals, then such complexity ought to be reflected in the policies and partnerships formed to address them. A growing number of analyses, beginning in the 1970s, would turn a spotlight on this complexity and fundamentally challenge the dominance of the biomedical/health care model and its capacity to solve the problems that beset public. These problems included the intractable burden of noncommunicable disease; growing levels of obesity; diminished psychological well-being; and, not least, stubborn and widening inequalities in the health and well-being of different social groups. Concern also mounted over containing rising, and potentially bankrupting, health care costs.

“A New Perspective on the Health of Canadians,” more commonly referred to as the Lalonde Report, after Canada’s then health minister Marc Lalonde, was published in 1974 (Lalonde, 1974 ). Despite its national focus, the report assumed wider relevance because of its analysis of one of public health’s greatest generic challenges, that of navigating among the many complex and interacting determinants of health to identify effective policies and actions. Implicitly offering a socioecological perspective, the Lalonde Report spoke of a “Health Field,” which included all matters that affect health and comprised four core elements: human biology, environment, lifestyle, and health care organization. Any issue, it was proposed, could be traced to one, or a combination, of these elements, allowing the creation of a “map of the health territory” for any problem (Lalonde, 1974 ). In this way, the contribution and interaction of the elements could be assessed. The analysis affirmed the health relevance of a complex environment comprising interacting physical and social dimensions in interaction with the human body. Lalonde’s message was logical and important, yet more than just an echo of an earlier, more inclusive, understanding of the determinants of health and disease. It recast these largely abandoned perspectives for a more scientific and sophisticated era. The proposal that thousands of “pieces” relevant to health and its determinants could be organized in “an orderly pattern” was alluring and progressive, as was the notion that the exercise alone would allow all contributors to more fully appreciate their roles and influence (Morris et al., 2006 ). In the ensuing years, Lalonde’s proposals for understanding and addressing complexity in the determinants of health have been refined and given greater policy relevance by others. In part, this has been through the development of conceptual models of the socioecological determinants of health. These models have been promoted as tools for presenting evidence that can make their implications more apparent (Evans & Stoddart, 1990 ; Dahlgren & Whitehead, 1991 ). In most of these representations, the local environment is accepted as a key driver of health and well-being (Morris et al., 2006 ).

Despite its inherent logic, the socioecological perspectives that emerged in the closing decades of the 20th century created scientific and policy challenges for all constituencies concerned with public health. There were obvious generic challenges, for example, around which of the models (each, necessarily, a gross simplification of a complex reality) might point to solutions (Morris et al., 2006 ; Evans & Stoddart, 1990 ; Reis et al., 2015 ); around the nature of evidence and its interpretation (Petticrew et al., 2004 ; Tannahill, 2008 ); and how, in practice, to traverse professional and policy silos to produce the interdisciplinary approaches that are inevitably required. In this connection, the task of motivating, supporting, and delivering effective intersectoral working, an abiding challenge for public health policy and practice, assumed a much higher profile in the late 20th century with the emergence of the socioecological model of health.

We emphasize that the continuing failure to adequately confront this challenge has the gravest implications for global public health. As Prüss-Üstün et al. recently observed, “Tackling environmental risks requires intersectoral collaboration. After nearly 50 years of actively promoting this concept, whether referred to as intersectoral action, breaking down silos or the nexus approach, it remains elusive as ever. The statement ‘intersectoral collaboration: loved by all, funded by no-one’ points to obstacles, mainly vested interests, that have burdened this approach ever since it was included as part of the WHO/UNICEF Alma Ata Declaration on Primary Health Care in 1978 . Environmental health, quintessentially intersectoral, has suffered most from this lack of progress” (Prüss-Üstün et al., 2016a ).

With specific reference to the role of the local environment, the recognition of socioecological complexity as the determinant of health meant that strict adherence to narrow hazard-focused and compartmentalized approaches became intellectually unsustainable. Yet, acceptance of the dynamic interaction of environment with other determinants of health demands a richer understanding of the environmental contribution than can be provided by toxicology or microbiology in isolation.

The Role of the Environment in Health Inequalities

The fact that the poorest, most degraded urban neighborhoods were those most blighted by disease and reduced life expectancy was clear even to the public health pioneers of the 19th century . Indeed, throughout much of the modern public health era, an acceptance of the importance of the environment for health and well-being has been accompanied by a recognition of the interplay between sociodemographic, economic, and physical factors in creating and sustaining health inequalities.

The term “health inequalities” refers to general differences in health, however caused. Where the differences in health are unfair, unjust, and avoidable, as they often are when linked to social variables, they should more properly be termed “health inequities.” However, in the extensive literature on the topic and in common usage, inequities are termed inequalities, and we adopt this convention here. Despite their importance, the emphasis on tackling health inequalities has varied considerably over time and according to place.

In 2008 , the final report of the Commission on the Social Determinants of Health (CSDH, 2008 ) elevated the global profile of health inequalities and emphasized the interplay of many societal-level factors in their creation in the 21st century . The significant achievements in public health across the world over nearly two centuries have not been shared equally between countries or by all social groups within countries. An important component has been the health-relevant differences in the physical context for people’s lives—the quality of the physical environment. Sometimes expressed in terms of environmental justice , or elsewhere as environmental health inequalities, attention to this area is key to tackling health inequalities across the world (CSDH, 2008 ; Morris & Braubach, 2012 ).

Estimates of the impact of environmental quality on health and well-being vary widely, depending on the definition of environment used. However, that impact is undeniable. Over a billion people in developing countries, for example, have inadequate access to water, and 2.6 billion lack basic sanitation . The World Health Organization estimates that environmental factors were responsible for 12.6 million deaths worldwide in 2012 , 23 percent of all deaths, and 22 percent of the total burden of disease. Addressing environmental risks could prevent 26 percent of all deaths of children under the age of 5 (Prüss-Üstün et al., 2016b ).

In addition, there is clear evidence that a “good” environment empowers health through access to environmental assets such as green spaces, access to a healthy diet, and safe environments in which to walk, cycle, play, and socialize. However, as these data suggest, there is also a fundamental equity dimension to the distribution of both the cause and distribution of environmental stressors, the susceptibility to exposure, and the adverse effects of those exposures. Deprived communities almost invariably live in poorer quality environments, with higher levels of indoor and outdoor air pollution, contaminated land, polluting industrial processes, overcrowded and poor quality housing, and lower levels of environmental assets (Prüss-Üstün et al., 2016a ; 2016b ; Royal College of Physicians, 2016 ; The Marmot Review Team, 2010 ). Populations in developed countries, including the former communist states of eastern Europe living in areas of high air pollution, are disproportionately deprived, for example (Kriger et al, 2014 ; Bell & Ebisu, 2012 ; Branis & Linhartova, 2012 ; Goodman et al., 2011 ). Poor indoor air quality is associated with unfit or inadequate housing standards, conditions that overwhelmingly affect the deprived (The Marmot Review Team, 2010 ). There is evidence that deprived communities are not only more exposed to environmental hazards but are also more susceptible to the effects of those exposures (Goodman et al., 2011 ; Carder et al., 2008 ; Richardson et al., 2011 ; 2013 ; Vinikoor-Imler et al., 2012 ). There are also concerns that stress, at both the individual and community level, can weaken the body’s defenses against external insult and influence the internal dose of toxicants (Gee & Payne-Sturges, 2004 ).

This effect is also seen in social and physical environments. An adequate and nutritious diet is essential to a healthy, productive, and fulfilling life, and it is a fundamental right predicated by a range of factors including personal knowledge, choice, convenience, availability, quality, cost, and social norms. The evidence is clear that deprivation compounds all these factors, with poorer people buying more unhealthy foods with fewer healthy components while being exposed to circumstances that make such “choices” inevitable (Rudge et al., 2013 ). The proportion of adults considered overweight or obese in 2008 in the 19 EU member states for which data were available ranged between 37 and 57 percent for women and between 51 and 69 percent for men ( EUROSTAT ). English children from deprived areas are almost twice as likely to be obese than those in affluent areas, and adult obesity is also associated with deprivation, particularly in women (Public Health England, 2016 ; National Obesity Observatory, 2013 ).

The poor in developed countries are adept at sourcing cheap calories and are exposed to a large numbers of local outlets selling cheap, calorie-dense takeaway food (Saunders et al., 2015 ). These meals are often super-sized and contain high levels of fats, sugar, and salt. At the same time, many of these areas provide limited access to healthy food options, creating a highly compromised public health environment (Saunders et al., 2015 ).

In addition, environmental stressors seem to have a cumulative impact, exacerbating this inequality. It is evident that poorer people have multiple health, social, and environmental stressors. It is entirely plausible that these stressors modify the effect of exposure to pollutants, as is reflected in the increased vulnerability of obese people to the effects of exposure to air pollutants, including increased risk of diseases such as cardiovascular events and respiratory symptoms (WHO, 2013 ; Jung et al., 2014 ). Long-term exposure to airborne pollutants has also been reported to increase the risk of obesity, and being overweight or obese is associated with an increased susceptibility to indoor air pollution in urban children with asthma (Lu et al., 2013 ).

The responsibility for, and relative benefits and costs of, environmental contamination are also important components of inequality. Environmental contamination may be tolerated by communities living in the vicinity of dirty industrial processes if they perceive a benefit in terms of local employment, although that trade-off has largely broken down in developed countries as those industries have declined in the 20th and 21st centuries. On a wider scale, the environmental consequences of contemporary affluent nations’ fuel economies are borne by those populations least able to bear them and with little or no responsibility for their causation (Patz et al., 2005 ). UNICEF has projected that 75–250 million Africans will be exposed to increased water stress due to climate change by 2020 (UNICEF, 2008 ), a phenomenon overwhelmingly caused by the First World. This is a gross injustice. These are also the same people with limited powers to prevent the dumping of rich countries’ waste in their communities. One appalling example is that of the “disposal” of 500 tons of toxic waste in and around Abidjan, the capital of Cote D’Ivoire, in 2006 . This poisonous cocktail of waste oil and contaminants was the result of the trading in, and processing of, hydrocarbon fuels by multinational commodity and shipping companies, criminal levels of cost cutting, and local political corruption, which led to 17 deaths and over 30,000 injuries in one of the poorest communities in the world (Bohand et al., 2007 ) There are many other examples, including the export, often illegally, of hundreds of thousands of tons of e-waste from Western countries to Africa, China, and Asia for recycling or disposal—transferring the costs and dangerous consequences of exposure to workers, including children, and local communities in these countries that do not have the technical or regulatory systems to deal safely with these toxic materials (ILO, 2012 ). Inuit mothers in northern Canada have elevated levels of chemicals such as PCBs—generated many hundreds, if not thousands, of miles away—in their breast milk (Johansen, 2002 ).

The redistribution of the environmental injustices historically endured by the poor also perversely appears to be affecting more affluent communities in the West. The huge expansion of “fracking” in North America, for example, may be leading to an export of risks from traditional “national sacrifice zones” to areas with no previous experience of such industry, creating “profound social, cultural, and economic shocks for middle class communities losing control over their environments” (Lave & Lutz, 2014 ). Despite their relative affluence, this would nonetheless be an injustice given the constraints on local democratic input and highly questionable direct economic benefits to those communities (Kinnaman, 2011 ; Lave & Lutz, 2014 ; Sovacool, 2014 ).

During a period when environmental catalysts for distress migrations are becoming more frequent (Thomas-Hope, 2011 ), there is a moral as well as a professional duty for the Environmental Health community to tackle these inequalities, which otherwise are likely to both widen and deepen.

The Health-Promoting Environment: Green, Blue, and Natural Spaces

While human communities have long valued access to natural resources such as green spaces, the industrialization of the 19th and early 20th centuries saw millions of people deprived of this access. This era did witness some far-sighted philanthropic gifting of areas of open recreational space for the working classes driven by a moral rather than evidence-based imperative. Though welcome, the distribution of, and access to, such resources was limited, inconsistent, unplanned, and vulnerable to the insecurities of voluntary funding. Subsequent local municipal development of parks and other open spaces increased access, and a greater understanding of the benefits of such access blossomed during the late 20th century as research demonstrated and quantified the public health dividends. Access to good-quality green spaces not only makes the places in which we live, work, and play more attractive, but also has a demonstrable effect on improving health and well-being. Green space is linked to lower levels of several diseases and conditions, including lower rates of mortality (Villeneuve et al., 2012 ), increased longevity in older people (Faculty of Public Health, 2011 ), improved mental health (Faculty of Public Health, 2011 ), better outcomes in disease treatment, and reduced medication (Faculty of Public Health, 2011 ), and it also helps reduce health inequalities (Mitchell & Popham, 2008 ; CABE, 2010 ). Plausible mechanisms for these benefits include the provision of a venue for physical activity, promotion of social contact, and the direct impacts of green spaces on psychological and physical health. Natural spaces also promote greater community cohesion and reduce social isolation, providing a platform for community activities, social interaction, physical activity, and recreation (Public Health England, 2014 ). Research from the United States has identified powerful associations between green space and major reductions in aggressive behavior, domestic abuse, and other crime in deprived urban areas (Kuo et al., 2001a , 2001b ).

And yet, there remain great inequalities in the distribution, use, and quality of this empowering resource. People living in the most deprived areas are less likely to live in the greenest areas and therefore have less opportunity to gain the health benefits of green space compared with people living in the least deprived areas (Public Health England, 2014 ). Children living in poor areas, for example, are nine times less likely than those living in affluent areas to have access to green space and places to play (National Children’s Bureau, 2013 ). It is entirely plausible that that this contributes to the sobering reality that children from deprived communities are up to three times as likely to be obese than those children growing up in affluent areas (National Children’s Bureau, 2013 ).

Accessibility, however, is not the same as availability or utility, nor is it simply a function of proximity. It is strongly impacted by the cost of access, whether it is actually physically available, opening times, and the ease of being able to get to it, for example, walking and good public transport. Deprived communities in particular appreciate the value of such spaces, but they tend to underuse them due to concerns about the safety and quality of the spaces (CABE, 2010 ). Experience has shown that quality of the green space is just as important, if not more so, than its size. Post-World War II urban developments in many countries have included large grassy areas, and substantially derelict former industrial sites have often been entirely grassed over. The sterility and sheer size of these sites, the cost of maintenance, and the lack of facilities have often led to misuse and subsequent abandonment by both communities and local municipalities.

The provision, maintenance, and promotion of good-quality and safe , publicly available spaces is not a subsidy; it is an investment delivering economic, health, and regeneration benefits . Research on Philadelphia estimated that maintaining city parks could achieve huge annual savings in health care costs, stormwater management, air pollution mitigation, and social cohesion benefits (The Trust for Public Land, 2008 ). The improved social cohesion associated with natural spaces also has economic benefits. A 2009 Scottish study estimated a £7.36 dividend for every £1 invested in conservation volunteering projects (Greenspace Scotland, 2009 ). It is clear from the evidence that increasing the use of good-quality green space for all social groups is likely to improve health outcomes and reduce health inequalities.

The Reemergence of the Infectious Threat

Among the developments that, for Western societies, consigned environment to the periphery of medical and public health interest in the post–World War II era, we highlighted the epidemiological transition in the mid- 20th century . Indeed, for a period in the 1960s and 1970s it seemed that infectious disease in the developed world had effectively been conquered (Fauci, 2001 ). It was even tempting to suggest that the developing world might eventually follow suit. Yet, within a relatively few years, the twin threats of emerging infectious disease and antibiotic resistance would shatter the earlier confidence and reestablish infection as a live threat to individuals, communities, and populations and one that presented, increasingly, on a global scale.

The term “emerging infectious disease” (EID) denotes an infectious disease, newly recognized as occurring in humans; one that has been previously recognized but is appearing for the first time in a new population or a different geographic area; one that now affects many more people; and/or one that is displaying new attributes, for example, in terms of its resistance or virulence ( adapted from The US Government & Global Emerging Infectious Disease Preparedness and Response ). Although the return of infection was not necessarily anticipated by a confident global community, many predisposing factors were clearly present. Changes in land use, growth and movement of populations, contacts between people and animals, international trade and travel, and, often, an absence of a public health infrastructure all played a part. Where such influences coincided, as in sub-Saharan Africa or parts of Asia, hotspots were created that were conducive to the emergence of infectious disease. Several hundred new infectious diseases appeared across the globe in the period between 1940 and 2004 , with the greatest number emerging in the 1980s (Jones et al., 2008 ). The 1980s was also the decade that notoriously witnessed the late 20th century ’s most sentinel infection event, the first reported cases of Human Immunodeficiency Virus/Acquired Immune Deficiency Syndrome (HIV/AIDS). By 2014 , AIDS alone would result in approximately 78 million cases worldwide . Although HIV/AIDS engendered particular alarm, the list of late- 20th-century EIDs of medical and public health significance is extensive. Variant Creutzfeldt-Jacob disease (vCJD), H5N1 Influenza and Ebola Virus Disease, the Northern Hemisphere debut of the mosquito-borne zoonotic viral disease, and West Nile Fever in New York City in 1999 were all public health and media events. The process continues unabated in the 21st century with the arrival of Severe Acute Respiratory Syndrome (SARS), H1N1 Influenza (“swine flu”), H7N9 Influenza (“bird flu”), and, despite having surfaced some 40 years earlier, Ebola revealed its potential as a global threat with the West African Outbreak of 2014–2015 . More recently still, the distressing incidence of microcephaly in South America putatively linked to the Zika virus simply emphasizes the abiding challenge posed by infection for public health and global economics (European Centre for Disease Control, 2016 ).

Antibiotic resistance has been a developing public health horror story over, perhaps, 50 years. The therapeutic use of antimicrobials and especially antibiotics was a key factor in slashing the burden of illness from infection in Western countries in the latter half of the 20th century . Yet all classes of organisms—fungi, protozoa, viruses, and bacteria—can develop antimicrobial resistance. Through their genetic processes, bacteria have derived multiple resistance mechanisms to antibiotics used in medicine and agriculture. The threat renders humankind vulnerable to a host of infections, notably in hospital settings where treatment options for many infections are now severely limited. As a consequence, even at the dawn of the 21st century , drug resistance was already being perceived as an increasing threat to global public health, involving all major microbial pathogens and antimicrobial drugs (Levy & Marshall, 2004 )

The challenges of EIDs and antimicrobial resistance are, unquestionably, game changers for medicine and public health in the 21st century . Importantly, they are among the factors that have revealed the true limitations of the biomedical model of health and disease in the 20th century and rekindled interest in the socioeconomic and environmental determinants of disease. HIV/AIDS merits special mention in this regard. Although it is believed to have origins in nonhuman primates in West Africa, it is not an environmental disease in the sense that there is a specific environmental reservoir. Medical sciences and epidemiology have shown transmission of the virus via unprotected sex, contaminated blood transfusions, hypodermic needles, and mother to child transmission during pregnancy, delivery, and breastfeeding. HIV (the infection) and AIDS (the disease) have shown the capacity to extend beyond the initially identified high-risk groups, potentially placing whole populations at risk. In some areas of sub-Saharan Africa where the infection is widespread, it impacts negatively on almost every aspect of society and the economy.

Over 30 years after it first emerged and despite concerted efforts, there is still no cure. In addition to banishing complacency, the infection and the disease call for a much wider perspective than that which took root in the postwar era of scientific positivism and medical paternalism. The failure to manage the threat stems in part from an incapacity to understand where to intervene to change behaviors and to see the disease in its social and environmental context.

