Air pollution has been a major threat to human health, ecosystems, and agricultural crops ever since the onset of widespread use of fossil fuel combustion and emissions of harmful substances into ambient air. As a basis for the development, implementation, and compliance assessment of air pollution control policies, monitoring networks for priority air pollutants were established, primarily for regulatory purposes. With increasing understanding of emission sources and the release and environmental fate of chemicals and toxic substances into ambient air, as well as atmospheric transport and chemical conversion processes, increasingly complex air pollution models have entered the scene. Today, highly accurate equipment is available to measure trace gases and aerosols in the atmosphere. In addition, sophisticated atmospheric chemistry transport models—which are routinely compared to and validated and assessed against measurements—are used to model dispersion and chemical processes affecting the composition of the atmosphere, and the resulting ambient concentrations of harmful pollutants. The models also provide methods to quantify the deposition of pollutants, such as acidifying and eutrophying substances, in vegetation, soils, and freshwater ecosystems. This article provides a general overview of the underlying concepts and key features of monitoring and modeling systems for outdoor air pollution.
Monitoring and Modeling of Outdoor Air Pollution
Exploring Air Pollution and COVID-19 Linkages in South Asia
Muthukumara Mani and Takahiro Yamada
South Asia is at the epicenter of the global air pollution problems and still evolving in COVID-19 cases and fatalities. There is growing evidence of increased rates of COVID-19 in areas with high levels of air pollution. Air pollution is found to cause cellular damage and inflammation throughout the body and has been linked to higher rates of diseases, including cancer, heart disease, stroke, diabetes, asthma, and other comorbidities. All these conditions also potentially increase the risk of death in COVID-19 patients. The causal link between the exposure to air pollution and COVID-19 is still under investigation around the world, underpinned by rigorous scientific research and peer-review processes. However, in terms of the approach after a careful review of the literature, the instrumental variable (IV) approach is a prospective candidate to establish causality in a reduced-form analysis to overcome endogeneity and measurement errors of air pollution level. An analysis, therefore, using sufficiently anonymized individual and household level information on COVID-19, household air pollution, and other individual and household socioeconomic endowments in the same primary sampling unit (PSU) of the individual and household survey would be necessary to establish the causality. The PSU data are usually available from demographic health surveys (DHS) with randomly displaced location information to maintain anonymity. Also, for the instrument of the exposure to ambient air pollution, the use of thermal inversions is suggested conditional on weather-related variables—for example, temperature, precipitation, wind velocity and direction, and humidity.
The Environmental Kuznets Curve
David I. Stern
The environmental Kuznets curve (EKC) is a hypothesized relationship between environmental degradation and GDP per capita. In the early stages of economic growth, pollution emissions and other human impacts on the environment increase, but beyond some level of GDP per capita (which varies for different indicators), the trend reverses, so that at high income levels, economic growth leads to environmental improvement. This implies that environmental impacts or emissions per capita are an inverted U-shaped function of GDP per capita. The EKC has been the dominant approach among economists to modeling ambient pollution concentrations and aggregate emissions since Grossman and Krueger introduced it in 1991 and is even found in introductory economics textbooks. Despite this, the EKC was criticized almost from the start on statistical and policy grounds, and debate continues. While concentrations and also emissions of some local pollutants, such as sulfur dioxide, have clearly declined in developed countries in recent decades, evidence for other pollutants, such as carbon dioxide, is much weaker. Initially, many understood the EKC to imply that environmental problems might be due to a lack of sufficient economic development, rather than the reverse, as was conventionally thought. This alarmed others because a simplistic policy prescription based on this idea, while perhaps addressing some issues like deforestation or local air pollution, could exacerbate environmental problems like climate change. Additionally, many of the econometric studies that supported the EKC were found to be statistically fragile. Some more recent research integrates the EKC with alternative approaches and finds that the relation between environmental impacts and development is subtler than the simple picture painted by the EKC. This research shows that usually, growth in the scale of the economy increases environmental impacts, all else held constant. However, the impact of growth might decline as countries get richer, and richer countries are likely to make more rapid progress in reducing environmental impacts. Finally, there is often convergence among countries, so that countries that have relatively high levels of impacts reduce them more quickly or increase them more slowly, all else held constant.