Ecological Public Health

Earlier in this article, we identified five issues that helped reestablish awareness of the environment as a key component in the production of human health and well-being in the late 20th century . These issues, and our understanding of them, continue to evolve to challenge the public health community and wider society in the 21st century . In the most general terms, progress seems most likely where issues and challenges are framed with reference to a much wider range of pertinent factors by developing new approaches to evidence and its synthesis; by aligning institutional, physical, and educational infrastructures to the task; and by building governance structures in which all players are accountable and yet are encouraged to unite in common cause.

However, society must now embrace an additional and potentially more devastating threat to health and well-being. Human activity, including economic activity, is now directly and indirectly driving changes to the ecosystems and planetary processes on which we rely for health, well-being, and existence. For too long, human beings have lived, moved, consumed, and pursued health and well-being as if humankind is distinct and separate from nature rather than integral to it. The consequences of this disconnect for the natural world were graphically expressed by Rachel Carson in the 1960s and many others in the ensuing years (e.g., see Rockström et al., 2009 ; Steffen et al., 2015 ). However, developments in science and technology now reveal the true extent of the crisis, its accelerating nature, and its consequences both now and in the medium and longer term.

The term “ecological public health” is increasingly being used to encapsulate a need to build health and well-being, henceforth, on ecological principles. Rayner and Lang ( 2012 ) observe that, despite appearing difficult and complex, Ecological public health “is now the 21st century ’s unavoidable task.” Thus, the already complex challenge of navigating human social complexity to deliver health, well-being, and greater equity, which has defined public health in Western society for several decades, is made more challenging still. The relationship of the environment and human health and well-being must be understood and addressed on vastly extended temporal and spatial scales.

The notion that the planet is a finite resource on which human activity can place intolerable pressure and that the consequences of doing so are potentially catastrophic has been around for some time (e.g., see Carson, 1962 ; Meadows et al., 1972 ). A contemporary evolution of this thinking is expressed by Rockstrom and colleagues. Their sentinel paper, first published in 2009 (Rockström et al., 2009 ) and updated in 2015 (Steffen et al., 2015 ), lists the large earth system processes that are urgently in need of stewardship if humanity is to remain safe into the future. Where applicable, it proposes thresholds beyond which nonlinear, abrupt, and potentially catastrophic changes in these systems might be expected. This thinking is used as a basis for defining a “safe operating space for humanity.” The authors propose nine “planetary boundaries.” Three of these—climate change, ocean acidification, and stratospheric ozone depletion—are major planetary systems where evidence exists of large-scale thresholds in the history of the planet history of the planet. Also included are systems of a rather different sort. These are the slow variables that buffer and regulate planetary resilience. These slow variables comprise interference with the nitrogen and phosphorus cycles; land-use change; rate of biodiversity loss; and freshwater use. Two parameters, air pollution and chemical pollution, are especially difficult to quantify, meaning that thresholds cannot yet be defined. It is emphasized that, while for understandable reasons, the nine systems are often discussed independently, they are interrelated in ways meaning that changes in one system have profound implications for the others. Rockstrom and colleagues observe that in the preindustrial era, all nine parameters were within the safe operating boundaries, and yet by the 1950s, change was underway, most evidently in the nitrogen cycle. By 2009 , according to their analysis, three planetary boundaries had been transgressed: climate change; rate of biodiversity loss; and the nitrogen cycle.

An implicit challenge in limiting global ecosystem damage and its multiple implications is how to achieve recognition among the public and policymakers that the choices they make either directly or indirectly cause ecosystem damage and related environmental change (Morris et al., 2015 ). Climate change is simply the most striking example, but comparable challenges over communication exist in relation to other planetary process and systems. The fundamental rethink of society, the economy, and the environment, which is necessary if health and well-being are to be built on ecological principles, will happen only if the true implications for health and well-being of a “business as usual” approach are understood, communicated, and challenged. For any population, the environmental changes that may ultimately have profound implications may take place in countries and regions well beyond their borders or may not occur for some time, conferring a temporal and/or spatial remoteness that diminishes the sense of urgency. Appreciating the importance of these “distal” pathways of ecosystem damage to human health and well-being demands a greater understanding of ecosystem services (the benefits human beings get from the natural environment) and of why they matter. It also demands a much fuller appreciation of the global connectivity of social, economic, and ecological systems (Morris et al., 2015 ; Adger et al., 2009 ).

When initiating our discussion of the role of environment in health, we observed that the modern public health era was built on an environmental conceptualization of public health. It is now inconceivable that health, well-being, health care, and equity in any of these domains can be delivered without rediscovering an environmental conceptualization of public health for the 21st century .

For Western society, ecological public health is likely to require a rethink of society, the economy, and our stewardship of the natural environment (Rayner & Lang, 2012 ). At the very least, it will demand pursuit, through policy and action, of outcomes that recognize a ‘quadruple bottom line’ measured in health and well-being, environmental quality, equity, and sustainability. The extent to which we embrace ecological principles will be evidenced in policies that address how we live (for example, the energy efficiency of our homes), how we move (particularly our reluctance to substitute travel in fossil-fueled cars with more active forms of travel); how we consume (notably how we source and produce food) and, of course how we obtain and conserve energy.

Taking Stock

Despite being necessarily selective, this article has sought to illustrate how perspectives on the role of the environment in human health and well-being have evolved over the course of the modern public health era. Perspectives can be seen to shift owing to changes in the nature of environmental hazards and risks that are themselves products of the evolution of how societies live, move around, consume, source their energy, and so on. Our understanding of the health relevance of the built and natural environments is also shaped by advances in scientific understanding and technology and a much wider economic, social, cultural, and even political context. In structuring our account, we have adopted a loose framework based on the “epidemiological eras,” elegantly articulated by two of the 20th century ’s leading epidemiologists (Susser & Susser, 1996 ). These eras are differentiated according to the dominant paradigm of the time concerning the causes of disease, each underpinned by analytical approaches to understand and prioritize risk.

The importance accorded to the environment as a mainstream public health issue arguably reached its lowest point in the decades following World War II when the tendency to regard health and disease as characteristics of individuals, rather than communities or populations, gained prominence. This approach diverted attention from social and environmental factors, divorcing health from place. Notions that humans are self-contained and impervious to context have now been largely swept away, not least because denial of a socioecological perspective hugely undermined attempts to address the most serious contemporary health challenges. Also instrumental in challenging the notion of the self-contained body has been an environmentalist movement with a particular interest in pesticide and other chemical contamination of the biosphere. The toxic effects of chemical contamination reinforce the reality of a body that is permeable and invariably in a state of intimate exchange with its surroundings. As Nash ( 2006 ) has observed, “ the singular and self-contained body of the early 20th century came, by the end of that century to seem distressingly porous and vulnerable to the modern landscape” (p. 13). We would simply add that humans exhibit comparable porosity and vulnerability to the social and economic context in which they exist.

We recognize that our account contains only limited reference to the regulatory context that has been so central to controlling the environment for public health. We consider it appropriate to sound a warning in this regard. The processes through which environment is monitored and regulated to protect human health and well-being are sometimes taken for granted. Yet, since the 1980s, pressures have mounted in most Western nations to ‘deregulate’ markets to maximize profit. These pressures have led to environmental and public health regulation being increasingly perceived by governments and markets as “red tape” and a barrier to economic enterprise. Pressure to loosen or even abandon aspects of environmental regulation has weakened formal controls, leaving society vulnerable to corporate excess and irresponsibility, with often serious impacts on public health (Oldenkamp et al., 2016 ). This is not to argue that regulation should be static. Rather, it should adapt to changing technological, social, and economic circumstances and should be appropriately funded whether it relates to the quality of the air we breathe, the water we drink, the buildings we live, learn, and work in, or the nutritional aspects of the food we eat. Neither do we deny the potential to exploit citizen science and the power of new technology to supplement conventional regulation (e.g., enabling vulnerable individuals to avoid hazardous exposures and the opportunities for personal pollution monitoring to improve research).

Mainly anthropogenic damage to planetary resources and ecosystems demands that, wherever we are in the world, public health agencies must understand not just the proximal threats to health and well-being that have been the targets of public health intervention throughout the modern public health era. They must also understand and move to prevent, counteract, and contain more distal threats to health and well-being. The distal threats derive from changes to environments that appear remote in space or time or involve a complex interaction of social, environmental, and economic influences. These are no longer abstract considerations. The unprecedented global connectivity of economic and social systems and the growing understanding of ecosystem interdependencies demand that the implications of human activity for health and well-being be recognized, understood, and addressed on a vastly extended temporal and spatial scale.

Only by build health and well-being on ecological principles (Ecological Public Health) will society effectively address the more distal threats to health and well-being from global ecosystem damage; the socioecological complexity of the proximal environment and the interconnections between these.

Conclusions

In this necessarily brief and artificially linear account, our intention has been to reinforce the enduring importance of the environment for health and well-being. Along the way, we have identified three factors that have marginalized the environment as a component of health and disease. We suggest that they continue to represent clear and present threats, undermining public health and, in the case of the latter, an existential threat to humankind.

The Threat from Medical Reductionism

This tendency to think of disease almost exclusively in terms of pathogenic agents and organic dysfunction marginalizes any influence outside the crucible of the laboratory. This trend was most evident in the decades following World War II but remains an ever-present threat.

The Separation of Health from Place

Closely related to medical reductionism is the tendency to downplay the importance of local context for life. The idea that if local environment matters, it does not matter much and, that when it comes to health and disease, the real action is not out there in the neighborhood and among the community but “over here” in the laboratory and at the level of the individual. Such perspectives are divisive. They create artificial barriers between many academic disciplines, including some medical specialties, and those working to manage and improve the local social and environmental context within which “permeable” human beings live out their lives.

The Denial of Ecology

Science now permits humans to understand the true extent to which their activities are plundering natural resources and harming the planetary systems and processes on which they depend. The pace of change is such that health, well-being, heath care, or anything approaching equity in these things will not be sustained in the medium to longer term without radically rethinking society, the environment, and the economy. The global connectivity of social, economic, and environmental systems means, ultimately, that no one is insulated from the threat whether by distance or socioeconomic circumstance. Ecological public health, the pursuit of health and well-being on ecological principles, has been described as the 21st century ’s unavoidable task. It demands recognition of the dynamic interconnections between people and their environment. Manifestly, we depend on the environment we inhabit, and we powerfully affect it. Among the clearest impediments to delivering ecological public health and preserving a viable environment for future generations are the belief that we can manipulate and conquer the natural environment without consequence, and the irresponsible capitalist imperative that subverts regulatory standards and damages and exploits the environment for profit. Both are revealed as transparent absurdities by an ecological understanding and analysis.

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REVIEW article

Environmental and health impacts of air pollution: a review.

• 1 Delphis S.A., Kifisia, Greece
• 2 Laboratory of Hygiene and Environmental Protection, Faculty of Medicine, Democritus University of Thrace, Alexandroupolis, Greece
• 3 Centre Hospitalier Universitaire Vaudois (CHUV), Service de Médicine Interne, Lausanne, Switzerland
• 4 School of Social and Political Sciences, University of Glasgow, Glasgow, United Kingdom

One of our era's greatest scourges is air pollution, on account not only of its impact on climate change but also its impact on public and individual health due to increasing morbidity and mortality. There are many pollutants that are major factors in disease in humans. Among them, Particulate Matter (PM), particles of variable but very small diameter, penetrate the respiratory system via inhalation, causing respiratory and cardiovascular diseases, reproductive and central nervous system dysfunctions, and cancer. Despite the fact that ozone in the stratosphere plays a protective role against ultraviolet irradiation, it is harmful when in high concentration at ground level, also affecting the respiratory and cardiovascular system. Furthermore, nitrogen oxide, sulfur dioxide, Volatile Organic Compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) are all considered air pollutants that are harmful to humans. Carbon monoxide can even provoke direct poisoning when breathed in at high levels. Heavy metals such as lead, when absorbed into the human body, can lead to direct poisoning or chronic intoxication, depending on exposure. Diseases occurring from the aforementioned substances include principally respiratory problems such as Chronic Obstructive Pulmonary Disease (COPD), asthma, bronchiolitis, and also lung cancer, cardiovascular events, central nervous system dysfunctions, and cutaneous diseases. Last but not least, climate change resulting from environmental pollution affects the geographical distribution of many infectious diseases, as do natural disasters. The only way to tackle this problem is through public awareness coupled with a multidisciplinary approach by scientific experts; national and international organizations must address the emergence of this threat and propose sustainable solutions.

Approach to the Problem

The interactions between humans and their physical surroundings have been extensively studied, as multiple human activities influence the environment. The environment is a coupling of the biotic (living organisms and microorganisms) and the abiotic (hydrosphere, lithosphere, and atmosphere).

Pollution is defined as the introduction into the environment of substances harmful to humans and other living organisms. Pollutants are harmful solids, liquids, or gases produced in higher than usual concentrations that reduce the quality of our environment.

Human activities have an adverse effect on the environment by polluting the water we drink, the air we breathe, and the soil in which plants grow. Although the industrial revolution was a great success in terms of technology, society, and the provision of multiple services, it also introduced the production of huge quantities of pollutants emitted into the air that are harmful to human health. Without any doubt, the global environmental pollution is considered an international public health issue with multiple facets. Social, economic, and legislative concerns and lifestyle habits are related to this major problem. Clearly, urbanization and industrialization are reaching unprecedented and upsetting proportions worldwide in our era. Anthropogenic air pollution is one of the biggest public health hazards worldwide, given that it accounts for about 9 million deaths per year ( 1 ).

Without a doubt, all of the aforementioned are closely associated with climate change, and in the event of danger, the consequences can be severe for mankind ( 2 ). Climate changes and the effects of global planetary warming seriously affect multiple ecosystems, causing problems such as food safety issues, ice and iceberg melting, animal extinction, and damage to plants ( 3 , 4 ).

Air pollution has various health effects. The health of susceptible and sensitive individuals can be impacted even on low air pollution days. Short-term exposure to air pollutants is closely related to COPD (Chronic Obstructive Pulmonary Disease), cough, shortness of breath, wheezing, asthma, respiratory disease, and high rates of hospitalization (a measurement of morbidity).

The long-term effects associated with air pollution are chronic asthma, pulmonary insufficiency, cardiovascular diseases, and cardiovascular mortality. According to a Swedish cohort study, diabetes seems to be induced after long-term air pollution exposure ( 5 ). Moreover, air pollution seems to have various malign health effects in early human life, such as respiratory, cardiovascular, mental, and perinatal disorders ( 3 ), leading to infant mortality or chronic disease in adult age ( 6 ).

National reports have mentioned the increased risk of morbidity and mortality ( 1 ). These studies were conducted in many places around the world and show a correlation between daily ranges of particulate matter (PM) concentration and daily mortality. Climate shifts and global planetary warming ( 3 ) could aggravate the situation. Besides, increased hospitalization (an index of morbidity) has been registered among the elderly and susceptible individuals for specific reasons. Fine and ultrafine particulate matter seems to be associated with more serious illnesses ( 6 ), as it can invade the deepest parts of the airways and more easily reach the bloodstream.

Air pollution mainly affects those living in large urban areas, where road emissions contribute the most to the degradation of air quality. There is also a danger of industrial accidents, where the spread of a toxic fog can be fatal to the populations of the surrounding areas. The dispersion of pollutants is determined by many parameters, most notably atmospheric stability and wind ( 6 ).

In developing countries ( 7 ), the problem is more serious due to overpopulation and uncontrolled urbanization along with the development of industrialization. This leads to poor air quality, especially in countries with social disparities and a lack of information on sustainable management of the environment. The use of fuels such as wood fuel or solid fuel for domestic needs due to low incomes exposes people to bad-quality, polluted air at home. It is of note that three billion people around the world are using the above sources of energy for their daily heating and cooking needs ( 8 ). In developing countries, the women of the household seem to carry the highest risk for disease development due to their longer duration exposure to the indoor air pollution ( 8 , 9 ). Due to its fast industrial development and overpopulation, China is one of the Asian countries confronting serious air pollution problems ( 10 , 11 ). The lung cancer mortality observed in China is associated with fine particles ( 12 ). As stated already, long-term exposure is associated with deleterious effects on the cardiovascular system ( 3 , 5 ). However, it is interesting to note that cardiovascular diseases have mostly been observed in developed and high-income countries rather than in the developing low-income countries exposed highly to air pollution ( 13 ). Extreme air pollution is recorded in India, where the air quality reaches hazardous levels. New Delhi is one of the more polluted cities in India. Flights in and out of New Delhi International Airport are often canceled due to the reduced visibility associated with air pollution. Pollution is occurring both in urban and rural areas in India due to the fast industrialization, urbanization, and rise in use of motorcycle transportation. Nevertheless, biomass combustion associated with heating and cooking needs and practices is a major source of household air pollution in India and in Nepal ( 14 , 15 ). There is spatial heterogeneity in India, as areas with diverse climatological conditions and population and education levels generate different indoor air qualities, with higher PM 2.5 observed in North Indian states (557–601 μg/m 3 ) compared to the Southern States (183–214 μg/m 3 ) ( 16 , 17 ). The cold climate of the North Indian areas may be the main reason for this, as longer periods at home and more heating are necessary compared to in the tropical climate of Southern India. Household air pollution in India is associated with major health effects, especially in women and young children, who stay indoors for longer periods. Chronic obstructive respiratory disease (CORD) and lung cancer are mostly observed in women, while acute lower respiratory disease is seen in young children under 5 years of age ( 18 ).

Accumulation of air pollution, especially sulfur dioxide and smoke, reaching 1,500 mg/m3, resulted in an increase in the number of deaths (4,000 deaths) in December 1952 in London and in 1963 in New York City (400 deaths) ( 19 ). An association of pollution with mortality was reported on the basis of monitoring of outdoor pollution in six US metropolitan cities ( 20 ). In every case, it seems that mortality was closely related to the levels of fine, inhalable, and sulfate particles more than with the levels of total particulate pollution, aerosol acidity, sulfur dioxide, or nitrogen dioxide ( 20 ).

Furthermore, extremely high levels of pollution are reported in Mexico City and Rio de Janeiro, followed by Milan, Ankara, Melbourne, Tokyo, and Moscow ( 19 ).

Based on the magnitude of the public health impact, it is certain that different kinds of interventions should be taken into account. Success and effectiveness in controlling air pollution, specifically at the local level, have been reported. Adequate technological means are applied considering the source and the nature of the emission as well as its impact on health and the environment. The importance of point sources and non-point sources of air pollution control is reported by Schwela and Köth-Jahr ( 21 ). Without a doubt, a detailed emission inventory must record all sources in a given area. Beyond considering the above sources and their nature, topography and meteorology should also be considered, as stated previously. Assessment of the control policies and methods is often extrapolated from the local to the regional and then to the global scale. Air pollution may be dispersed and transported from one region to another area located far away. Air pollution management means the reduction to acceptable levels or possible elimination of air pollutants whose presence in the air affects our health or the environmental ecosystem. Private and governmental entities and authorities implement actions to ensure the air quality ( 22 ). Air quality standards and guidelines were adopted for the different pollutants by the WHO and EPA as a tool for the management of air quality ( 1 , 23 ). These standards have to be compared to the emissions inventory standards by causal analysis and dispersion modeling in order to reveal the problematic areas ( 24 ). Inventories are generally based on a combination of direct measurements and emissions modeling ( 24 ).

As an example, we state here the control measures at the source through the use of catalytic converters in cars. These are devices that turn the pollutants and toxic gases produced from combustion engines into less-toxic pollutants by catalysis through redox reactions ( 25 ). In Greece, the use of private cars was restricted by tracking their license plates in order to reduce traffic congestion during rush hour ( 25 ).

Concerning industrial emissions, collectors and closed systems can keep the air pollution to the minimal standards imposed by legislation ( 26 ).

Current strategies to improve air quality require an estimation of the economic value of the benefits gained from proposed programs. These proposed programs by public authorities, and directives are issued with guidelines to be respected.

In Europe, air quality limit values AQLVs (Air Quality Limit Values) are issued for setting off planning claims ( 27 ). In the USA, the NAAQS (National Ambient Air Quality Standards) establish the national air quality limit values ( 27 ). While both standards and directives are based on different mechanisms, significant success has been achieved in the reduction of overall emissions and associated health and environmental effects ( 27 ). The European Directive identifies geographical areas of risk exposure as monitoring/assessment zones to record the emission sources and levels of air pollution ( 27 ), whereas the USA establishes global geographical air quality criteria according to the severity of their air quality problem and records all sources of the pollutants and their precursors ( 27 ).

In this vein, funds have been financing, directly or indirectly, projects related to air quality along with the technical infrastructure to maintain good air quality. These plans focus on an inventory of databases from air quality environmental planning awareness campaigns. Moreover, pollution measures of air emissions may be taken for vehicles, machines, and industries in urban areas.

Technological innovation can only be successful if it is able to meet the needs of society. In this sense, technology must reflect the decision-making practices and procedures of those involved in risk assessment and evaluation and act as a facilitator in providing information and assessments to enable decision makers to make the best decisions possible. Summarizing the aforementioned in order to design an effective air quality control strategy, several aspects must be considered: environmental factors and ambient air quality conditions, engineering factors and air pollutant characteristics, and finally, economic operating costs for technological improvement and administrative and legal costs. Considering the economic factor, competitiveness through neoliberal concepts is offering a solution to environmental problems ( 22 ).

The development of environmental governance, along with technological progress, has initiated the deployment of a dialogue. Environmental politics has created objections and points of opposition between different political parties, scientists, media, and governmental and non-governmental organizations ( 22 ). Radical environmental activism actions and movements have been created ( 22 ). The rise of the new information and communication technologies (ICTs) are many times examined as to whether and in which way they have influenced means of communication and social movements such as activism ( 28 ). Since the 1990s, the term “digital activism” has been used increasingly and in many different disciplines ( 29 ). Nowadays, multiple digital technologies can be used to produce a digital activism outcome on environmental issues. More specifically, devices with online capabilities such as computers or mobile phones are being used as a way to pursue change in political and social affairs ( 30 ).

In the present paper, we focus on the sources of environmental pollution in relation to public health and propose some solutions and interventions that may be of interest to environmental legislators and decision makers.

Sources of Exposure

It is known that the majority of environmental pollutants are emitted through large-scale human activities such as the use of industrial machinery, power-producing stations, combustion engines, and cars. Because these activities are performed at such a large scale, they are by far the major contributors to air pollution, with cars estimated to be responsible for approximately 80% of today's pollution ( 31 ). Some other human activities are also influencing our environment to a lesser extent, such as field cultivation techniques, gas stations, fuel tanks heaters, and cleaning procedures ( 32 ), as well as several natural sources, such as volcanic and soil eruptions and forest fires.

The classification of air pollutants is based mainly on the sources producing pollution. Therefore, it is worth mentioning the four main sources, following the classification system: Major sources, Area sources, Mobile sources, and Natural sources.

Major sources include the emission of pollutants from power stations, refineries, and petrochemicals, the chemical and fertilizer industries, metallurgical and other industrial plants, and, finally, municipal incineration.

Indoor area sources include domestic cleaning activities, dry cleaners, printing shops, and petrol stations.

Mobile sources include automobiles, cars, railways, airways, and other types of vehicles.

Finally, natural sources include, as stated previously, physical disasters ( 33 ) such as forest fire, volcanic erosion, dust storms, and agricultural burning.

However, many classification systems have been proposed. Another type of classification is a grouping according to the recipient of the pollution, as follows:

Air pollution is determined as the presence of pollutants in the air in large quantities for long periods. Air pollutants are dispersed particles, hydrocarbons, CO, CO 2 , NO, NO 2 , SO 3 , etc.

Water pollution is organic and inorganic charge and biological charge ( 10 ) at high levels that affect the water quality ( 34 , 35 ).

Soil pollution occurs through the release of chemicals or the disposal of wastes, such as heavy metals, hydrocarbons, and pesticides.

Air pollution can influence the quality of soil and water bodies by polluting precipitation, falling into water and soil environments ( 34 , 36 ). Notably, the chemistry of the soil can be amended due to acid precipitation by affecting plants, cultures, and water quality ( 37 ). Moreover, movement of heavy metals is favored by soil acidity, and metals are so then moving into the watery environment. It is known that heavy metals such as aluminum are noxious to wildlife and fishes. Soil quality seems to be of importance, as soils with low calcium carbonate levels are at increased jeopardy from acid rain. Over and above rain, snow and particulate matter drip into watery ' bodies ( 36 , 38 ).

Lastly, pollution is classified following type of origin:

Radioactive and nuclear pollution , releasing radioactive and nuclear pollutants into water, air, and soil during nuclear explosions and accidents, from nuclear weapons, and through handling or disposal of radioactive sewage.

Radioactive materials can contaminate surface water bodies and, being noxious to the environment, plants, animals, and humans. It is known that several radioactive substances such as radium and uranium concentrate in the bones and can cause cancers ( 38 , 39 ).

Noise pollution is produced by machines, vehicles, traffic noises, and musical installations that are harmful to our hearing.

The World Health Organization introduced the term DALYs. The DALYs for a disease or health condition is defined as the sum of the Years of Life Lost (YLL) due to premature mortality in the population and the Years Lost due to Disability (YLD) for people living with the health condition or its consequences ( 39 ). In Europe, air pollution is the main cause of disability-adjusted life years lost (DALYs), followed by noise pollution. The potential relationships of noise and air pollution with health have been studied ( 40 ). The study found that DALYs related to noise were more important than those related to air pollution, as the effects of environmental noise on cardiovascular disease were independent of air pollution ( 40 ). Environmental noise should be counted as an independent public health risk ( 40 ).

Environmental pollution occurs when changes in the physical, chemical, or biological constituents of the environment (air masses, temperature, climate, etc.) are produced.

Pollutants harm our environment either by increasing levels above normal or by introducing harmful toxic substances. Primary pollutants are directly produced from the above sources, and secondary pollutants are emitted as by-products of the primary ones. Pollutants can be biodegradable or non-biodegradable and of natural origin or anthropogenic, as stated previously. Moreover, their origin can be a unique source (point-source) or dispersed sources.

Pollutants have differences in physical and chemical properties, explaining the discrepancy in their capacity for producing toxic effects. As an example, we state here that aerosol compounds ( 41 – 43 ) have a greater toxicity than gaseous compounds due to their tiny size (solid or liquid) in the atmosphere; they have a greater penetration capacity. Gaseous compounds are eliminated more easily by our respiratory system ( 41 ). These particles are able to damage lungs and can even enter the bloodstream ( 41 ), leading to the premature deaths of millions of people yearly. Moreover, the aerosol acidity ([H+]) seems to considerably enhance the production of secondary organic aerosols (SOA), but this last aspect is not supported by other scientific teams ( 38 ).

Climate and Pollution

Air pollution and climate change are closely related. Climate is the other side of the same coin that reduces the quality of our Earth ( 44 ). Pollutants such as black carbon, methane, tropospheric ozone, and aerosols affect the amount of incoming sunlight. As a result, the temperature of the Earth is increasing, resulting in the melting of ice, icebergs, and glaciers.

In this vein, climatic changes will affect the incidence and prevalence of both residual and imported infections in Europe. Climate and weather affect the duration, timing, and intensity of outbreaks strongly and change the map of infectious diseases in the globe ( 45 ). Mosquito-transmitted parasitic or viral diseases are extremely climate-sensitive, as warming firstly shortens the pathogen incubation period and secondly shifts the geographic map of the vector. Similarly, water-warming following climate changes leads to a high incidence of waterborne infections. Recently, in Europe, eradicated diseases seem to be emerging due to the migration of population, for example, cholera, poliomyelitis, tick-borne encephalitis, and malaria ( 46 ).

The spread of epidemics is associated with natural climate disasters and storms, which seem to occur more frequently nowadays ( 47 ). Malnutrition and disequilibration of the immune system are also associated with the emerging infections affecting public health ( 48 ).

The Chikungunya virus “took the airplane” from the Indian Ocean to Europe, as outbreaks of the disease were registered in Italy ( 49 ) as well as autochthonous cases in France ( 50 ).

An increase in cryptosporidiosis in the United Kingdom and in the Czech Republic seems to have occurred following flooding ( 36 , 51 ).

As stated previously, aerosols compounds are tiny in size and considerably affect the climate. They are able to dissipate sunlight (the albedo phenomenon) by dispersing a quarter of the sun's rays back to space and have cooled the global temperature over the last 30 years ( 52 ).

Air Pollutants

The World Health Organization (WHO) reports on six major air pollutants, namely particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Air pollution can have a disastrous effect on all components of the environment, including groundwater, soil, and air. Additionally, it poses a serious threat to living organisms. In this vein, our interest is mainly to focus on these pollutants, as they are related to more extensive and severe problems in human health and environmental impact. Acid rain, global warming, the greenhouse effect, and climate changes have an important ecological impact on air pollution ( 53 ).

Particulate Matter (PM) and Health

Studies have shown a relationship between particulate matter (PM) and adverse health effects, focusing on either short-term (acute) or long-term (chronic) PM exposure.

Particulate matter (PM) is usually formed in the atmosphere as a result of chemical reactions between the different pollutants. The penetration of particles is closely dependent on their size ( 53 ). Particulate Matter (PM) was defined as a term for particles by the United States Environmental Protection Agency ( 54 ). Particulate matter (PM) pollution includes particles with diameters of 10 micrometers (μm) or smaller, called PM 10 , and extremely fine particles with diameters that are generally 2.5 micrometers (μm) and smaller.

Particulate matter contains tiny liquid or solid droplets that can be inhaled and cause serious health effects ( 55 ). Particles <10 μm in diameter (PM 10 ) after inhalation can invade the lungs and even reach the bloodstream. Fine particles, PM 2.5 , pose a greater risk to health ( 6 , 56 ) ( Table 1 ).

Table 1 . Penetrability according to particle size.

Multiple epidemiological studies have been performed on the health effects of PM. A positive relation was shown between both short-term and long-term exposures of PM 2.5 and acute nasopharyngitis ( 56 ). In addition, long-term exposure to PM for years was found to be related to cardiovascular diseases and infant mortality.

Those studies depend on PM 2.5 monitors and are restricted in terms of study area or city area due to a lack of spatially resolved daily PM 2.5 concentration data and, in this way, are not representative of the entire population. Following a recent epidemiological study by the Department of Environmental Health at Harvard School of Public Health (Boston, MA) ( 57 ), it was reported that, as PM 2.5 concentrations vary spatially, an exposure error (Berkson error) seems to be produced, and the relative magnitudes of the short- and long-term effects are not yet completely elucidated. The team developed a PM 2.5 exposure model based on remote sensing data for assessing short- and long-term human exposures ( 57 ). This model permits spatial resolution in short-term effects plus the assessment of long-term effects in the whole population.

Moreover, respiratory diseases and affection of the immune system are registered as long-term chronic effects ( 58 ). It is worth noting that people with asthma, pneumonia, diabetes, and respiratory and cardiovascular diseases are especially susceptible and vulnerable to the effects of PM. PM 2.5 , followed by PM 10 , are strongly associated with diverse respiratory system diseases ( 59 ), as their size permits them to pierce interior spaces ( 60 ). The particles produce toxic effects according to their chemical and physical properties. The components of PM 10 and PM 2.5 can be organic (polycyclic aromatic hydrocarbons, dioxins, benzene, 1-3 butadiene) or inorganic (carbon, chlorides, nitrates, sulfates, metals) in nature ( 55 ).

Particulate Matter (PM) is divided into four main categories according to type and size ( 61 ) ( Table 2 ).

Table 2 . Types and sizes of particulate Matter (PM).

Gas contaminants include PM in aerial masses.

Particulate contaminants include contaminants such as smog, soot, tobacco smoke, oil smoke, fly ash, and cement dust.

Biological Contaminants are microorganisms (bacteria, viruses, fungi, mold, and bacterial spores), cat allergens, house dust and allergens, and pollen.

Types of Dust include suspended atmospheric dust, settling dust, and heavy dust.

Finally, another fact is that the half-lives of PM 10 and PM 2.5 particles in the atmosphere is extended due to their tiny dimensions; this permits their long-lasting suspension in the atmosphere and even their transfer and spread to distant destinations where people and the environment may be exposed to the same magnitude of pollution ( 53 ). They are able to change the nutrient balance in watery ecosystems, damage forests and crops, and acidify water bodies.

As stated, PM 2.5 , due to their tiny size, are causing more serious health effects. These aforementioned fine particles are the main cause of the “haze” formation in different metropolitan areas ( 12 , 13 , 61 ).

Ozone Impact in the Atmosphere

Ozone (O 3 ) is a gas formed from oxygen under high voltage electric discharge ( 62 ). It is a strong oxidant, 52% stronger than chlorine. It arises in the stratosphere, but it could also arise following chain reactions of photochemical smog in the troposphere ( 63 ).

Ozone can travel to distant areas from its initial source, moving with air masses ( 64 ). It is surprising that ozone levels over cities are low in contrast to the increased amounts occuring in urban areas, which could become harmful for cultures, forests, and vegetation ( 65 ) as it is reducing carbon assimilation ( 66 ). Ozone reduces growth and yield ( 47 , 48 ) and affects the plant microflora due to its antimicrobial capacity ( 67 , 68 ). In this regard, ozone acts upon other natural ecosystems, with microflora ( 69 , 70 ) and animal species changing their species composition ( 71 ). Ozone increases DNA damage in epidermal keratinocytes and leads to impaired cellular function ( 72 ).

Ground-level ozone (GLO) is generated through a chemical reaction between oxides of nitrogen and VOCs emitted from natural sources and/or following anthropogenic activities.

Ozone uptake usually occurs by inhalation. Ozone affects the upper layers of the skin and the tear ducts ( 73 ). A study of short-term exposure of mice to high levels of ozone showed malondialdehyde formation in the upper skin (epidermis) but also depletion in vitamins C and E. It is likely that ozone levels are not interfering with the skin barrier function and integrity to predispose to skin disease ( 74 ).

Due to the low water-solubility of ozone, inhaled ozone has the capacity to penetrate deeply into the lungs ( 75 ).

Toxic effects induced by ozone are registered in urban areas all over the world, causing biochemical, morphologic, functional, and immunological disorders ( 76 ).

The European project (APHEA2) focuses on the acute effects of ambient ozone concentrations on mortality ( 77 ). Daily ozone concentrations compared to the daily number of deaths were reported from different European cities for a 3-year period. During the warm period of the year, an observed increase in ozone concentration was associated with an increase in the daily number of deaths (0.33%), in the number of respiratory deaths (1.13%), and in the number of cardiovascular deaths (0.45%). No effect was observed during wintertime.

Carbon Monoxide (CO)

Carbon monoxide is produced by fossil fuel when combustion is incomplete. The symptoms of poisoning due to inhaling carbon monoxide include headache, dizziness, weakness, nausea, vomiting, and, finally, loss of consciousness.

The affinity of carbon monoxide to hemoglobin is much greater than that of oxygen. In this vein, serious poisoning may occur in people exposed to high levels of carbon monoxide for a long period of time. Due to the loss of oxygen as a result of the competitive binding of carbon monoxide, hypoxia, ischemia, and cardiovascular disease are observed.

Carbon monoxide affects the greenhouses gases that are tightly connected to global warming and climate. This should lead to an increase in soil and water temperatures, and extreme weather conditions or storms may occur ( 68 ).

However, in laboratory and field experiments, it has been seen to produce increased plant growth ( 78 ).

Nitrogen Oxide (NO 2 )

Nitrogen oxide is a traffic-related pollutant, as it is emitted from automobile motor engines ( 79 , 80 ). It is an irritant of the respiratory system as it penetrates deep in the lung, inducing respiratory diseases, coughing, wheezing, dyspnea, bronchospasm, and even pulmonary edema when inhaled at high levels. It seems that concentrations over 0.2 ppm produce these adverse effects in humans, while concentrations higher than 2.0 ppm affect T-lymphocytes, particularly the CD8+ cells and NK cells that produce our immune response ( 81 ).It is reported that long-term exposure to high levels of nitrogen dioxide can be responsible for chronic lung disease. Long-term exposure to NO 2 can impair the sense of smell ( 81 ).

However, systems other than respiratory ones can be involved, as symptoms such as eye, throat, and nose irritation have been registered ( 81 ).

High levels of nitrogen dioxide are deleterious to crops and vegetation, as they have been observed to reduce crop yield and plant growth efficiency. Moreover, NO 2 can reduce visibility and discolor fabrics ( 81 ).

Sulfur Dioxide (SO 2 )

Sulfur dioxide is a harmful gas that is emitted mainly from fossil fuel consumption or industrial activities. The annual standard for SO 2 is 0.03 ppm ( 82 ). It affects human, animal, and plant life. Susceptible people as those with lung disease, old people, and children, who present a higher risk of damage. The major health problems associated with sulfur dioxide emissions in industrialized areas are respiratory irritation, bronchitis, mucus production, and bronchospasm, as it is a sensory irritant and penetrates deep into the lung converted into bisulfite and interacting with sensory receptors, causing bronchoconstriction. Moreover, skin redness, damage to the eyes (lacrimation and corneal opacity) and mucous membranes, and worsening of pre-existing cardiovascular disease have been observed ( 81 ).

Environmental adverse effects, such as acidification of soil and acid rain, seem to be associated with sulfur dioxide emissions ( 83 ).

Lead is a heavy metal used in different industrial plants and emitted from some petrol motor engines, batteries, radiators, waste incinerators, and waste waters ( 84 ).

Moreover, major sources of lead pollution in the air are metals, ore, and piston-engine aircraft. Lead poisoning is a threat to public health due to its deleterious effects upon humans, animals, and the environment, especially in the developing countries.

Exposure to lead can occur through inhalation, ingestion, and dermal absorption. Trans- placental transport of lead was also reported, as lead passes through the placenta unencumbered ( 85 ). The younger the fetus is, the more harmful the toxic effects. Lead toxicity affects the fetal nervous system; edema or swelling of the brain is observed ( 86 ). Lead, when inhaled, accumulates in the blood, soft tissue, liver, lung, bones, and cardiovascular, nervous, and reproductive systems. Moreover, loss of concentration and memory, as well as muscle and joint pain, were observed in adults ( 85 , 86 ).

Children and newborns ( 87 ) are extremely susceptible even to minimal doses of lead, as it is a neurotoxicant and causes learning disabilities, impairment of memory, hyperactivity, and even mental retardation.

Elevated amounts of lead in the environment are harmful to plants and crop growth. Neurological effects are observed in vertebrates and animals in association with high lead levels ( 88 ).

Polycyclic Aromatic Hydrocarbons(PAHs)

The distribution of PAHs is ubiquitous in the environment, as the atmosphere is the most important means of their dispersal. They are found in coal and in tar sediments. Moreover, they are generated through incomplete combustion of organic matter as in the cases of forest fires, incineration, and engines ( 89 ). PAH compounds, such as benzopyrene, acenaphthylene, anthracene, and fluoranthene are recognized as toxic, mutagenic, and carcinogenic substances. They are an important risk factor for lung cancer ( 89 ).