Housing, Indoor Air Pollution, and Health in High-Income Countries
Richard Sharpe, Nicholas Osborne, Cheryl Paterson, Timothy Taylor, Lora Fleming, and George Morris
Despite the overwhelming evidence that living in poor-quality housing and built environments are significant contributors to public health problems, housing issues persist and represent a considerable societal and economic burden worldwide. The complex interaction between multiple behavioral, lifestyle, and environmental factors influencing health throughout the “life-course” (i.e., from childhood to adulthood) in high-income countries has limited the ability to develop more salutogenic housing interventions. The resultant, usually negative, health outcomes depend on many specific housing factors including housing quality and standards, affordability, overcrowding, the type of tenure and property. The immediate outdoor environment also plays an important role in health and wellbeing at the population level, which includes air (indoor and outdoor), noise pollution and the quality of accessible natural environments. These exposures are particularly important for more vulnerable populations, such as the elderly or infirm, and those living in insecure accommodation or in fuel poverty (i.e., being unable to heat the home adequately). Being homeless also is associated with increased risks in a number of health problems. Investigating pathways to protecting health and wellbeing has led to a range of studies examining the potential benefits resulting from accessing more natural environments, more sustainable communities, and housing interventions such as “green construction” techniques. Built environment interventions focusing on the provision of adequate housing designs that incorporate a “life-course” approach, affordable and environmentally sustainable homes, and urban regeneration along with active community engagement, appear capable of improving the overall physical and mental health of residents. While some interventions have resulted in improved public health outcomes in more high-income countries, others have led to a range of unintended consequences that can adversely affect residents’ health and wellbeing. Furthering understanding into four interrelated factors such as housing-specific issues, the immediate environment and housing, vulnerable populations, and natural spaces and sustainable communities can help to inform the development of future interventions.
Atmospheric Brown Clouds
Sumit Sharma, Liliana Nunez, and Veerabhadran Ramanathan
Atmospheric brown clouds (ABCs) are widespread pollution clouds that can at times span an entire continent or an ocean basin. ABCs extend vertically from the ground upward to as high as 3 km, and they consist of both aerosols and gases. ABCs consist of anthropogenic aerosols such as sulfates, nitrates, organics, and black carbon and natural dust aerosols. Gaseous pollutants that contribute to the formation of ABCs are NOx (nitrogen oxides), SOx (sulfur oxides), VOCs (volatile organic compounds), CO (carbon monoxide), CH4 (methane), and O3 (ozone). The brownish color of the cloud (which is visible when looking at the horizon) is due to absorption of solar radiation at short wavelengths (green, blue, and UV) by organic and black carbon aerosols as well as by NOx. While the local nature of ABCs around polluted cities has been known since the early 1900s, the widespread transoceanic and transcontinental nature of ABCs as well as their large-scale effects on climate, hydrological cycle, and agriculture were discovered inadvertently by The Indian Ocean Experiment (INDOEX), an international experiment conducted in the 1990s over the Indian Ocean. A major discovery of INDOEX was that ABCs caused drastic dimming at the surface. The magnitude of the dimming was as large as 10–20% (based on a monthly average) over vast areas of land and ocean regions. The dimming was shown to be accompanied by significant atmospheric absorption of solar radiation by black and brown carbon (a form of organic carbon). Black and brown carbon, ozone and methane contribute as much as 40% to anthropogenic radiative forcing. The dimming by sulfates, nitrates, and carbonaceous (black and organic carbon) species has been shown to disrupt and weaken the monsoon circulation over southern Asia. In addition, the ozone in ABCs leads to a significant decrease in agriculture yields (by as much as 20–40%) in the polluted regions. Most significantly, the aerosols (in ABCs) near the ground lead to about 4 million premature mortalities every year. Technological and regulatory measures are available to mitigate most of the pollution resulting from ABCs. The importance of ABCs to global environmental problems led the United Nations Environment Programme (UNEP) to form the international ABC program. This ABC program subsequently led to the identification of short-lived climate pollutants as potent mitigation agents of climate change, and in recognition, UNEP formed the Climate and Clean Air Coalition to deal with these pollutants.