Volatile Organic Compounds(VOCs)

Volatile organic compounds (VOCs), such as toluene, benzene, ethylbenzene, and xylene ( 90 ), have been found to be associated with cancer in humans ( 91 ). The use of new products and materials has actually resulted in increased concentrations of VOCs. VOCs pollute indoor air ( 90 ) and may have adverse effects on human health ( 91 ). Short-term and long-term adverse effects on human health are observed. VOCs are responsible for indoor air smells. Short-term exposure is found to cause irritation of eyes, nose, throat, and mucosal membranes, while those of long duration exposure include toxic reactions ( 92 ). Predictable assessment of the toxic effects of complex VOC mixtures is difficult to estimate, as these pollutants can have synergic, antagonistic, or indifferent effects ( 91 , 93 ).

Dioxins originate from industrial processes but also come from natural processes, such as forest fires and volcanic eruptions. They accumulate in foods such as meat and dairy products, fish and shellfish, and especially in the fatty tissue of animals ( 94 ).

Short-period exhibition to high dioxin concentrations may result in dark spots and lesions on the skin ( 94 ). Long-term exposure to dioxins can cause developmental problems, impairment of the immune, endocrine and nervous systems, reproductive infertility, and cancer ( 94 ).

Without any doubt, fossil fuel consumption is responsible for a sizeable part of air contamination. This contamination may be anthropogenic, as in agricultural and industrial processes or transportation, while contamination from natural sources is also possible. Interestingly, it is of note that the air quality standards established through the European Air Quality Directive are somewhat looser than the WHO guidelines, which are stricter ( 95 ).

Effect of Air Pollution on Health

The most common air pollutants are ground-level ozone and Particulates Matter (PM). Air pollution is distinguished into two main types:

Outdoor pollution is the ambient air pollution.

Indoor pollution is the pollution generated by household combustion of fuels.

People exposed to high concentrations of air pollutants experience disease symptoms and states of greater and lesser seriousness. These effects are grouped into short- and long-term effects affecting health.

Susceptible populations that need to be aware of health protection measures include old people, children, and people with diabetes and predisposing heart or lung disease, especially asthma.

As extensively stated previously, according to a recent epidemiological study from Harvard School of Public Health, the relative magnitudes of the short- and long-term effects have not been completely clarified ( 57 ) due to the different epidemiological methodologies and to the exposure errors. New models are proposed for assessing short- and long-term human exposure data more successfully ( 57 ). Thus, in the present section, we report the more common short- and long-term health effects but also general concerns for both types of effects, as these effects are often dependent on environmental conditions, dose, and individual susceptibility.

Short-term effects are temporary and range from simple discomfort, such as irritation of the eyes, nose, skin, throat, wheezing, coughing and chest tightness, and breathing difficulties, to more serious states, such as asthma, pneumonia, bronchitis, and lung and heart problems. Short-term exposure to air pollution can also cause headaches, nausea, and dizziness.

These problems can be aggravated by extended long-term exposure to the pollutants, which is harmful to the neurological, reproductive, and respiratory systems and causes cancer and even, rarely, deaths.

The long-term effects are chronic, lasting for years or the whole life and can even lead to death. Furthermore, the toxicity of several air pollutants may also induce a variety of cancers in the long term ( 96 ).

As stated already, respiratory disorders are closely associated with the inhalation of air pollutants. These pollutants will invade through the airways and will accumulate at the cells. Damage to target cells should be related to the pollutant component involved and its source and dose. Health effects are also closely dependent on country, area, season, and time. An extended exposure duration to the pollutant should incline to long-term health effects in relation also to the above factors.

Particulate Matter (PMs), dust, benzene, and O 3 cause serious damage to the respiratory system ( 97 ). Moreover, there is a supplementary risk in case of existing respiratory disease such as asthma ( 98 ). Long-term effects are more frequent in people with a predisposing disease state. When the trachea is contaminated by pollutants, voice alterations may be remarked after acute exposure. Chronic obstructive pulmonary disease (COPD) may be induced following air pollution, increasing morbidity and mortality ( 99 ). Long-term effects from traffic, industrial air pollution, and combustion of fuels are the major factors for COPD risk ( 99 ).

Multiple cardiovascular effects have been observed after exposure to air pollutants ( 100 ). Changes occurred in blood cells after long-term exposure may affect cardiac functionality. Coronary arteriosclerosis was reported following long-term exposure to traffic emissions ( 101 ), while short-term exposure is related to hypertension, stroke, myocardial infracts, and heart insufficiency. Ventricle hypertrophy is reported to occur in humans after long-time exposure to nitrogen oxide (NO 2 ) ( 102 , 103 ).

Neurological effects have been observed in adults and children after extended-term exposure to air pollutants.

Psychological complications, autism, retinopathy, fetal growth, and low birth weight seem to be related to long-term air pollution ( 83 ). The etiologic agent of the neurodegenerative diseases (Alzheimer's and Parkinson's) is not yet known, although it is believed that extended exposure to air pollution seems to be a factor. Specifically, pesticides and metals are cited as etiological factors, together with diet. The mechanisms in the development of neurodegenerative disease include oxidative stress, protein aggregation, inflammation, and mitochondrial impairment in neurons ( 104 ) ( Figure 1 ).

Figure 1 . Impact of air pollutants on the brain.

Brain inflammation was observed in dogs living in a highly polluted area in Mexico for a long period ( 105 ). In human adults, markers of systemic inflammation (IL-6 and fibrinogen) were found to be increased as an immediate response to PNC on the IL-6 level, possibly leading to the production of acute-phase proteins ( 106 ). The progression of atherosclerosis and oxidative stress seem to be the mechanisms involved in the neurological disturbances caused by long-term air pollution. Inflammation comes secondary to the oxidative stress and seems to be involved in the impairment of developmental maturation, affecting multiple organs ( 105 , 107 ). Similarly, other factors seem to be involved in the developmental maturation, which define the vulnerability to long-term air pollution. These include birthweight, maternal smoking, genetic background and socioeconomic environment, as well as education level.

However, diet, starting from breast-feeding, is another determinant factor. Diet is the main source of antioxidants, which play a key role in our protection against air pollutants ( 108 ). Antioxidants are free radical scavengers and limit the interaction of free radicals in the brain ( 108 ). Similarly, genetic background may result in a differential susceptibility toward the oxidative stress pathway ( 60 ). For example, antioxidant supplementation with vitamins C and E appears to modulate the effect of ozone in asthmatic children homozygous for the GSTM1 null allele ( 61 ). Inflammatory cytokines released in the periphery (e.g., respiratory epithelia) upregulate the innate immune Toll-like receptor 2. Such activation and the subsequent events leading to neurodegeneration have recently been observed in lung lavage in mice exposed to ambient Los Angeles (CA, USA) particulate matter ( 61 ). In children, neurodevelopmental morbidities were observed after lead exposure. These children developed aggressive and delinquent behavior, reduced intelligence, learning difficulties, and hyperactivity ( 109 ). No level of lead exposure seems to be “safe,” and the scientific community has asked the Centers for Disease Control and Prevention (CDC) to reduce the current screening guideline of 10 μg/dl ( 109 ).

It is important to state that impact on the immune system, causing dysfunction and neuroinflammation ( 104 ), is related to poor air quality. Yet, increases in serum levels of immunoglobulins (IgA, IgM) and the complement component C3 are observed ( 106 ). Another issue is that antigen presentation is affected by air pollutants, as there is an upregulation of costimulatory molecules such as CD80 and CD86 on macrophages ( 110 ).

As is known, skin is our shield against ultraviolet radiation (UVR) and other pollutants, as it is the most exterior layer of our body. Traffic-related pollutants, such as PAHs, VOCs, oxides, and PM, may cause pigmented spots on our skin ( 111 ). On the one hand, as already stated, when pollutants penetrate through the skin or are inhaled, damage to the organs is observed, as some of these pollutants are mutagenic and carcinogenic, and, specifically, they affect the liver and lung. On the other hand, air pollutants (and those in the troposphere) reduce the adverse effects of ultraviolet radiation UVR in polluted urban areas ( 111 ). Air pollutants absorbed by the human skin may contribute to skin aging, psoriasis, acne, urticaria, eczema, and atopic dermatitis ( 111 ), usually caused by exposure to oxides and photochemical smoke ( 111 ). Exposure to PM and cigarette smoking act as skin-aging agents, causing spots, dyschromia, and wrinkles. Lastly, pollutants have been associated with skin cancer ( 111 ).

Higher morbidity is reported to fetuses and children when exposed to the above dangers. Impairment in fetal growth, low birth weight, and autism have been reported ( 112 ).

Another exterior organ that may be affected is the eye. Contamination usually comes from suspended pollutants and may result in asymptomatic eye outcomes, irritation ( 112 ), retinopathy, or dry eye syndrome ( 113 , 114 ).

Environmental Impact of Air Pollution

Air pollution is harming not only human health but also the environment ( 115 ) in which we live. The most important environmental effects are as follows.

Acid rain is wet (rain, fog, snow) or dry (particulates and gas) precipitation containing toxic amounts of nitric and sulfuric acids. They are able to acidify the water and soil environments, damage trees and plantations, and even damage buildings and outdoor sculptures, constructions, and statues.

Haze is produced when fine particles are dispersed in the air and reduce the transparency of the atmosphere. It is caused by gas emissions in the air coming from industrial facilities, power plants, automobiles, and trucks.

Ozone , as discussed previously, occurs both at ground level and in the upper level (stratosphere) of the Earth's atmosphere. Stratospheric ozone is protecting us from the Sun's harmful ultraviolet (UV) rays. In contrast, ground-level ozone is harmful to human health and is a pollutant. Unfortunately, stratospheric ozone is gradually damaged by ozone-depleting substances (i.e., chemicals, pesticides, and aerosols). If this protecting stratospheric ozone layer is thinned, then UV radiation can reach our Earth, with harmful effects for human life (skin cancer) ( 116 ) and crops ( 117 ). In plants, ozone penetrates through the stomata, inducing them to close, which blocks CO 2 transfer and induces a reduction in photosynthesis ( 118 ).

Global climate change is an important issue that concerns mankind. As is known, the “greenhouse effect” keeps the Earth's temperature stable. Unhappily, anthropogenic activities have destroyed this protecting temperature effect by producing large amounts of greenhouse gases, and global warming is mounting, with harmful effects on human health, animals, forests, wildlife, agriculture, and the water environment. A report states that global warming is adding to the health risks of poor people ( 119 ).

People living in poorly constructed buildings in warm-climate countries are at high risk for heat-related health problems as temperatures mount ( 119 ).

Wildlife is burdened by toxic pollutants coming from the air, soil, or the water ecosystem and, in this way, animals can develop health problems when exposed to high levels of pollutants. Reproductive failure and birth effects have been reported.

Eutrophication is occurring when elevated concentrations of nutrients (especially nitrogen) stimulate the blooming of aquatic algae, which can cause a disequilibration in the diversity of fish and their deaths.

Without a doubt, there is a critical concentration of pollution that an ecosystem can tolerate without being destroyed, which is associated with the ecosystem's capacity to neutralize acidity. The Canada Acid Rain Program established this load at 20 kg/ha/yr ( 120 ).

Hence, air pollution has deleterious effects on both soil and water ( 121 ). Concerning PM as an air pollutant, its impact on crop yield and food productivity has been reported. Its impact on watery bodies is associated with the survival of living organisms and fishes and their productivity potential ( 121 ).

An impairment in photosynthetic rhythm and metabolism is observed in plants exposed to the effects of ozone ( 121 ).

Sulfur and nitrogen oxides are involved in the formation of acid rain and are harmful to plants and marine organisms.

Last but not least, as mentioned above, the toxicity associated with lead and other metals is the main threat to our ecosystems (air, water, and soil) and living creatures ( 121 ).

In 2018, during the first WHO Global Conference on Air Pollution and Health, the WHO's General Director, Dr. Tedros Adhanom Ghebreyesus, called air pollution a “silent public health emergency” and “the new tobacco” ( 122 ).

Undoubtedly, children are particularly vulnerable to air pollution, especially during their development. Air pollution has adverse effects on our lives in many different respects.

Diseases associated with air pollution have not only an important economic impact but also a societal impact due to absences from productive work and school.

Despite the difficulty of eradicating the problem of anthropogenic environmental pollution, a successful solution could be envisaged as a tight collaboration of authorities, bodies, and doctors to regularize the situation. Governments should spread sufficient information and educate people and should involve professionals in these issues so as to control the emergence of the problem successfully.

Technologies to reduce air pollution at the source must be established and should be used in all industries and power plants. The Kyoto Protocol of 1997 set as a major target the reduction of GHG emissions to below 5% by 2012 ( 123 ). This was followed by the Copenhagen summit, 2009 ( 124 ), and then the Durban summit of 2011 ( 125 ), where it was decided to keep to the same line of action. The Kyoto protocol and the subsequent ones were ratified by many countries. Among the pioneers who adopted this important protocol for the world's environmental and climate “health” was China ( 3 ). As is known, China is a fast-developing economy and its GDP (Gross Domestic Product) is expected to be very high by 2050, which is defined as the year of dissolution of the protocol for the decrease in gas emissions.

A more recent international agreement of crucial importance for climate change is the Paris Agreement of 2015, issued by the UNFCCC (United Nations Climate Change Committee). This latest agreement was ratified by a plethora of UN (United Nations) countries as well as the countries of the European Union ( 126 ). In this vein, parties should promote actions and measures to enhance numerous aspects around the subject. Boosting education, training, public awareness, and public participation are some of the relevant actions for maximizing the opportunities to achieve the targets and goals on the crucial matter of climate change and environmental pollution ( 126 ). Without any doubt, technological improvements makes our world easier and it seems difficult to reduce the harmful impact caused by gas emissions, we could limit its use by seeking reliable approaches.

Synopsizing, a global prevention policy should be designed in order to combat anthropogenic air pollution as a complement to the correct handling of the adverse health effects associated with air pollution. Sustainable development practices should be applied, together with information coming from research in order to handle the problem effectively.

At this point, international cooperation in terms of research, development, administration policy, monitoring, and politics is vital for effective pollution control. Legislation concerning air pollution must be aligned and updated, and policy makers should propose the design of a powerful tool of environmental and health protection. As a result, the main proposal of this essay is that we should focus on fostering local structures to promote experience and practice and extrapolate these to the international level through developing effective policies for sustainable management of ecosystems.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

IM is employed by the company Delphis S.A.

The remaining authors declare that the present review paper was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Keywords: air pollution, environment, health, public health, gas emission, policy

Citation: Manisalidis I, Stavropoulou E, Stavropoulos A and Bezirtzoglou E (2020) Environmental and Health Impacts of Air Pollution: A Review. Front. Public Health 8:14. doi: 10.3389/fpubh.2020.00014

Received: 17 October 2019; Accepted: 17 January 2020; Published: 20 February 2020.

Reviewed by:

Copyright © 2020 Manisalidis, Stavropoulou, Stavropoulos and Bezirtzoglou. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Ioannis Manisalidis, giannismanisal@gmail.com ; Elisavet Stavropoulou, elisabeth.stavropoulou@gmail.com

† These authors have contributed equally to this work

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Environmental and Health Impacts of Air Pollution: A Review

Ioannis manisalidis.

1 Delphis S.A., Kifisia, Greece

2 Laboratory of Hygiene and Environmental Protection, Faculty of Medicine, Democritus University of Thrace, Alexandroupolis, Greece

Elisavet Stavropoulou

3 Centre Hospitalier Universitaire Vaudois (CHUV), Service de Médicine Interne, Lausanne, Switzerland

Agathangelos Stavropoulos

4 School of Social and Political Sciences, University of Glasgow, Glasgow, United Kingdom

Eugenia Bezirtzoglou

One of our era's greatest scourges is air pollution, on account not only of its impact on climate change but also its impact on public and individual health due to increasing morbidity and mortality. There are many pollutants that are major factors in disease in humans. Among them, Particulate Matter (PM), particles of variable but very small diameter, penetrate the respiratory system via inhalation, causing respiratory and cardiovascular diseases, reproductive and central nervous system dysfunctions, and cancer. Despite the fact that ozone in the stratosphere plays a protective role against ultraviolet irradiation, it is harmful when in high concentration at ground level, also affecting the respiratory and cardiovascular system. Furthermore, nitrogen oxide, sulfur dioxide, Volatile Organic Compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) are all considered air pollutants that are harmful to humans. Carbon monoxide can even provoke direct poisoning when breathed in at high levels. Heavy metals such as lead, when absorbed into the human body, can lead to direct poisoning or chronic intoxication, depending on exposure. Diseases occurring from the aforementioned substances include principally respiratory problems such as Chronic Obstructive Pulmonary Disease (COPD), asthma, bronchiolitis, and also lung cancer, cardiovascular events, central nervous system dysfunctions, and cutaneous diseases. Last but not least, climate change resulting from environmental pollution affects the geographical distribution of many infectious diseases, as do natural disasters. The only way to tackle this problem is through public awareness coupled with a multidisciplinary approach by scientific experts; national and international organizations must address the emergence of this threat and propose sustainable solutions.

Approach to the Problem

The interactions between humans and their physical surroundings have been extensively studied, as multiple human activities influence the environment. The environment is a coupling of the biotic (living organisms and microorganisms) and the abiotic (hydrosphere, lithosphere, and atmosphere).

Pollution is defined as the introduction into the environment of substances harmful to humans and other living organisms. Pollutants are harmful solids, liquids, or gases produced in higher than usual concentrations that reduce the quality of our environment.

Human activities have an adverse effect on the environment by polluting the water we drink, the air we breathe, and the soil in which plants grow. Although the industrial revolution was a great success in terms of technology, society, and the provision of multiple services, it also introduced the production of huge quantities of pollutants emitted into the air that are harmful to human health. Without any doubt, the global environmental pollution is considered an international public health issue with multiple facets. Social, economic, and legislative concerns and lifestyle habits are related to this major problem. Clearly, urbanization and industrialization are reaching unprecedented and upsetting proportions worldwide in our era. Anthropogenic air pollution is one of the biggest public health hazards worldwide, given that it accounts for about 9 million deaths per year ( 1 ).

Without a doubt, all of the aforementioned are closely associated with climate change, and in the event of danger, the consequences can be severe for mankind ( 2 ). Climate changes and the effects of global planetary warming seriously affect multiple ecosystems, causing problems such as food safety issues, ice and iceberg melting, animal extinction, and damage to plants ( 3 , 4 ).

Air pollution has various health effects. The health of susceptible and sensitive individuals can be impacted even on low air pollution days. Short-term exposure to air pollutants is closely related to COPD (Chronic Obstructive Pulmonary Disease), cough, shortness of breath, wheezing, asthma, respiratory disease, and high rates of hospitalization (a measurement of morbidity).

The long-term effects associated with air pollution are chronic asthma, pulmonary insufficiency, cardiovascular diseases, and cardiovascular mortality. According to a Swedish cohort study, diabetes seems to be induced after long-term air pollution exposure ( 5 ). Moreover, air pollution seems to have various malign health effects in early human life, such as respiratory, cardiovascular, mental, and perinatal disorders ( 3 ), leading to infant mortality or chronic disease in adult age ( 6 ).

National reports have mentioned the increased risk of morbidity and mortality ( 1 ). These studies were conducted in many places around the world and show a correlation between daily ranges of particulate matter (PM) concentration and daily mortality. Climate shifts and global planetary warming ( 3 ) could aggravate the situation. Besides, increased hospitalization (an index of morbidity) has been registered among the elderly and susceptible individuals for specific reasons. Fine and ultrafine particulate matter seems to be associated with more serious illnesses ( 6 ), as it can invade the deepest parts of the airways and more easily reach the bloodstream.

Air pollution mainly affects those living in large urban areas, where road emissions contribute the most to the degradation of air quality. There is also a danger of industrial accidents, where the spread of a toxic fog can be fatal to the populations of the surrounding areas. The dispersion of pollutants is determined by many parameters, most notably atmospheric stability and wind ( 6 ).

In developing countries ( 7 ), the problem is more serious due to overpopulation and uncontrolled urbanization along with the development of industrialization. This leads to poor air quality, especially in countries with social disparities and a lack of information on sustainable management of the environment. The use of fuels such as wood fuel or solid fuel for domestic needs due to low incomes exposes people to bad-quality, polluted air at home. It is of note that three billion people around the world are using the above sources of energy for their daily heating and cooking needs ( 8 ). In developing countries, the women of the household seem to carry the highest risk for disease development due to their longer duration exposure to the indoor air pollution ( 8 , 9 ). Due to its fast industrial development and overpopulation, China is one of the Asian countries confronting serious air pollution problems ( 10 , 11 ). The lung cancer mortality observed in China is associated with fine particles ( 12 ). As stated already, long-term exposure is associated with deleterious effects on the cardiovascular system ( 3 , 5 ). However, it is interesting to note that cardiovascular diseases have mostly been observed in developed and high-income countries rather than in the developing low-income countries exposed highly to air pollution ( 13 ). Extreme air pollution is recorded in India, where the air quality reaches hazardous levels. New Delhi is one of the more polluted cities in India. Flights in and out of New Delhi International Airport are often canceled due to the reduced visibility associated with air pollution. Pollution is occurring both in urban and rural areas in India due to the fast industrialization, urbanization, and rise in use of motorcycle transportation. Nevertheless, biomass combustion associated with heating and cooking needs and practices is a major source of household air pollution in India and in Nepal ( 14 , 15 ). There is spatial heterogeneity in India, as areas with diverse climatological conditions and population and education levels generate different indoor air qualities, with higher PM 2.5 observed in North Indian states (557–601 μg/m 3 ) compared to the Southern States (183–214 μg/m 3 ) ( 16 , 17 ). The cold climate of the North Indian areas may be the main reason for this, as longer periods at home and more heating are necessary compared to in the tropical climate of Southern India. Household air pollution in India is associated with major health effects, especially in women and young children, who stay indoors for longer periods. Chronic obstructive respiratory disease (CORD) and lung cancer are mostly observed in women, while acute lower respiratory disease is seen in young children under 5 years of age ( 18 ).

Accumulation of air pollution, especially sulfur dioxide and smoke, reaching 1,500 mg/m3, resulted in an increase in the number of deaths (4,000 deaths) in December 1952 in London and in 1963 in New York City (400 deaths) ( 19 ). An association of pollution with mortality was reported on the basis of monitoring of outdoor pollution in six US metropolitan cities ( 20 ). In every case, it seems that mortality was closely related to the levels of fine, inhalable, and sulfate particles more than with the levels of total particulate pollution, aerosol acidity, sulfur dioxide, or nitrogen dioxide ( 20 ).

Furthermore, extremely high levels of pollution are reported in Mexico City and Rio de Janeiro, followed by Milan, Ankara, Melbourne, Tokyo, and Moscow ( 19 ).

Based on the magnitude of the public health impact, it is certain that different kinds of interventions should be taken into account. Success and effectiveness in controlling air pollution, specifically at the local level, have been reported. Adequate technological means are applied considering the source and the nature of the emission as well as its impact on health and the environment. The importance of point sources and non-point sources of air pollution control is reported by Schwela and Köth-Jahr ( 21 ). Without a doubt, a detailed emission inventory must record all sources in a given area. Beyond considering the above sources and their nature, topography and meteorology should also be considered, as stated previously. Assessment of the control policies and methods is often extrapolated from the local to the regional and then to the global scale. Air pollution may be dispersed and transported from one region to another area located far away. Air pollution management means the reduction to acceptable levels or possible elimination of air pollutants whose presence in the air affects our health or the environmental ecosystem. Private and governmental entities and authorities implement actions to ensure the air quality ( 22 ). Air quality standards and guidelines were adopted for the different pollutants by the WHO and EPA as a tool for the management of air quality ( 1 , 23 ). These standards have to be compared to the emissions inventory standards by causal analysis and dispersion modeling in order to reveal the problematic areas ( 24 ). Inventories are generally based on a combination of direct measurements and emissions modeling ( 24 ).

As an example, we state here the control measures at the source through the use of catalytic converters in cars. These are devices that turn the pollutants and toxic gases produced from combustion engines into less-toxic pollutants by catalysis through redox reactions ( 25 ). In Greece, the use of private cars was restricted by tracking their license plates in order to reduce traffic congestion during rush hour ( 25 ).

Concerning industrial emissions, collectors and closed systems can keep the air pollution to the minimal standards imposed by legislation ( 26 ).

Current strategies to improve air quality require an estimation of the economic value of the benefits gained from proposed programs. These proposed programs by public authorities, and directives are issued with guidelines to be respected.

In Europe, air quality limit values AQLVs (Air Quality Limit Values) are issued for setting off planning claims ( 27 ). In the USA, the NAAQS (National Ambient Air Quality Standards) establish the national air quality limit values ( 27 ). While both standards and directives are based on different mechanisms, significant success has been achieved in the reduction of overall emissions and associated health and environmental effects ( 27 ). The European Directive identifies geographical areas of risk exposure as monitoring/assessment zones to record the emission sources and levels of air pollution ( 27 ), whereas the USA establishes global geographical air quality criteria according to the severity of their air quality problem and records all sources of the pollutants and their precursors ( 27 ).

In this vein, funds have been financing, directly or indirectly, projects related to air quality along with the technical infrastructure to maintain good air quality. These plans focus on an inventory of databases from air quality environmental planning awareness campaigns. Moreover, pollution measures of air emissions may be taken for vehicles, machines, and industries in urban areas.

Technological innovation can only be successful if it is able to meet the needs of society. In this sense, technology must reflect the decision-making practices and procedures of those involved in risk assessment and evaluation and act as a facilitator in providing information and assessments to enable decision makers to make the best decisions possible. Summarizing the aforementioned in order to design an effective air quality control strategy, several aspects must be considered: environmental factors and ambient air quality conditions, engineering factors and air pollutant characteristics, and finally, economic operating costs for technological improvement and administrative and legal costs. Considering the economic factor, competitiveness through neoliberal concepts is offering a solution to environmental problems ( 22 ).

The development of environmental governance, along with technological progress, has initiated the deployment of a dialogue. Environmental politics has created objections and points of opposition between different political parties, scientists, media, and governmental and non-governmental organizations ( 22 ). Radical environmental activism actions and movements have been created ( 22 ). The rise of the new information and communication technologies (ICTs) are many times examined as to whether and in which way they have influenced means of communication and social movements such as activism ( 28 ). Since the 1990s, the term “digital activism” has been used increasingly and in many different disciplines ( 29 ). Nowadays, multiple digital technologies can be used to produce a digital activism outcome on environmental issues. More specifically, devices with online capabilities such as computers or mobile phones are being used as a way to pursue change in political and social affairs ( 30 ).

In the present paper, we focus on the sources of environmental pollution in relation to public health and propose some solutions and interventions that may be of interest to environmental legislators and decision makers.

Sources of Exposure

It is known that the majority of environmental pollutants are emitted through large-scale human activities such as the use of industrial machinery, power-producing stations, combustion engines, and cars. Because these activities are performed at such a large scale, they are by far the major contributors to air pollution, with cars estimated to be responsible for approximately 80% of today's pollution ( 31 ). Some other human activities are also influencing our environment to a lesser extent, such as field cultivation techniques, gas stations, fuel tanks heaters, and cleaning procedures ( 32 ), as well as several natural sources, such as volcanic and soil eruptions and forest fires.

The classification of air pollutants is based mainly on the sources producing pollution. Therefore, it is worth mentioning the four main sources, following the classification system: Major sources, Area sources, Mobile sources, and Natural sources.

Major sources include the emission of pollutants from power stations, refineries, and petrochemicals, the chemical and fertilizer industries, metallurgical and other industrial plants, and, finally, municipal incineration.

Indoor area sources include domestic cleaning activities, dry cleaners, printing shops, and petrol stations.

Mobile sources include automobiles, cars, railways, airways, and other types of vehicles.

Finally, natural sources include, as stated previously, physical disasters ( 33 ) such as forest fire, volcanic erosion, dust storms, and agricultural burning.

However, many classification systems have been proposed. Another type of classification is a grouping according to the recipient of the pollution, as follows:

Air pollution is determined as the presence of pollutants in the air in large quantities for long periods. Air pollutants are dispersed particles, hydrocarbons, CO, CO 2 , NO, NO 2 , SO 3 , etc.

Water pollution is organic and inorganic charge and biological charge ( 10 ) at high levels that affect the water quality ( 34 , 35 ).

Soil pollution occurs through the release of chemicals or the disposal of wastes, such as heavy metals, hydrocarbons, and pesticides.

Air pollution can influence the quality of soil and water bodies by polluting precipitation, falling into water and soil environments ( 34 , 36 ). Notably, the chemistry of the soil can be amended due to acid precipitation by affecting plants, cultures, and water quality ( 37 ). Moreover, movement of heavy metals is favored by soil acidity, and metals are so then moving into the watery environment. It is known that heavy metals such as aluminum are noxious to wildlife and fishes. Soil quality seems to be of importance, as soils with low calcium carbonate levels are at increased jeopardy from acid rain. Over and above rain, snow and particulate matter drip into watery ' bodies ( 36 , 38 ).

Lastly, pollution is classified following type of origin:

Radioactive and nuclear pollution , releasing radioactive and nuclear pollutants into water, air, and soil during nuclear explosions and accidents, from nuclear weapons, and through handling or disposal of radioactive sewage.

Radioactive materials can contaminate surface water bodies and, being noxious to the environment, plants, animals, and humans. It is known that several radioactive substances such as radium and uranium concentrate in the bones and can cause cancers ( 38 , 39 ).

Noise pollution is produced by machines, vehicles, traffic noises, and musical installations that are harmful to our hearing.

The World Health Organization introduced the term DALYs. The DALYs for a disease or health condition is defined as the sum of the Years of Life Lost (YLL) due to premature mortality in the population and the Years Lost due to Disability (YLD) for people living with the health condition or its consequences ( 39 ). In Europe, air pollution is the main cause of disability-adjusted life years lost (DALYs), followed by noise pollution. The potential relationships of noise and air pollution with health have been studied ( 40 ). The study found that DALYs related to noise were more important than those related to air pollution, as the effects of environmental noise on cardiovascular disease were independent of air pollution ( 40 ). Environmental noise should be counted as an independent public health risk ( 40 ).

Environmental pollution occurs when changes in the physical, chemical, or biological constituents of the environment (air masses, temperature, climate, etc.) are produced.

Pollutants harm our environment either by increasing levels above normal or by introducing harmful toxic substances. Primary pollutants are directly produced from the above sources, and secondary pollutants are emitted as by-products of the primary ones. Pollutants can be biodegradable or non-biodegradable and of natural origin or anthropogenic, as stated previously. Moreover, their origin can be a unique source (point-source) or dispersed sources.

Pollutants have differences in physical and chemical properties, explaining the discrepancy in their capacity for producing toxic effects. As an example, we state here that aerosol compounds ( 41 – 43 ) have a greater toxicity than gaseous compounds due to their tiny size (solid or liquid) in the atmosphere; they have a greater penetration capacity. Gaseous compounds are eliminated more easily by our respiratory system ( 41 ). These particles are able to damage lungs and can even enter the bloodstream ( 41 ), leading to the premature deaths of millions of people yearly. Moreover, the aerosol acidity ([H+]) seems to considerably enhance the production of secondary organic aerosols (SOA), but this last aspect is not supported by other scientific teams ( 38 ).

Climate and Pollution

Air pollution and climate change are closely related. Climate is the other side of the same coin that reduces the quality of our Earth ( 44 ). Pollutants such as black carbon, methane, tropospheric ozone, and aerosols affect the amount of incoming sunlight. As a result, the temperature of the Earth is increasing, resulting in the melting of ice, icebergs, and glaciers.

In this vein, climatic changes will affect the incidence and prevalence of both residual and imported infections in Europe. Climate and weather affect the duration, timing, and intensity of outbreaks strongly and change the map of infectious diseases in the globe ( 45 ). Mosquito-transmitted parasitic or viral diseases are extremely climate-sensitive, as warming firstly shortens the pathogen incubation period and secondly shifts the geographic map of the vector. Similarly, water-warming following climate changes leads to a high incidence of waterborne infections. Recently, in Europe, eradicated diseases seem to be emerging due to the migration of population, for example, cholera, poliomyelitis, tick-borne encephalitis, and malaria ( 46 ).

The spread of epidemics is associated with natural climate disasters and storms, which seem to occur more frequently nowadays ( 47 ). Malnutrition and disequilibration of the immune system are also associated with the emerging infections affecting public health ( 48 ).

The Chikungunya virus “took the airplane” from the Indian Ocean to Europe, as outbreaks of the disease were registered in Italy ( 49 ) as well as autochthonous cases in France ( 50 ).

An increase in cryptosporidiosis in the United Kingdom and in the Czech Republic seems to have occurred following flooding ( 36 , 51 ).

As stated previously, aerosols compounds are tiny in size and considerably affect the climate. They are able to dissipate sunlight (the albedo phenomenon) by dispersing a quarter of the sun's rays back to space and have cooled the global temperature over the last 30 years ( 52 ).

Air Pollutants

The World Health Organization (WHO) reports on six major air pollutants, namely particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Air pollution can have a disastrous effect on all components of the environment, including groundwater, soil, and air. Additionally, it poses a serious threat to living organisms. In this vein, our interest is mainly to focus on these pollutants, as they are related to more extensive and severe problems in human health and environmental impact. Acid rain, global warming, the greenhouse effect, and climate changes have an important ecological impact on air pollution ( 53 ).

Particulate Matter (PM) and Health

Studies have shown a relationship between particulate matter (PM) and adverse health effects, focusing on either short-term (acute) or long-term (chronic) PM exposure.

Particulate matter (PM) is usually formed in the atmosphere as a result of chemical reactions between the different pollutants. The penetration of particles is closely dependent on their size ( 53 ). Particulate Matter (PM) was defined as a term for particles by the United States Environmental Protection Agency ( 54 ). Particulate matter (PM) pollution includes particles with diameters of 10 micrometers (μm) or smaller, called PM 10 , and extremely fine particles with diameters that are generally 2.5 micrometers (μm) and smaller.

Particulate matter contains tiny liquid or solid droplets that can be inhaled and cause serious health effects ( 55 ). Particles <10 μm in diameter (PM 10 ) after inhalation can invade the lungs and even reach the bloodstream. Fine particles, PM 2.5 , pose a greater risk to health ( 6 , 56 ) ( Table 1 ).

Penetrability according to particle size.

Multiple epidemiological studies have been performed on the health effects of PM. A positive relation was shown between both short-term and long-term exposures of PM 2.5 and acute nasopharyngitis ( 56 ). In addition, long-term exposure to PM for years was found to be related to cardiovascular diseases and infant mortality.

Those studies depend on PM 2.5 monitors and are restricted in terms of study area or city area due to a lack of spatially resolved daily PM 2.5 concentration data and, in this way, are not representative of the entire population. Following a recent epidemiological study by the Department of Environmental Health at Harvard School of Public Health (Boston, MA) ( 57 ), it was reported that, as PM 2.5 concentrations vary spatially, an exposure error (Berkson error) seems to be produced, and the relative magnitudes of the short- and long-term effects are not yet completely elucidated. The team developed a PM 2.5 exposure model based on remote sensing data for assessing short- and long-term human exposures ( 57 ). This model permits spatial resolution in short-term effects plus the assessment of long-term effects in the whole population.

Moreover, respiratory diseases and affection of the immune system are registered as long-term chronic effects ( 58 ). It is worth noting that people with asthma, pneumonia, diabetes, and respiratory and cardiovascular diseases are especially susceptible and vulnerable to the effects of PM. PM 2.5 , followed by PM 10 , are strongly associated with diverse respiratory system diseases ( 59 ), as their size permits them to pierce interior spaces ( 60 ). The particles produce toxic effects according to their chemical and physical properties. The components of PM 10 and PM 2.5 can be organic (polycyclic aromatic hydrocarbons, dioxins, benzene, 1-3 butadiene) or inorganic (carbon, chlorides, nitrates, sulfates, metals) in nature ( 55 ).

Particulate Matter (PM) is divided into four main categories according to type and size ( 61 ) ( Table 2 ).

Types and sizes of particulate Matter (PM).

Gas contaminants include PM in aerial masses.

Particulate contaminants include contaminants such as smog, soot, tobacco smoke, oil smoke, fly ash, and cement dust.

Biological Contaminants are microorganisms (bacteria, viruses, fungi, mold, and bacterial spores), cat allergens, house dust and allergens, and pollen.

Types of Dust include suspended atmospheric dust, settling dust, and heavy dust.

Finally, another fact is that the half-lives of PM 10 and PM 2.5 particles in the atmosphere is extended due to their tiny dimensions; this permits their long-lasting suspension in the atmosphere and even their transfer and spread to distant destinations where people and the environment may be exposed to the same magnitude of pollution ( 53 ). They are able to change the nutrient balance in watery ecosystems, damage forests and crops, and acidify water bodies.

As stated, PM 2.5 , due to their tiny size, are causing more serious health effects. These aforementioned fine particles are the main cause of the “haze” formation in different metropolitan areas ( 12 , 13 , 61 ).

Ozone Impact in the Atmosphere

Ozone (O 3 ) is a gas formed from oxygen under high voltage electric discharge ( 62 ). It is a strong oxidant, 52% stronger than chlorine. It arises in the stratosphere, but it could also arise following chain reactions of photochemical smog in the troposphere ( 63 ).

Ozone can travel to distant areas from its initial source, moving with air masses ( 64 ). It is surprising that ozone levels over cities are low in contrast to the increased amounts occuring in urban areas, which could become harmful for cultures, forests, and vegetation ( 65 ) as it is reducing carbon assimilation ( 66 ). Ozone reduces growth and yield ( 47 , 48 ) and affects the plant microflora due to its antimicrobial capacity ( 67 , 68 ). In this regard, ozone acts upon other natural ecosystems, with microflora ( 69 , 70 ) and animal species changing their species composition ( 71 ). Ozone increases DNA damage in epidermal keratinocytes and leads to impaired cellular function ( 72 ).

Ground-level ozone (GLO) is generated through a chemical reaction between oxides of nitrogen and VOCs emitted from natural sources and/or following anthropogenic activities.

Ozone uptake usually occurs by inhalation. Ozone affects the upper layers of the skin and the tear ducts ( 73 ). A study of short-term exposure of mice to high levels of ozone showed malondialdehyde formation in the upper skin (epidermis) but also depletion in vitamins C and E. It is likely that ozone levels are not interfering with the skin barrier function and integrity to predispose to skin disease ( 74 ).

Due to the low water-solubility of ozone, inhaled ozone has the capacity to penetrate deeply into the lungs ( 75 ).

Toxic effects induced by ozone are registered in urban areas all over the world, causing biochemical, morphologic, functional, and immunological disorders ( 76 ).

The European project (APHEA2) focuses on the acute effects of ambient ozone concentrations on mortality ( 77 ). Daily ozone concentrations compared to the daily number of deaths were reported from different European cities for a 3-year period. During the warm period of the year, an observed increase in ozone concentration was associated with an increase in the daily number of deaths (0.33%), in the number of respiratory deaths (1.13%), and in the number of cardiovascular deaths (0.45%). No effect was observed during wintertime.

Carbon Monoxide (CO)

Carbon monoxide is produced by fossil fuel when combustion is incomplete. The symptoms of poisoning due to inhaling carbon monoxide include headache, dizziness, weakness, nausea, vomiting, and, finally, loss of consciousness.