Impacts of Megacities on Air Quality: Challenges and Opportunities
Luisa T. Molina, Tong Zhu, Wei Wan, and Bhola R. Gurjar
Megacities (metropolitan areas with populations over 10 million) and large urban centers present a major challenge for the global environment. Transportation, industrial activities, and energy demand have increased in megacities due to population growth and unsustainable urban development, leading to increasing levels of air pollution that subject the residents to the health risks associated with harmful pollutants, and impose heavy economic and social costs. Although much progress has been made in reducing air pollution in developed and some developing world megacities, there are many remaining challenges in achieving cleaner and breathable air for their residents. As centers of economic growth, scientific advancement, and technology innovation, however, these urban settings also offer unique opportunities to capitalize on the multiple benefits that can be achieved by optimizing energy use, reducing atmospheric pollution, minimizing greenhouse gas emissions, and bringing many social benefits. Realizing such benefits will, however, require strong and wide-ranging institutional cooperation, public awareness, and multi-stakeholder involvement. This is especially critical as the phenomenon of urbanization continues in virtually all countries of the world, and more megacities will be added to the world, with the majority of them located in developing countries. The air quality and emission mitigation strategies of eight megacities—Mexico City, Beijing, Shanghai, Shenzhen, Chengdu, Delhi, Kolkata, and Mumbai—are presented as examples of the environmental challenges experienced by large urban centers. While these megacities share common problems of air pollution due to the rapid growth in population and urbanization, each city has its own unique circumstances—geographical location, meteorology, sources of emissions, human and financial resources, and institutional capacity—to address them. Nevertheless, the need for an integrated multidisciplinary approach to air quality management is the same. Mexico City’s air pollution problem was considered among the worst in the world in the 1980s due to rapid population growth, uncontrolled urban development, and energy consumption. After three decades of implementing successive comprehensive air quality management programs that combined regulatory actions with technological change and were based on scientific, technical, social, and political considerations, Mexico City has made significant progress in improving its air quality; however, ozone and particulate matter are still at levels above the respective Mexican air quality standards. Beijing, Shanghai, Shenzhen, and Chengdu are microcosms of megacities in the People’s Republic of China, with rapid socioeconomic development, expanding urbanization, and swift industrialization since the era of reform and opening up began in the late 1970s, leading to severe air pollution. In 2013, the Chinese government issued the Action Plan for Air Pollution Prevention and Control. Through scientific research and regional coordinated air pollution control actions implemented by the Chinese government authority, the concentration of atmospheric pollutants in several major cities has decreased substantially. About 20% of total megacities’ populations in the world reside in Indian megacities; the population is projected to increase, with Delhi becoming the largest megacity by 2030. The increased demands of energy and transportation, as well as other sources such as biomass burning, have led to severe air pollution. The air quality trends for some pollutants have reduced as a result of emissions control measures implemented by the Indian government; however, the level of particulate matter is still higher than the national standards and is one of the leading causes of premature deaths. The examples of the eight cities illustrate that although most air pollution problems are caused by local or regional sources of emissions, air pollutants are transported from state to state and across international borders; therefore, international coordination and collaboration should be strongly encouraged. Based on the available technical-scientific information, the regulations, standards, and policies for the reduction of polluting emissions can be formulated and implemented, which combined with adequate surveillance, enforcement, and compliance, would lead to progressive air quality improvement that benefits the population and the environment. The experience and the lessons learned from the eight megacities can be valuable for other large urban centers confronting similar air pollution challenges.
Air Pollution and Weather Interaction in East Asia
Aijun Ding, Xin Huang, and Congbin Fu
Air pollution is one of the grand environmental challenges in developing countries, especially those with high population density like China. High concentrations of primary and secondary trace gases and particulate matter (PM) are frequently observed in the industrialized and urbanized regions, causing negative effects on the health of humans, plants, and the ecosystem. Meteorological conditions are among the most important factors influencing day-to-day air quality. Synoptic weather and boundary layer dynamics control the dispersion capacity and transport of air pollutants, while the main meteorological parameters, such as air temperature, radiation, and relative humidity, influence the chemical transformation of secondary air pollutants at the same time. Intense air pollution, especially high concentration of radiatively important aerosols, can substantially influence meteorological parameters, boundary layer dynamics, synoptic weather, and even regional climate through their strong radiative effects. As one of the main monsoon regions, with the most intense human activities in the world, East Asia is a region experiencing complex air pollution, with sources from anthropogenic fossil fuel combustion, biomass burning, dust storms, and biogenic emissions. A mixture of these different plumes can cause substantial two-way interactions and feedbacks in the formation of air pollutants under various weather conditions. Improving the understanding of such interactions needs more field measurements using integrated multiprocess measurement platforms, as well as more efforts in developing numerical models, especially for those with online coupled processes. All these efforts are very important for policymaking from the perspectives of environmental protection and mitigation of climate change.