The affinity of carbon monoxide to hemoglobin is much greater than that of oxygen. In this vein, serious poisoning may occur in people exposed to high levels of carbon monoxide for a long period of time. Due to the loss of oxygen as a result of the competitive binding of carbon monoxide, hypoxia, ischemia, and cardiovascular disease are observed.

Carbon monoxide affects the greenhouses gases that are tightly connected to global warming and climate. This should lead to an increase in soil and water temperatures, and extreme weather conditions or storms may occur ( 68 ).

However, in laboratory and field experiments, it has been seen to produce increased plant growth ( 78 ).

Nitrogen Oxide (NO 2 )

Nitrogen oxide is a traffic-related pollutant, as it is emitted from automobile motor engines ( 79 , 80 ). It is an irritant of the respiratory system as it penetrates deep in the lung, inducing respiratory diseases, coughing, wheezing, dyspnea, bronchospasm, and even pulmonary edema when inhaled at high levels. It seems that concentrations over 0.2 ppm produce these adverse effects in humans, while concentrations higher than 2.0 ppm affect T-lymphocytes, particularly the CD8+ cells and NK cells that produce our immune response ( 81 ).It is reported that long-term exposure to high levels of nitrogen dioxide can be responsible for chronic lung disease. Long-term exposure to NO 2 can impair the sense of smell ( 81 ).

However, systems other than respiratory ones can be involved, as symptoms such as eye, throat, and nose irritation have been registered ( 81 ).

High levels of nitrogen dioxide are deleterious to crops and vegetation, as they have been observed to reduce crop yield and plant growth efficiency. Moreover, NO 2 can reduce visibility and discolor fabrics ( 81 ).

Sulfur Dioxide (SO 2 )

Sulfur dioxide is a harmful gas that is emitted mainly from fossil fuel consumption or industrial activities. The annual standard for SO 2 is 0.03 ppm ( 82 ). It affects human, animal, and plant life. Susceptible people as those with lung disease, old people, and children, who present a higher risk of damage. The major health problems associated with sulfur dioxide emissions in industrialized areas are respiratory irritation, bronchitis, mucus production, and bronchospasm, as it is a sensory irritant and penetrates deep into the lung converted into bisulfite and interacting with sensory receptors, causing bronchoconstriction. Moreover, skin redness, damage to the eyes (lacrimation and corneal opacity) and mucous membranes, and worsening of pre-existing cardiovascular disease have been observed ( 81 ).

Environmental adverse effects, such as acidification of soil and acid rain, seem to be associated with sulfur dioxide emissions ( 83 ).

Lead is a heavy metal used in different industrial plants and emitted from some petrol motor engines, batteries, radiators, waste incinerators, and waste waters ( 84 ).

Moreover, major sources of lead pollution in the air are metals, ore, and piston-engine aircraft. Lead poisoning is a threat to public health due to its deleterious effects upon humans, animals, and the environment, especially in the developing countries.

Exposure to lead can occur through inhalation, ingestion, and dermal absorption. Trans- placental transport of lead was also reported, as lead passes through the placenta unencumbered ( 85 ). The younger the fetus is, the more harmful the toxic effects. Lead toxicity affects the fetal nervous system; edema or swelling of the brain is observed ( 86 ). Lead, when inhaled, accumulates in the blood, soft tissue, liver, lung, bones, and cardiovascular, nervous, and reproductive systems. Moreover, loss of concentration and memory, as well as muscle and joint pain, were observed in adults ( 85 , 86 ).

Children and newborns ( 87 ) are extremely susceptible even to minimal doses of lead, as it is a neurotoxicant and causes learning disabilities, impairment of memory, hyperactivity, and even mental retardation.

Elevated amounts of lead in the environment are harmful to plants and crop growth. Neurological effects are observed in vertebrates and animals in association with high lead levels ( 88 ).

Polycyclic Aromatic Hydrocarbons(PAHs)

The distribution of PAHs is ubiquitous in the environment, as the atmosphere is the most important means of their dispersal. They are found in coal and in tar sediments. Moreover, they are generated through incomplete combustion of organic matter as in the cases of forest fires, incineration, and engines ( 89 ). PAH compounds, such as benzopyrene, acenaphthylene, anthracene, and fluoranthene are recognized as toxic, mutagenic, and carcinogenic substances. They are an important risk factor for lung cancer ( 89 ).

Volatile Organic Compounds(VOCs)

Volatile organic compounds (VOCs), such as toluene, benzene, ethylbenzene, and xylene ( 90 ), have been found to be associated with cancer in humans ( 91 ). The use of new products and materials has actually resulted in increased concentrations of VOCs. VOCs pollute indoor air ( 90 ) and may have adverse effects on human health ( 91 ). Short-term and long-term adverse effects on human health are observed. VOCs are responsible for indoor air smells. Short-term exposure is found to cause irritation of eyes, nose, throat, and mucosal membranes, while those of long duration exposure include toxic reactions ( 92 ). Predictable assessment of the toxic effects of complex VOC mixtures is difficult to estimate, as these pollutants can have synergic, antagonistic, or indifferent effects ( 91 , 93 ).

Dioxins originate from industrial processes but also come from natural processes, such as forest fires and volcanic eruptions. They accumulate in foods such as meat and dairy products, fish and shellfish, and especially in the fatty tissue of animals ( 94 ).

Short-period exhibition to high dioxin concentrations may result in dark spots and lesions on the skin ( 94 ). Long-term exposure to dioxins can cause developmental problems, impairment of the immune, endocrine and nervous systems, reproductive infertility, and cancer ( 94 ).

Without any doubt, fossil fuel consumption is responsible for a sizeable part of air contamination. This contamination may be anthropogenic, as in agricultural and industrial processes or transportation, while contamination from natural sources is also possible. Interestingly, it is of note that the air quality standards established through the European Air Quality Directive are somewhat looser than the WHO guidelines, which are stricter ( 95 ).

Effect of Air Pollution on Health

The most common air pollutants are ground-level ozone and Particulates Matter (PM). Air pollution is distinguished into two main types:

Outdoor pollution is the ambient air pollution.

Indoor pollution is the pollution generated by household combustion of fuels.

People exposed to high concentrations of air pollutants experience disease symptoms and states of greater and lesser seriousness. These effects are grouped into short- and long-term effects affecting health.

Susceptible populations that need to be aware of health protection measures include old people, children, and people with diabetes and predisposing heart or lung disease, especially asthma.

As extensively stated previously, according to a recent epidemiological study from Harvard School of Public Health, the relative magnitudes of the short- and long-term effects have not been completely clarified ( 57 ) due to the different epidemiological methodologies and to the exposure errors. New models are proposed for assessing short- and long-term human exposure data more successfully ( 57 ). Thus, in the present section, we report the more common short- and long-term health effects but also general concerns for both types of effects, as these effects are often dependent on environmental conditions, dose, and individual susceptibility.

Short-term effects are temporary and range from simple discomfort, such as irritation of the eyes, nose, skin, throat, wheezing, coughing and chest tightness, and breathing difficulties, to more serious states, such as asthma, pneumonia, bronchitis, and lung and heart problems. Short-term exposure to air pollution can also cause headaches, nausea, and dizziness.

These problems can be aggravated by extended long-term exposure to the pollutants, which is harmful to the neurological, reproductive, and respiratory systems and causes cancer and even, rarely, deaths.

The long-term effects are chronic, lasting for years or the whole life and can even lead to death. Furthermore, the toxicity of several air pollutants may also induce a variety of cancers in the long term ( 96 ).

As stated already, respiratory disorders are closely associated with the inhalation of air pollutants. These pollutants will invade through the airways and will accumulate at the cells. Damage to target cells should be related to the pollutant component involved and its source and dose. Health effects are also closely dependent on country, area, season, and time. An extended exposure duration to the pollutant should incline to long-term health effects in relation also to the above factors.

Particulate Matter (PMs), dust, benzene, and O 3 cause serious damage to the respiratory system ( 97 ). Moreover, there is a supplementary risk in case of existing respiratory disease such as asthma ( 98 ). Long-term effects are more frequent in people with a predisposing disease state. When the trachea is contaminated by pollutants, voice alterations may be remarked after acute exposure. Chronic obstructive pulmonary disease (COPD) may be induced following air pollution, increasing morbidity and mortality ( 99 ). Long-term effects from traffic, industrial air pollution, and combustion of fuels are the major factors for COPD risk ( 99 ).

Multiple cardiovascular effects have been observed after exposure to air pollutants ( 100 ). Changes occurred in blood cells after long-term exposure may affect cardiac functionality. Coronary arteriosclerosis was reported following long-term exposure to traffic emissions ( 101 ), while short-term exposure is related to hypertension, stroke, myocardial infracts, and heart insufficiency. Ventricle hypertrophy is reported to occur in humans after long-time exposure to nitrogen oxide (NO 2 ) ( 102 , 103 ).

Neurological effects have been observed in adults and children after extended-term exposure to air pollutants.

Psychological complications, autism, retinopathy, fetal growth, and low birth weight seem to be related to long-term air pollution ( 83 ). The etiologic agent of the neurodegenerative diseases (Alzheimer's and Parkinson's) is not yet known, although it is believed that extended exposure to air pollution seems to be a factor. Specifically, pesticides and metals are cited as etiological factors, together with diet. The mechanisms in the development of neurodegenerative disease include oxidative stress, protein aggregation, inflammation, and mitochondrial impairment in neurons ( 104 ) ( Figure 1 ).

Impact of air pollutants on the brain.

Brain inflammation was observed in dogs living in a highly polluted area in Mexico for a long period ( 105 ). In human adults, markers of systemic inflammation (IL-6 and fibrinogen) were found to be increased as an immediate response to PNC on the IL-6 level, possibly leading to the production of acute-phase proteins ( 106 ). The progression of atherosclerosis and oxidative stress seem to be the mechanisms involved in the neurological disturbances caused by long-term air pollution. Inflammation comes secondary to the oxidative stress and seems to be involved in the impairment of developmental maturation, affecting multiple organs ( 105 , 107 ). Similarly, other factors seem to be involved in the developmental maturation, which define the vulnerability to long-term air pollution. These include birthweight, maternal smoking, genetic background and socioeconomic environment, as well as education level.

However, diet, starting from breast-feeding, is another determinant factor. Diet is the main source of antioxidants, which play a key role in our protection against air pollutants ( 108 ). Antioxidants are free radical scavengers and limit the interaction of free radicals in the brain ( 108 ). Similarly, genetic background may result in a differential susceptibility toward the oxidative stress pathway ( 60 ). For example, antioxidant supplementation with vitamins C and E appears to modulate the effect of ozone in asthmatic children homozygous for the GSTM1 null allele ( 61 ). Inflammatory cytokines released in the periphery (e.g., respiratory epithelia) upregulate the innate immune Toll-like receptor 2. Such activation and the subsequent events leading to neurodegeneration have recently been observed in lung lavage in mice exposed to ambient Los Angeles (CA, USA) particulate matter ( 61 ). In children, neurodevelopmental morbidities were observed after lead exposure. These children developed aggressive and delinquent behavior, reduced intelligence, learning difficulties, and hyperactivity ( 109 ). No level of lead exposure seems to be “safe,” and the scientific community has asked the Centers for Disease Control and Prevention (CDC) to reduce the current screening guideline of 10 μg/dl ( 109 ).

It is important to state that impact on the immune system, causing dysfunction and neuroinflammation ( 104 ), is related to poor air quality. Yet, increases in serum levels of immunoglobulins (IgA, IgM) and the complement component C3 are observed ( 106 ). Another issue is that antigen presentation is affected by air pollutants, as there is an upregulation of costimulatory molecules such as CD80 and CD86 on macrophages ( 110 ).

As is known, skin is our shield against ultraviolet radiation (UVR) and other pollutants, as it is the most exterior layer of our body. Traffic-related pollutants, such as PAHs, VOCs, oxides, and PM, may cause pigmented spots on our skin ( 111 ). On the one hand, as already stated, when pollutants penetrate through the skin or are inhaled, damage to the organs is observed, as some of these pollutants are mutagenic and carcinogenic, and, specifically, they affect the liver and lung. On the other hand, air pollutants (and those in the troposphere) reduce the adverse effects of ultraviolet radiation UVR in polluted urban areas ( 111 ). Air pollutants absorbed by the human skin may contribute to skin aging, psoriasis, acne, urticaria, eczema, and atopic dermatitis ( 111 ), usually caused by exposure to oxides and photochemical smoke ( 111 ). Exposure to PM and cigarette smoking act as skin-aging agents, causing spots, dyschromia, and wrinkles. Lastly, pollutants have been associated with skin cancer ( 111 ).

Higher morbidity is reported to fetuses and children when exposed to the above dangers. Impairment in fetal growth, low birth weight, and autism have been reported ( 112 ).

Another exterior organ that may be affected is the eye. Contamination usually comes from suspended pollutants and may result in asymptomatic eye outcomes, irritation ( 112 ), retinopathy, or dry eye syndrome ( 113 , 114 ).

Environmental Impact of Air Pollution

Air pollution is harming not only human health but also the environment ( 115 ) in which we live. The most important environmental effects are as follows.

Acid rain is wet (rain, fog, snow) or dry (particulates and gas) precipitation containing toxic amounts of nitric and sulfuric acids. They are able to acidify the water and soil environments, damage trees and plantations, and even damage buildings and outdoor sculptures, constructions, and statues.

Haze is produced when fine particles are dispersed in the air and reduce the transparency of the atmosphere. It is caused by gas emissions in the air coming from industrial facilities, power plants, automobiles, and trucks.

Ozone , as discussed previously, occurs both at ground level and in the upper level (stratosphere) of the Earth's atmosphere. Stratospheric ozone is protecting us from the Sun's harmful ultraviolet (UV) rays. In contrast, ground-level ozone is harmful to human health and is a pollutant. Unfortunately, stratospheric ozone is gradually damaged by ozone-depleting substances (i.e., chemicals, pesticides, and aerosols). If this protecting stratospheric ozone layer is thinned, then UV radiation can reach our Earth, with harmful effects for human life (skin cancer) ( 116 ) and crops ( 117 ). In plants, ozone penetrates through the stomata, inducing them to close, which blocks CO 2 transfer and induces a reduction in photosynthesis ( 118 ).

Global climate change is an important issue that concerns mankind. As is known, the “greenhouse effect” keeps the Earth's temperature stable. Unhappily, anthropogenic activities have destroyed this protecting temperature effect by producing large amounts of greenhouse gases, and global warming is mounting, with harmful effects on human health, animals, forests, wildlife, agriculture, and the water environment. A report states that global warming is adding to the health risks of poor people ( 119 ).

People living in poorly constructed buildings in warm-climate countries are at high risk for heat-related health problems as temperatures mount ( 119 ).

Wildlife is burdened by toxic pollutants coming from the air, soil, or the water ecosystem and, in this way, animals can develop health problems when exposed to high levels of pollutants. Reproductive failure and birth effects have been reported.

Eutrophication is occurring when elevated concentrations of nutrients (especially nitrogen) stimulate the blooming of aquatic algae, which can cause a disequilibration in the diversity of fish and their deaths.

Without a doubt, there is a critical concentration of pollution that an ecosystem can tolerate without being destroyed, which is associated with the ecosystem's capacity to neutralize acidity. The Canada Acid Rain Program established this load at 20 kg/ha/yr ( 120 ).

Hence, air pollution has deleterious effects on both soil and water ( 121 ). Concerning PM as an air pollutant, its impact on crop yield and food productivity has been reported. Its impact on watery bodies is associated with the survival of living organisms and fishes and their productivity potential ( 121 ).

An impairment in photosynthetic rhythm and metabolism is observed in plants exposed to the effects of ozone ( 121 ).

Sulfur and nitrogen oxides are involved in the formation of acid rain and are harmful to plants and marine organisms.

Last but not least, as mentioned above, the toxicity associated with lead and other metals is the main threat to our ecosystems (air, water, and soil) and living creatures ( 121 ).

In 2018, during the first WHO Global Conference on Air Pollution and Health, the WHO's General Director, Dr. Tedros Adhanom Ghebreyesus, called air pollution a “silent public health emergency” and “the new tobacco” ( 122 ).

Undoubtedly, children are particularly vulnerable to air pollution, especially during their development. Air pollution has adverse effects on our lives in many different respects.

Diseases associated with air pollution have not only an important economic impact but also a societal impact due to absences from productive work and school.

Despite the difficulty of eradicating the problem of anthropogenic environmental pollution, a successful solution could be envisaged as a tight collaboration of authorities, bodies, and doctors to regularize the situation. Governments should spread sufficient information and educate people and should involve professionals in these issues so as to control the emergence of the problem successfully.

Technologies to reduce air pollution at the source must be established and should be used in all industries and power plants. The Kyoto Protocol of 1997 set as a major target the reduction of GHG emissions to below 5% by 2012 ( 123 ). This was followed by the Copenhagen summit, 2009 ( 124 ), and then the Durban summit of 2011 ( 125 ), where it was decided to keep to the same line of action. The Kyoto protocol and the subsequent ones were ratified by many countries. Among the pioneers who adopted this important protocol for the world's environmental and climate “health” was China ( 3 ). As is known, China is a fast-developing economy and its GDP (Gross Domestic Product) is expected to be very high by 2050, which is defined as the year of dissolution of the protocol for the decrease in gas emissions.

A more recent international agreement of crucial importance for climate change is the Paris Agreement of 2015, issued by the UNFCCC (United Nations Climate Change Committee). This latest agreement was ratified by a plethora of UN (United Nations) countries as well as the countries of the European Union ( 126 ). In this vein, parties should promote actions and measures to enhance numerous aspects around the subject. Boosting education, training, public awareness, and public participation are some of the relevant actions for maximizing the opportunities to achieve the targets and goals on the crucial matter of climate change and environmental pollution ( 126 ). Without any doubt, technological improvements makes our world easier and it seems difficult to reduce the harmful impact caused by gas emissions, we could limit its use by seeking reliable approaches.

Synopsizing, a global prevention policy should be designed in order to combat anthropogenic air pollution as a complement to the correct handling of the adverse health effects associated with air pollution. Sustainable development practices should be applied, together with information coming from research in order to handle the problem effectively.

At this point, international cooperation in terms of research, development, administration policy, monitoring, and politics is vital for effective pollution control. Legislation concerning air pollution must be aligned and updated, and policy makers should propose the design of a powerful tool of environmental and health protection. As a result, the main proposal of this essay is that we should focus on fostering local structures to promote experience and practice and extrapolate these to the international level through developing effective policies for sustainable management of ecosystems.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

IM is employed by the company Delphis S.A. The remaining authors declare that the present review paper was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Human Impacts on the Environment

Humans impact the physical environment in many ways: overpopulation, pollution, burning fossil fuels, and deforestation. Changes like these have triggered climate change, soil erosion, poor air quality, and undrinkable water. These negative impacts can affect human behavior and can prompt mass migrations or battles over clean water.

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An Extended Family Perspective on Intergenerational Human Capital Transmission in China

• Original Research
• Published: 06 September 2024

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• Wei Zou   ORCID: orcid.org/0000-0001-9413-1957 1 &
• Ruiqi Ma   ORCID: orcid.org/0000-0002-0014-2039 1  

Intergenerational transmission is a crucial indicator for measuring the equality of opportunities in society. Higher levels of intergenerational transmission can suppress individual efforts and have severe negative impacts on various aspects such as distribution, growth, and innovation. This paper synthesises theoretical and empirical work on intergenerational human capital transmission from an extended family perspective in China. The majority of existent literature on intergenerational transmission only explores the correlation of parents and their own offspring in aspects of income or social status, and may lead to underestimation of the level of intergenerational transmission in society since it negelcts the potential impacts from clan networks. Building on the theoretical analysis, the study is designed to build up an extended family framework and provide insights into the overall intergenerational transmission of human capital in China, where historical tradition is deeprooted and clan networks are closely linked, using (CHIP) Chinese Household Income Project Survey, a longitudinal household-level dataset. We compile data from extended families including parents and grandparents, as well as parents’ siblings, and estimate the intergenerational transmission of human capital with two different measures, the schooling years and an index of occupational status. Our main findings show that the traditional parent–child intergenerational transmission model represents the lower bound of the intergenerational transmission coefficient, and parental siblings have a significant and positive effect on offspring’s human capital attainment, and the results remain solid after a bunch of robustness tests. We find significant heterogeneity in the intergenerational transmission of human capital by gender, birth cohorts or rural–urban residency. The multigenerational effects of grandparents on descendants are not significant after controlling for the human capital levels of parents and their siblings. Using a sample of non-biological offspring as a comparison group, we find that the acquired family environment plays a more significant role than innate genes. Fundamentally, this study helps establish policies to enhance intergenerational mobility and social opportunities.

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Acknowledgements

The authors acknowledge the support of the National Social Science Foundation of China under a major project entitled "Research on the Long-term Mechanism of Relative Poverty Alleviation" (20&ZD168) and the National Natural Science Foundation of China under the project entitled "Intergenerational Transmission, Neighbourhood Effect and Educational Poverty: A Social Network Economics Perspective" (71973102). We would like to thank Editage ( www.editage.cn ) for English language editing.

National Social Science Foundation of China,20&amp;ZD168,Wei Zou,National Natural Science Foundation of China,71973102,Wei Zou

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S1: Mathematical Derivation and Monte Carlo Simulation

For ease of understanding, a binary linear regression model (assuming K  = 2) is set up in this study to derive this conclusion.

Let \(K=M=2\) in Eq. ( 1 ), and the proxies for the explanatory variables be \({x}_{1}\) and \({x}_{2}\) , respectively. \({x}_{1}\) and \({x}_{2}\) are standardized series with a mean of 0 and variance of 1. Both explanatory and explained variables are subject to measurement bias: \(y = y^{*} + v\) , \(x_{1} = x_{1}^{*} + u_{1}\) , \(x_{2} = x_{2}^{*} + u_{2}\) . Assumptions: \(u_{1} , u_{2} , v\) are uncorrelated with \({x}_{1}^{*}\) , \({x}_{2}^{*}\) , \({y}^{*}\) , respectively; \(\beta_{k} \ge 0, \forall k > 0\) ; \(Cov\left( {x_{1} ,x_{2} } \right) = \rho \ge 0\) define \(Var\left( {u_{1} } \right) = \lambda_{1}\) , \(Var\left( {u_{2} } \right) = \lambda_{2}\) , \(Var\left( {e + v} \right) = \delta^{2}\) . Substituting the observable variables into Eq. ( 1 ) yields the equation: \(y = \beta_{1} x_{1} + \beta_{2} x_{2} + w\) , where \(w = e + v - \beta_{1} \mu_{1} - \beta_{2} \mu_{2}\) .

It is not difficult to derive: \(Cov\left( {x_{1} ,w} \right) = - \beta_{1} \;\mu_{1}\) , \(Cov\left( {x_{2} ,w} \right) = - \beta_{2} \mu_{2}\) , \(Cov\left( {y,x_{1} } \right) = \beta_{1} + \beta_{2} \rho - \beta_{1} \lambda_{1}\) , \(Cov\left( {y,x_{2} } \right) = \beta_{2} + \beta_{1} \rho - \beta_{2} \lambda_{2}\) . Using OLS to estimate Eq. ( 1 ) yields the following set of normal equations.

According to asymptotic theory, the sample moments are consistent estimates of the overall moments under large samples: \(\frac{1}{n}\sum {x_{k} x_{{k^{\prime}}} \to _{p} Cov(x_{k} , x_{{k^{\prime}}} )}\) . Finding the probability limit for both sides of the above system of normal equations simultaneously gives:

The OLS estimates \({\widehat{\beta }}_{1}\) and \({\widehat{\beta }}_{2}\) are not consistent estimates of the true parameters, and the direction of bias is uncertain. If \({\widehat{\beta }}_{1}\) and \({\widehat{\beta }}_{2}\) are added together, this gives:

The sum of the OLS estimates is a lower bound on the actual intergenerational mobility coefficient. The above equation holds for an equal number of explanatory variables and their proxies \(\left( {M = K = 2} \right)\) , and when there is only one proxy variable in the OLS regression equation \(\left( {M = 1 < K} \right)\) , we can still reach a similar conclusion: \(p\lim (\hat{\beta }_{1} + \hat{\beta }_{2} ) = \beta_{1} \left( {1 - \lambda_{1} } \right) + \beta_{2} \rho \le \beta_{1} + \beta_{2}\) . Similarly, when \(\beta_{2} = 0\) \(\left( {M > K = 1} \right)\) , we still have \(p\lim (\hat{\beta }_{1} + \hat{\beta }_{2} ) = \beta_{1} \left[ {1 - \frac{{\lambda_{1} }}{{\left( {1 + \rho } \right)}}} \right] \le \beta_{1}\) .

The assumption that the explanatory variables’ measurement bias \({u}_{1}\) and \({u}_{2}\) are irrelevant can be further relaxed when continuing with the assumptions \(\beta_{k} \ge \beta_{k + 1}\) and \(\lambda_{k} \ge \lambda_{k + 1}\) .

The hypothesis is empirically reasonable in reality, where an increase in  k  means that that parents' generation relative is more distantly related to the offspring and, therefore, likely to have a minor impact on the human capital of the offspring. A more distant kinship means more individuals can be obtained to match them, thus reducing the variance of the measurement bias. Defining \(Cov\left( {u_{1} ,u_{2} } \right) = \sigma\) , the probability limit for the sum of the OLS estimates is derived as:

According to Eq. ( 8 ), either \(\sigma >0\) or \(\sigma <0\) yields \(\mathit{plim}\left({\widehat{\beta }}_{1}+{\widehat{\beta }}_{2}\right)\le {\beta }_{1}+{\beta }_{2}.\) By the Cauchy–Schwarz inequality, we have \(\left|\sigma \right|\le \sqrt{{\lambda }_{1}{\lambda }_{2}}\) . Even considering the worst case, when \(\sigma\) takes the smallest value in the interval, we have \(\sigma =-\sqrt{{\lambda }_{1}{\lambda }_{2}}\) , such that \({\beta }_{1}\left({\lambda }_{1}+\sigma \right)+{\beta }_{2}\left({\lambda }_{2}+\sigma \right)=({\beta }_{1}{\sqrt{\lambda }}_{1}-{\beta }_{2}{\sqrt{\lambda }}_{2})({\sqrt{\lambda }}_{1}-{\sqrt{\lambda }}_{2})\ge 0\) .

The above conclusions still hold for multivariate linear models. Adermon (2021) provides a procedure for deriving proof that the conclusion holds in the general case ( \(\forall M,K>0\) ) using matrix algebra. The general conclusion that the sum of the OLS estimated coefficients is a lower bound on the sum of the actual parameters is obtained here by Monte Carlo simulations using STATA software.

Before conducting Monte Carlo simulations, this study makes the following arrangements: first, we set \({\beta }_{k}, {\lambda }_{k}\sim U(\text{0,1})\) , \(e\sim N(\text{0,1})\) , \({{\varvec{x}}}^{*}\sim {\varvec{N}}(0,{\boldsymbol{\Sigma }}^{*})\) , \({\varvec{u}}\sim {\varvec{N}}(0,\boldsymbol{\Sigma })\) , where \({\boldsymbol{\Sigma }}^{*}\) is the covariance matrix of the true explanatory variable \({{\varvec{x}}}^{*}\) , whose main diagonal element is \(1-{\lambda }_{k}\) , and \(\boldsymbol{\Sigma }\) is the covariance matrix of the measurement bias \({\varvec{u}}\) , whose main diagonal element is \({\lambda }_{k}\) . Second, if we want to randomly generate the \({{\varvec{x}}}^{*}\) and \({\varvec{u}}\) , we must first specify their covariance matrix (symmetric positive definite matrix); thus, we first randomly generate a random matrix \({\varvec{A}}\) with diagonal elements of 1 and other elements obeying uniform distribution, then convert this random matrix into a symmetric matrix \({\varvec{C}}\) by the formula \(({\varvec{A}}+{{\varvec{A}}}{\prime})/2\) , and judge whether the symmetric random matrix \({\varvec{C}}\) is a positive definite matrix. If the positive definite matrix is judged to be true, then continue; otherwise, re-execute the above steps until the positive definite matrix is judged to be true. Third, multiply \(\sqrt{(1-{\lambda }_{i})(1-{\lambda }_{j})}\) by the elements of the i -th row and j -th column of the symmetric positive random matrix C to obtain the covariance matrix \({\boldsymbol{\Sigma }}^{*}\) of \({{\varvec{x}}}^{*}\) ; then multiply \(\sqrt{{\lambda }_{i}{\lambda }_{j}}\) by the elements of the i -th row and j -th column of the symmetric positive random matrix C to obtain the covariance matrix \(\boldsymbol{\Sigma }\) of \({\varvec{u}}\) ; and finally, random variables with joint normal distribution are generated from the covariance matrix of \({{\varvec{x}}}^{*}\) and \({\varvec{u}}\) for Monte Carlo simulation.

We set the sample size of the simulation to 100,000, the number of simulations to 1,000, the number of actual explanatory variables K to 5, and the number of proxies M to be in the range of 1 to 5. The number of proxies was chosen separately for OLS estimation, and the sum of the coefficients of the proxies was estimated and compared with the sum of the actual parameters. The specific results are shown in Fig.  2 .

Monte Carlo simulation of OLS estimation

The red dashed lines in (a) and (b) in Fig.  2 are the 45-degree lines, showing the scatter plots of the sum of the OLS coefficient estimates and the sum of the actual parameters when the number of proxies  M  is 1, 3, and 5 (the darker the scatter, the higher the number of proxies) for the two cases of no correlation between the measurement biases and positive correlation between the measurement biases, respectively. As can be seen, the scatter points are all below the 45-degree line, which means that the resulting sum of OLS coefficient estimates is smaller than the sum of the actual parameters, indicating that the direction of bias of the sum of OLS coefficient estimates is determined as a lower bound of the sum of the actual parameters. (c) and (d) in Fig.  2 shows the bias distribution in the sum of OLS coefficient estimates for different numbers of proxies  M for the  two scenarios of measured bias correlation described above, respectively. The bias in the sum of OLS coefficient estimates decreases as the number of proxies M increases, but when there is a positive correlation between the measurement biases, the estimated bias does not decrease as fast as when there is no correlation between the measurement biases.

If parental human capital \({x}_{1}\) is set as the endogenous variable and the human capital variables of other parents' generation relatives are set as instrumental variables, estimating the coefficient of \({x}_{1}\) by two-stage least squares (TSLS) can obtain an upper bound on the long-term intergenerational mobility coefficient. Continuing with the binary linear regression model set above, when we set \({x}_{2}\) as the instrumental variable (IV) for \({x}_{1}\) , using the TSLS estimating equation \(y=\beta {x}_{1}+\varsigma\) yields the TSLS estimate for \(\beta\) as:

At this point, \({\widehat{\beta }}_{TSLS}\ge {\beta }_{1}+{\beta }_{2}\) , indicates that the TSLS estimate for \(\beta\) is an upper bound on the sum of the actual coefficients. According to the Cauchy–Schwarz inequality, there is \(\rho \le \sqrt{(1-{\lambda }_{1})(1-{\lambda }_{2})}\) , and when assuming \({\lambda }_{k}\ge {\lambda }_{k+1}\) , there is \(\rho \le 1-{\lambda }_{2}\) . This is also intuitively easy to understand, as any variable related to parental human capital may have an independent effect on the human capital of the offspring, which leads to an upward bias in the intergenerational transferability TSLS estimates. Thus, the TSLS estimates can provide an upper bound on the true intergenerational transmission coefficient (Blanden, 2013 ).  

Monte Carlo simulation of TSLS coefficient estimation

Figure 3 shows the results of a Monte Carlo simulation with a sample size of 100,000 and a number of simulations of 1,000. In this case, the true data generation process (DGP) is set to be a binary linear equation, and the TSLS estimate of \(\beta\) is obtained by setting \({x}_{2}\) as an instrumental variable for \({x}_{1}\) at the time of estimation. To avoid the problem of weak instrumental variables, we set the correlation coefficients of \({x}_{1}\) and \({x}_{2}\) to obey uniformly distributed random numbers of no less than 0.5.

The simulation results are provided in (a) and (b) of Fig.  3 for unrestricted \({\lambda }_{k}\ge {\lambda }_{k+1}\) to hold and restricted \({\lambda }_{k}\ge {\lambda }_{k+1}\) to hold, respectively. When unrestricted \({\lambda }_{k}\ge {\lambda }_{k+1}\) holds, most of the TSLS estimates lie above the 45-degree line, while when restricted \({\lambda }_{k}\ge {\lambda }_{k+1}\) holds, all of the TSLS estimates lie above the 45-degree line, indicating that the TSLS estimates are greater than the sum of the actual coefficients.

S2: Appendix Tables

Tables 12 , 13 , 14 , 15 , 16 .

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Zou, W., Ma, R. An Extended Family Perspective on Intergenerational Human Capital Transmission in China. Soc Indic Res (2024). https://doi.org/10.1007/s11205-024-03427-3

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DOI : https://doi.org/10.1007/s11205-024-03427-3

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Research on low-light environment object detection algorithm based on yolo_gd.

1. Introduction

2. related works, 2.1. object detection methods, 2.2. attention mechanism, 3. the yolo_gd network model, 3.1. the bi-level routing spatial attention module, 3.2. cross-layer feature fusion method based on information aggregation and distribution mechanism, 3.3. cross-stage local feature fusion module, 3.4. anchor box loss function with integrated minimum point distance mechanism, 4. experimental results, 4.1. experimental datasets, 4.2. experimental environment and parameter settings, 4.3. evaluation metrics, 4.4. model training results, 4.5. quantitative analysis, 4.6. qualitative analysis, 4.7. ablation experiments, 4.8. supplementary experiments on the uavdt dataset, 5. conclusions and future studies, author contributions, data availability statement, conflicts of interest.

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Li, J.; Wang, X.; Chang, Q.; Wang, Y.; Chen, H. Research on Low-Light Environment Object Detection Algorithm Based on YOLO_GD. Electronics 2024 , 13 , 3527. https://doi.org/10.3390/electronics13173527

Li J, Wang X, Chang Q, Wang Y, Chen H. Research on Low-Light Environment Object Detection Algorithm Based on YOLO_GD. Electronics . 2024; 13(17):3527. https://doi.org/10.3390/electronics13173527

Li, Jian, Xin Wang, Qi Chang, Yongshan Wang, and Haifeng Chen. 2024. "Research on Low-Light Environment Object Detection Algorithm Based on YOLO_GD" Electronics 13, no. 17: 3527. https://doi.org/10.3390/electronics13173527

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Reconstructing early human colonization of the Balearic Islands in the western Mediterranean is challenging due to limited archaeological evidence. Current understanding places human arrival ~4400 years ago. Here, U-series data from phreatic overgrowth on speleothems are combined with the discovery of a submerged bridge in Genovesa Cave that exhibits a distinctive coloration band near its top. The band is at the same depth as the phreatic overgrowth on speleothems (−1.1 meters), both of which indicate a sea-level stillstand between ~6000 and ~5400 years ago. Integrating the bridge depth with a high-resolution Holocene sea-level curve for Mallorca and the dated phreatic overgrowth on speleothems level constrains the construction of the bridge between ~6000 and ~5600 years ago. Subsequent sea-level rise flooded the archeological structure, ruling out later construction dates. This provides evidence for early human presence on the island dating at least 5600 and possibly beyond ~6000 years ago.

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Introduction.

Mallorca, the main island of the Balearic Archipelago, is the sixth largest in the Mediterranean Sea, yet it was among the last to be colonized 1 . An in-depth discussion concerning the earliest colonization of various Mediterranean islands, including Mallorca, may be found in Cherry and Leppard 1 , Dawson 2 , and Simmons 3 . Despite extensive research on this topic, there has been considerable disagreement about the timing of the earliest colonization of Mallorca. Radiocarbon dating of bone material excavated from Cova (Cave) de Moleta indicate human presence on the island as early as 7000 calibrated years before present (cal B.P.) 4 . Subsequent age determinations from findings in Cova de Canet, further extended the timeline, suggesting human occupation dating back to approximately 9000 cal B.P. 5 . A series of publications 6 , 7 , 8 , 9 , 10 , 11 revealed inconsistencies regarding the exact stratigraphic position and context of the dated bone (sample KBN-640d 12 ) in Cova de Moleta. Due to the overall poor preservation of the samples and the lack of clear and specific information on this particular radiocarbon-dated sample, Ramis and Alcover 7 suggested that the bone fragment, initially identified as human, might actually belong to M. balearicus , an endemic bovid. Consequently, this sample was considered not relevant for determining the timing of the island’s colonization. Similarly, the radiocarbon dates from Cova de Canet were considered highly controversial because they originate from a charcoal layer that lacks clear evidence of human activity 7 , 8 . Furthermore, in neither of these caves do the M. balearicus bones show butchery marks, making it difficult to establish a clear link to contemporary human presence 2 . Due to the aforementioned issues these early results were deemed unreliable 1 , 8 , 13 .

Several studies have reevaluated most of the previously dated materials and supplemented them with new radiocarbon dates obtained from charcoal, ash, and bones 6 , 7 , 9 , 10 . Based on these new results, there is now a consensus that the timeframe for earliest human settlement on the island is between 4600 and 4200 cal B.P. 14 .

Dawson 2 presents a synthesis of the various lines of argument regarding arrival models in the Balearic islands that includes: (1) Early (~9000 cal B.P.), (2) Intermediate (~7600 cal B.P.), and (3) Late (~5000 cal B.P.) arrival phases. The last two models suggest the existence of stable settlements, yet only the third one has been deemed plausible in the local archeological literature 7 , 8 , 14 .

While there has been a growing body of evidence revealing progressively earlier human settlements on many islands in the Mediterranean basin, the timeline for the initial human colonization in Mallorca has seen relatively minor adjustments over the past decades 8 , 15 , 16 . The latest research suggests that this colonization occurred approximately 4400 cal B.P., coinciding with the human-mediated extinction of Myotragus balearicus 14 . This conclusion is based on two radiocarbon ages, which provide a relatively narrow time window of 350 years ( p  > 90%) between the last documented Myotragus bone (4581–4417 cal B.P.) and the first dated sheep bone (4417–4231 cal B.P.). However, it remains challenging to confirm whether the ages of these paleontological remains represent the latest or the earliest such occurrences on the island. Subsequent field work may shed light on this matter.