Environmental Degradation: Estimating the Health Effects of Ambient PM2.5 Air Pollution in Developing Countries
Ernesto Sánchez-Triana, Bjorn Larsen, Santiago Enriquez, and Andreia Costa Santos
Air pollution of fine particulates (PM2.5) is a leading cause of mortality worldwide. It is estimated that ambient PM2.5 air pollution results in between 4.1 million and 8.9 million premature deaths annually. According to the World Bank, the health effects of ambient PM2.5 air pollution had a cost of $6.4 trillion in purchasing power parity (PPP) adjusted dollars in 2019, equivalent to 4.8% of global gross domestic product (PPP adjusted) that year. Estimating the health effects and cost of ambient PM2.5 air pollution involves three steps: (1) estimating population exposure to pollution; (2) estimating the health effects of such exposure; and (3) assigning a monetary value to the illnesses and premature deaths caused by ambient air pollution. Estimating population exposure to ambient PM2,5 has gone from predominantly using ground level monitoring data mainly in larger cities to estimates of nationwide population weighted exposures based on satellite imagery and chemical transport models along with ground level monitoring data. The Global Burden of Disease 2010 (GBD 2010) provided for the first time national, regional and global estimates of exposures to ambient PM2.5. The GBD exposure estimates have also evolved substantially from 2010 to 2019, especially national estimates in South Asia, the Middle East and North Africa, Sub-Saharan Africa and Latin America and the Caribbean. Estimation of health effects of ambient PM2.5 has also undergone substantial developments during the last two decades. These developments involve: i) going from largely estimating health effects associated with variations in daily exposures to estimating health effects of annual exposure; ii) going from estimating all-cause mortality or mortality from broad disease categories (i.e., cardiopulmonary diseases) to estimating mortality from specific diseases; and iii) being able to estimate health effects over a wide range of exposure that reflect ambient and household air pollution exposure levels in low- and middle-income countries. As to monetary valuation of health effects of ambient air pollution, estimates in most low- and middle-income countries still rely on benefit transfer of values of statistical life (VSL) from high-income countries.
Household Air Pollution in Low and Middle Income Countries
Caroline A. Ochieng, Cathryn Tonne, Sotiris Vardoulakis, and Jan Semenza
Household air pollution from use of solid fuels (biomass fuels and coal) is a major problem in low and middle income countries, where 90% of the population relies on these fuels as the primary source of domestic energy. Use of solid fuels has multiple impacts, on individuals and households, and on the local and global environment. For individuals, the impact on health can be considerable, as household air pollution from solid fuel use has been associated with acute lower respiratory infections, chronic obstructive pulmonary disease, lung cancer, and other illnesses. Household-level impacts include the work, time, and high opportunity costs involved in biomass fuel collection and processing. Harvesting and burning biomass fuels affects local environments by contributing to deforestation and outdoor air pollution. At a global level, inefficient burning of solid fuels contributes to climate change. Improved biomass cookstoves have for a long time been considered the most feasible immediate intervention in resource-poor settings. Their ability to reduce exposure to household air pollution to levels that meet health standards is however questionable. In addition, adoption of improved cookstoves has been low, and there is limited evidence on how the barriers to adoption and use can be overcome. However, the issue of household air pollution in low and middle income countries has gained considerable attention in recent years, with a range of international initiatives in place to address it. These initiatives could enable a transition from biomass to cleaner fuels, but such a transition also requires an enabling policy environment, especially at the national level, and new modes of financing technology delivery. More research is also needed to guide policy and interventions, especially on exposure-response relationships with various health outcomes and on how to overcome poverty and other barriers to wide-scale transition from biomass fuels to cleaner forms of energy.