Our study site is a submerged archeological structure in the Genovesa Cave (also known as Cova de’n Bessó; 39°31’32” N, 3°19’2” E), situated in the eastern part of Mallorca (Fig.  1a, b ). The cave hosts ceramic sherds and stone constructions. The latter includes a stone-paved path that connects the cave entrance to the first underground lake (Fig.  1d ), a cyclopean stone wall running parallel to the path, and an 8.62 m long 17 and 0.5 m high stone walkway (hereafter referred as to bridge) oriented NE–SW (Fig.  1c, e , Supplementary Fig.  1 , Supplementary Table  1 ). This last structure was built across a lake by stacking large limestone breakdown blocks on top of each other, without the use of mortar or cement. The uppermost layer comprises flat boulders of considerable size (Supplementary Fig.  1b ). The largest stone measures 1.63 m in length and 0.6 m in width. Relative to the preindustrial (pre-1900 CE) sea level, the bridge is submerged by 1.05 ± 0.1 m of water at its upper part (Figs.  1 e, 2 ). However, at the time of its construction, it served as an access path to the only other dry chamber in the cave (Sala de les Rates-pinyades, i.e., Bats Room), where pottery, tentatively attributed to the Naviform period (ca. 3550–3000 cal B.P.) was discovered 18 , 19 . The bridge structure was inferred to have been built around the same period 20 .

a Map showing Mallorca in the western Mediterranean (black square). b Location of Genovesa and Drac caves; CCG: Closos de Can Gaià archeological site. c Plan of Genovesa Cave showing the location of the phreatic overgrowth on speleothems samples (yellow circles) and the survey station (red dot). d Photograph of the stone-paved path leading to the bridge (person height = 167 cm). e Cross-section (x–x’) indicating the location of the submerged bridge relative to the cave entrance and the present sea level. Maps ( a , b ) are available under CC Public Domain License from https://pixabay.com/illustrations/map-europe-world-earth-continent-2672639/ and https://pixabay.com/illustrations/mallorca-map-land-country-europe-968363/ , respectively.

The cross sections depict the spatial relationship between the submerged bridge and the U-series dated samples (phreatic overgrowth on speleothems: orange/yellow spindle; soda straw tips: red circle) from Genovesa and Drac caves. The vertical scale applies uniformly to all samples from both caves. All ages are reported as thousands of years (kyr) before present, where present is defined as 1950 CE.

Here, we integrate uranium-series (U-series) age data acquired from phreatic overgrowth on speleothems and stalactite tips in Genovesa and Drac caves, along with Late Holocene relative sea level (RSL) information available for Mallorca 21 . Additionally, we consider the presence of the bridge, the coloration mark on its upper part, and the depths at which these respective features occur. This combined evidence contributes valuable insights to the ongoing debate surrounding the timing of human colonization on Mallorca.

Results and discussion

Speleothems and sea level.

Proxies for cave-based sea-level reconstructions include mineralogical (sediments, speleothems) 22 , 23 , archeological (fish tanks, saltpans, submerged structures, etc.) 24 , and biological (borings, worm tubes, etc.) 24 records. In the case of Genovesa Cave, a typical coastal karst feature situated <450 m from the shoreline, both mineralogical and archeological records are present. Many of its well-decorated passages, galleries, and chambers are now flooded due to rising sea levels 20 . Because of the cave’s proximity to the coast and the high permeability of the Upper Miocene host rock 25 , the hydraulic gradient is negligible (9 × 10 -5  m /m) for such short distances (see Methods), and thus the water table in the cave is, and was in the past, coincident with sea level 26 , 27 . During times of high sea level stillstands, when the cave was partly flooded, distinct encrustations of calcite and aragonite accumulated over preexisting stalactites, forming the so-called phreatic overgrowths on speleothems 28 (POS). This is a particularly useful proxy for precisely and accurately reconstructing sea-level changes across various timescales 21 , 29 . Furthermore, ordinary stalactites, which form in cave passages above the water table and later become submerged as sea-levels rise are also valuable in this process since they document the moment when the cave shifted from being air- to water-filled 22 .

A distinct light-colored band (~15 cm wide) is visible along the entire bridge at its upper part (Fig.  2 , Supplementary Fig.  1a ). This coloration mark bears a resemblance to a “bathtub ring” and its presence is likely related to a relatively short-lived stable water table that allowed the precipitation of a sub-millimeter calcite crust at the water/air interface. When the water level increased, the calcite did not disappear since the water below the water table remained somewhat saturated with respect to calcium carbonate. As discussed later, this feature along with the new POS ages and their elevation play a crucial role in determining when this bridge, now submerged, was constructed.

Geochronology

The U-series ages ( n  = 34; 28 for POS and 6 from stalactites) are given in Supplementary Table  2 and are all reported as years before present (BP), where present is 1950 CE. Ten of these ages are from POS samples GE-D8 (Genovesa Cave; Supplementary Fig.  2 ) and DR-D15 dated as part of a prior study 21 . The latter was collected in Drac Cave (39°32’9” N, 3°19’49” E), located 1.6 km to the north-east of Genovesa Cave (Fig.  1c , Supplementary Figs.  S3 – S4 ).

Regardless of the sampling depth, all the vadose stalactites on which the POS formed in both caves, produced ages older than 8200 years B.P. (Fig.  2 , Supplementary Fig.  5 ). The phreatic overgrowth samples GE-D6, GE-D7, and DR-D23 (Supplementary Figs.  S6 – S8 ), precipitated at ~1.10 ± 0.1 m below the preindustrial sea level (mbpsl). A 232 Th/ 238 U- 234 U/ 238 U- 230 Th/ 238 U (plotted as a Rosholt A type) isochron age of 5479 ± 120 years B.P. ( n  = 3 of 4; hereafter, ± refers to 2 σ uncertainty) was measured for GE-D6 (Supplemental Table  2 , Supplementary Fig.  9a ). GE-D7, in the same room and at the same elevation as GE-D6, yielded a weighted average age of 5510 ± 549 years B.P. using the same correction (initial 230 Th/ 232 Th atomic ratio = 5.1 ± 0.4 ppm) generated by the GE-D6 isochron age. Onac et al. 21 . used a slightly higher initial for GE-D8 (8 ppm) that was located at a higher elevation than GE-D6 & -D7. For DR-D23, we obtained a 232 Th/ 238 U- 234 U/ 238 U- 230 Th/ 238 U (plotted as a Rosholt A type) isochron age of 5824 ± 140 years B.P. ( n  = 6) (Supplementary Table  2 , Supplementary Fig.  9b ). This isochron shows an exceptionally high initial 230 Th/ 232 Th atomic ratio = 527.5 ± 22.1 ppm, more than 10x higher than used for DR-D15 (44 ppm) from the same cave but at a different elevation 21 . The fluffy fibrous cotton-candy texture of the two sub-samples with high U component of DR-D23 may have something to do with the high initial 230 Th/ 232 Th. The isochron ages were necessary to produce accurate ages with smaller uncertainties.

Collectively, the POS data from Genovesa and the nearby Drac, reveal three distinct periods of relative sea-level stability (Fig.  2 ). One occurred at 0 ± 0.04 m from 2720 ± 11 to 296 ± 18 years B.P. The second period lasting from 3703 ± 14 to 3368 ± 8 years B.P., corresponds to a sea level of 0.25 mbpsl. Lastly, a third period at ~1.1 ± 0.1 mbpsl is documented between 5820 ± 140 and 5479 ± 120 years B.P. (Figs.  2 , 3 ). By adding the uncertainty to the older age and subtracting the uncertainty from the younger age, the maximum time span of POS growth at 1.1 mbpsl ranges from 5964 – 5359 years B.P. During this interval, both the POS and the coloration mark formed. For the latter to develop, the bridge must have been submerged, at least to its upper surface, allowing calcite to precipitate during the sea-level stillstand. Therefore, this period is of particular interest because it may aid in providing the timeline of the bridge construction as detailed below.

Timing of bridge construction

The assembly date of the bridge in Genovesa Cave remains uncertain due to the absence of written records or a robust time-stratigraphic context. In order to constrain the building time of this archeological structure, we rely on a well-defined Late Holocene sea-level curve generated by Onac et al. 21 . for Mallorca (depicted by the solid blue line in Fig.  3 ) and the ages and depths at which POS grew and coloration mark formed. First, we assess previous assumptions regarding the timing of the submerged bridge construction using this curve. Then, we examine our new sea-level data in conjunction with the timing of the earliest human arrival model proposed by Bover et al. 14 .

The prehistoric pottery discovered in Sala de les Rates-pinyades of the Genovesa Cave has been linked to the Naviform period (3550–3000 cal B.P.). This attribution is based on typological similarities between the ceramics found in Genovesa and those documented at the Closos de Can Gaià, a Bronze Age site located ~10 km south of our cave (Fig.  1b ). The archeological horizon in which comparable pottery was discovered at the latter site was dated to ~3600 cal B.P 30 . However, Costa and Guerrero 31 argue that Closos de Can Gaià excavation required a reassessment of the chronological framework, due to issues with the radiocarbon dates. Despite this, adopting the previously reported radiocarbon age, Gràcia et al. 20 suggested that the construction of the bridge likely occurred toward the end of the Naviform period.

However, the RSL curve (Fig.  3 ) indicates that sea level was ~0.25 ± 0.1 m below the preindustrial baseline ~3500 years ago 21 , implying a total water depth of ~1.3 m in the cave lake. The vertical height of the bridge is 0.5 m, and thus it was submerged by 0.8 m of water at this time (Fig.  3 ). The construction of the bridge around 4400 years ago, the time suggested by Bover et al. 14 to be the earliest evidence of human presence on the island, is also improbable. At that time, relative sea level in Mallorca was ~0.35 ± 0.1 m below preindustrial level, and the bridge would have been submerged by 0.7 m. Building a bridge below water level is a highly unlikely scenario, and thus it was likely built at an earlier time, when sea level was lower. The predicted relative sea-level curve for Mallorca (Fig.  3 ) indicates that the top of the bridge would have been close to water level no earlier than 5600 years ago and this provides an approximate lower bound on the age of the feature. The distinct coloration mark on the bridge also provides strong evidence of an age greater than the ages estimated by Gràcia et al. 20 and Bover et al. 14 . As discussed earlier and according to the POS-based relative sea-level record, this mark would not have developed if the top of the bridge was well below the water level, i.e., at times more recent than ~5500 years ago. However, an age older than 6000 years for the feature can be ruled out since sea level was even lower (Fig.  3 ), and the construction of a bridge at its current height would have been unnecessary.

Comparison between the position of the submerged bridge, phreatic overgrowth on speleothems (POS), coloration mark, and the RSL prediction (blue curve) 21 based on a Glacial Isostatic Adjustment model that uses the ICE-6G (VM 5) ice history with an upper mantle viscosity of 1.3 × 1020 Pa s. Solid symbols with age and depth uncertainties represent POS elevations. The brown rectangle depicts the bridge with its coloration band in the upper part. The insets show an underwater image of the bridge (Photo courtesy of R. Landreth) and a close-up view on the RSL position of samples GE-D6, GE-D7, and DR-D23 that grew at 1.1 mbpsl. The uncertainties for GE-D6, GE-D7, and DR-D23 are absolute 2 σ error bars based on three dimensional isochron ages or weighted average (GE-D7). The dotted blue line is a sea-level rise scenario that includes the brief stillstand inferred from the POS growth.

The phreatic overgrowths GE-D6, GE-D7, and DR-D23 from Genovesa and Drac caves formed at a relative sea level of 1.1 mbpsl, which is 5 cm below the upper part of the bridge. The two more precise isochron ages suggest sea-level remained relatively constant for a few hundreds of years between ~5964 and 5359 years B.P. The relative brevity of this time frame might explain why the morphology and size of the POS are somehow atypical and smaller compared to those POS that developed when the sea level was stable at 0 m for over 2000 years. Furthermore, this <600 year period of nearly constant sea level was sufficient to develop the coloration mark. Given that the occurrence of this feature correlates directly with the previously mentioned sea-level stillstand position, it suggests that the bridge was already in place. In fact, its construction could have commenced as early as ~6000 years ago when the water depth in the lake was ~0.25 m. However, it had to be completed before ~5600 years ago when the sea-level rose to the top surface of the bridge.

Conclusions

We have integrated new U-series data from POS in Genovesa Cave with the observation of a unique coloration mark found on a submerged bridge within the same cave. This mark developed during a period of sea-level stability that was responsible for the precipitation of a POS level at 1.1 mbpsl. By combining this evidence with a high-resolution Late Holocene sea-level curve for Mallorca, we offer a more accurate timeframe for the construction of this archeological structure.

The history of the bridge construction appears to be closely associated with rapid Holocene sea-level rise just prior to 6000 years ago and a brief sea-level stillstand that led to some upper sections of the cave being flooded. According to our POS chronology, the sea-level rise ceased and remained stable for several hundred years between ~5964 and 5359 years B.P. During this time, POS formed in the cave lake, and a distinctive “bathtub ring” developed on the bridge.

The building of the bridge likely began early during this period, when crossing the 0.25 m-deep lake required its construction. However, the structure must have been completed before ~5600 years when the upper part of the bridge became submerged.

Evidence indicates that humans constructed a stone-paved pathway leading to the cave’s water pool and a robust bridge, facilitating access to the only other dry section of the cave situated beyond the lake, in the Sala d’Entrada. The exact reasons behind the construction of these structures in Genovesa Cave remain elusive. Nevertheless, the chronological constraints posed by the depth of the bridge, coupled with the similar depth at which POS and the coloration mark occur, support the idea of an early human presence on the island by 5600 years B.P. and potentially dating back as far as 6000 years ago.

Location, recovery, and storage of samples

Underwater mapping to locate each sample was carried out using a Suunto SK-8 compass, a pre-knotted diving line, and a rollable measuring tape (accuracy of ±1 cm). To measure underwater sample depths, we utilized the pressure depth-meter built into the Suunto Gekko steel diving computer having an accuracy of ± 1% and resolution of 0.1 m. One of the authors (FG) employed technical scuba diving procedures to collect submerged phreatic overgrowth on speleothems and stalactites situated at depths ranging from 0.75 to 2 m below present sea level. We selected this specific depth range to precisely determine the sea level’s relationship with the bridge and its coloration mark.

All samples used in this study are housed within the University of Balearic Islands Repository of Phreatic Overgrowth on Speleothems, under the curation of the Earth Sciences Research Group.

Hydraulic gradient measurement

The calculation of the hydraulic gradient requires two variables: the change in head (elevation) and the change in distance. Measurements for both were performed using a Leica TC 405 total station, with a standard deviation of 5” (arcseconds) for horizontal and vertical angles and 2 mm + 2 ppm (parts per million of distance measured) for electronic distance measurements. In our study, we found that the head change between sea level and the cave lake surface was 0.037 m. Additionally, the distance between the survey station located on the coastline at the sea level and the one adjacent to the cave lake surface measured 412 m. Dividing the change in head (0.037 m) by the change in distance (412 m), we derived the hydraulic gradient (9 x 10 −5  m /m) for Genovesa Cave. This value is even lower for Drac Cave, which is located <100 m from the coast.

Submerged bridge setting

The precise location and elevation of the submerged bridge and its coloration band were accurately established during the cave mapping process using a Suunto Tandem 360PC compass/clinometer (accuracy 0.33°/0.25°) and a Bosch GLM-50 Laser distance measurer (accuracy 1.5 mm). This location was confirmed during measurements of the hydraulic gradient with a Leica TC 405 total station, yielding discrepancies of <1.5° in azimuth and 10 mm in distance. The lake surface elevation was determined with a precision of better than 2 mm (see Hydraulic gradient measurement), enabling the direct measurement of the depth of the bridge and its coloration band from the lake surface. These depths were measured directly by our diver coauthor (FG) using a millimeter-graded measuring tape, with an error margin within 5 mm.

U-series dating

Each POS was first halved and subsamples were collected by milling 10–200 mg of powders if samples were amendable to drilling. Pieces were extracted from the tip of the stalactite and from parts of the POS. All subsamples were completely dissolved in 15 N HNO 3 in Teflon beakers and spiked with a mixed solution of 229 Th, 233 U, and 236 U tracers. We added a few drops of HClO 4 , and then the sample-spike mixture was fluxed for ~1 h and dried to ensure thorough mixing and removal of organic matter from the samples. After cooling, the subsample crust was dissolved in 7 N HNO 3 for anion resin (Eichrom 1 x 8200–400 mesh, chloride form) column chemistry. Subsamples were purified in the columns using 7 N HNO 3 to remove matrix constituents. The Th fraction was eluted with 6 N HCl, and the U fraction was obtained using H 2 O.

The analysis of U and Th was carried out separately using a Thermo Scientific Neptune Plus multi-collector inductively coupled plasma mass spectrometer in static mode. This mode was chosen because our instrument has enough detectors for all the required peaks. The U and Th solutions were introduced using an Aridus II desolvating nebulizer, which enhanced signal intensity by approximately a factor of 4. All isotopes, except 230 Th and 234 U, were measured on Faraday cups equipped with 10 -10 , 10 -11 , and 10 -12 -ohm resistors, with the selection based on signal intensity. For 230 Th and 234 U, measurements were conducted using a secondary electron multiplier with a retardation potential that offered low abundance sensitivity (~5 × 10 -7 ). Gain calibration between the Faraday cups and scanning electron microscope was achieved using the U standard CRM-112 and an in-house 230 Th- 229 Th standard. The obtained dates were calculated using the decay constants reported by Cheng et al. 32 (2013). The 232 Th/ 238 U- 234 U/ 238 U- 230 Th/ 238 U isochron ages were calculated using IsoplotR 33 , a modified version of Isoplot 34 . We chose the Rosholt A type of isochron plots, which is the first option in IsoplotR software.

Data availability

All data generated or analyzed during this study are included in this published article and its Supplementary Materials. The full U-series dataset is available at https://doi.org/10.5281/zenodo.12609107 .

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Acknowledgements

This research was supported by NSF Awards AGS 2202683 (B.P.O), 0326902 and 2202712 (V.J.P. and Y.A.), 2202698 (J.X.M.), and Spanish MCIN/AEI, PID2020-112720GB-I00 (J.J.F.). We extend our gratitude to the Conselleria de Medi Ambient i Territori (Govern de les Illes Balears) for granting us permission to collect geological samples from Genovesa and Drac caves. We thank Oana A. Dumitru and Giuseppe Lucia for their support during fieldwork, as well as Bogdan Tomuș for conducting the measurements necessary for calculating the hydraulic gradient and precisely locating the submerged bridge.

Author information

Deceased: Angel Ginés.

Authors and Affiliations

Karst Research Group, School of Geosciences, University of South Florida, 4202 E. Fowler Ave., Tampa, FL, 33620, USA

Bogdan P. Onac

Emil G. Racoviță Institute, Babeș-Bolyai University, Clinicilor 5-7, 400006, Cluj-Napoca, Romania

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, 87131, USA

Victor J. Polyak & Yemane Asmerom

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 02138, USA

Jerry X. Mitrovica

Earth Sciences Research Group, Universitat de les Illes Balears, Ctra. Valldemossa km 7.5, 07122, Palma (Mallorca), Spain

Joaquín Ginés, Joan J. Fornós & Angel Ginés

Societat Espeleològica Balear, Margarida Xirgu, 16, 07011, Palma (Mallorca), Spain

Francesc Gràcia

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Contributions

B.P.O., J.G., J.J.F., A.G. designed the research. F.G., B.P.O., J.G., and J.J.F. measured the depth of the bridge and its coloration band. F.G. collected and measured the elevations of underwater samples. V.J.P. and Y.A. generated the U-series ages of the phreatic overgrowth samples. J.X.M. produced the GIA models. B.P.O., V.J.P., and J.X.M. drafted the manuscript with contributions from all authors.

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Correspondence to Bogdan P. Onac .

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Onac, B.P., Polyak, V.J., Mitrovica, J.X. et al. Submerged bridge constructed at least 5600 years ago indicates early human arrival in Mallorca, Spain. Commun Earth Environ 5 , 457 (2024). https://doi.org/10.1038/s43247-024-01584-4

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Received : 20 February 2024

Accepted : 25 July 2024

Published : 30 August 2024

DOI : https://doi.org/10.1038/s43247-024-01584-4

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