Ecosystem Services and Human Health
Elisabet Lindgren and Thomas Elmqvist
Ecosystem services refer to benefits for human societies and well-being obtained from ecosystems. Research on health effects of ecosystem services have until recently mostly focused on beneficial effects on physical and mental health from spending time in nature or having access to urban green space. However, nearly all of the different ecosystem services may have impacts on health, either directly or indirectly. Ecosystem services can be divided into provisioning services that provide food and water; regulating services that provide, for example, clean air, moderate extreme events, and regulate the local climate; supporting services that help maintain biodiversity and infectious disease control; and cultural services. With a rapidly growing global population, the demand for food and water will increase. Knowledge about ecosystems will provide opportunities for sustainable agriculture production in both terrestrial and marine environments. Diarrheal diseases and associated childhood deaths are strongly linked to poor water quality, sanitation, and hygiene. Even though improvements are being made, nearly 750 million people still lack access to reliable water sources. Ecosystems such as forests, wetlands, and lakes capture, filter, and store water used for drinking, irrigation, and other human purposes. Wetlands also store and treat solid waste and wastewater, and such ecosystem services could become of increasing use for sustainable development. Ecosystems contribute to local climate regulation and are of importance for climate change mitigation and adaptation. Coastal ecosystems, such as mangrove and coral reefs, act as natural barriers against storm surges and flooding. Flooding is associated with increased risk of deaths, epidemic outbreaks, and negative health impacts from destroyed infrastructure. Vegetation reduces the risk of flooding, also in cities, by increasing permeability and reducing surface runoff following precipitation events. The urban heat island effect will increase city-center temperatures during heatwaves. The elderly, people with chronic cardiovascular and respiratory diseases, and outdoor workers in cities where temperatures soar during heatwaves are in particular vulnerable to heat. Vegetation and especially trees help in different ways to reduce temperatures by shading and evapotranspiration. Air pollution increases the mortality and morbidity risks during heatwaves. Vegetation has been shown also to contribute to improved air quality by, depending on plant species, filtering out gases and airborne particulates. Greenery also has a noise-reducing effect, thereby decreasing noise-related illnesses and annoyances. Biological control uses the knowledge of ecosystems and biodiversity to help control human and animal diseases. Natural surroundings and urban parks and gardens have direct beneficial effects on people’s physical and mental health and well-being. Increased physical activities have well-known health benefits. Spending time in natural environments has also been linked to aesthetic benefits, life enrichments, social cohesion, and spiritual experience. Even living close to or with a view of nature has been shown to reduce stress and increase a sense of well-being.
Air Pollution, Science, Policy, and International Negotiations
In the course of time, the framing of the air pollution issue has undergone a transformation. It is no longer viewed as either a local health issue or a transboundary problem affecting ecosystems but as a global issue that manifests at various levels and has links to various problems. This poses a challenge for processes fostering data collection, international cooperation, and science and policy networking to deal with the issue in its various manifestations. The experience at the Air Convention, officially the Convention on Long-range Transboundary Air Pollution (CLRTAP) of the United Nations Economic Commission for Europe (UN-ECE), shows that interaction between science and policymaking at various levels of scale can enhance each other if certain conditions are met. Alignment of, for example, air policy, climate policy, nitrogen policy, health policy, and biodiversity policy not only asks for cooperation at different scales (i.e., at the local, national, regional, and global levels) but also between different arenas of decision-making and negotiation. This means that joint processes of science and policy development are needed to identify where problem formulations meet, how procedures for data collection match or which indicators are comparable, and what is possible with regard to aligning sequence and focus of policymaking. These do not necessarily need to be, or even should be, processes leading to full integration of policymaking or scientific assessment. However, successful joint processes make clear to decision-makers what the (co-)benefits of certain emission reduction measures are for various policy problems while providing a more complete picture of the cost-effectiveness of these measures. History has shown that decision-makers start acting when they can see the benefits of certain policy options or when the costs of inaction exceed those of action. Policy options might range from emission reduction measures to investments in scientific infrastructure and international cooperation. It also helps when problems are viewed as relevant by those who have the power and resources to act. Observations, measurements, and scientific assessment have the potential to point to this relevance but so does informed, critical public opinion. Current international cooperation is aimed at maintaining a network of experts and continuing efforts in capacity building in countries. Also in cities, capacity building is crucial, which is more and more supported by citizen-led air quality monitoring initiatives.