In a review article published in the journal Science, the authors discuss the health effects of air pollution and challenges associated with monitoring population exposure to air pollution, particularly in low—and middle-income countries and among vulnerable populations. The authors highlight the urgent need to prioritize interdisciplinary research on air pollution exposure and risk assessment to develop evidence-based intervention policies and risk communication strategies.
Review: Toward cleaner air and better health: Current state, challenges, and priorities. Image Credit: CI Photos / Shutterstock
Background
Exposure to ambient air pollution, including particulate matter (PM) and greenhouse gases, is associated with increased risks of various non-communicable diseases. Ambient PM has been classified as a group I human carcinogen for lung cancer. An estimated 5.2 million deaths occur each year globally due to air pollution. The World Health Organization (WHO) updated the Global Air Quality Guidelines (AQGs) for air pollutants in 2021 in order to reduce the public health burden. These guidelines are based on evidence from recent studies investigating the adverse health effects of global air pollution.
Non-communicable diseases are associated with a significant percentage of global mortality and disability, accounting for almost 41 million deaths per year, with 77% occurring in low- and middle-income countries. About 99% of the global population is continuously exposed to poor-quality air that exceeds the AQG limits. Populations with increased vulnerability due to age, sex, and disease status are at higher risk from the damaging effects of air pollution exposure. Racial and ethnic disparities in air pollution-attributable premature mortality have also increased over the decades.
Assessment of Population Exposure to Air Pollution
Air pollution is primarily caused by complex mixtures of PM, including black carbon and polycyclic aromatic hydrocarbons, various greenhouse gases (NO2, SO2, CO), and volatile organic compounds (VOCs). Air pollutants are generated from a range of man-made sources, including residential and commercial energy use, agricultural and industrial emissions, biomass burning, and vehicle emissions, as well as natural sources, such as wildfires and vulcanism. The components and sources of PM air pollution, particularly PM2.5, vary greatly and may include primary combustion as well as secondarily formed particles, which may alter as geographical and meteorological conditions change.
Air pollutants can cross international boundaries and interact with other locally formed substances, which can alter ambient air's toxic properties. Assessment of the air pollution sources and emission scenarios in the Group of Twenty (G20) countries found a statistically significant connection between transboundary air pollution and premature deaths. Despite reductions in concentrations of ambient PM2.5 and O3 in high-income countries (HICs), pollution exposure levels remain relatively high across most of the world, particularly in regions of South Asia.
Currently, most countries worldwide routinely monitor ambient air pollution for regulatory purposes. Although levels of PM and ozone have decreased in high-income countries in recent times, air pollution exposure levels remain high in many countries, especially in South Asia. Ground-based air quality monitoring systems have found a significant induction in annual levels of ambient NO2, ammonia (NH3), and reactive VOCs in most tropical cities of Africa and Asia, which are almost exclusively generated from man-made sources. Similarly, roadside monitoring systems have found high levels of NO2 and NOX in urban areas of Europe.
One of the major challenges in accurately estimating global air pollution exposure is that ground-based air quality monitoring is primarily done in urban areas. In low- and middle-income countries, the development of local emission inventories is needed to predict anthropogenic emission sources and trends accurately. Rapid alterations in agricultural activity should be considered for improving emission estimates' accuracy. The development of inventories for regional emissions can better depict sources and trends of anthropogenic emissions, but the accuracy of emission estimates relies on the methodology used and the kind and quality of the data included.
Improved exposure models combining satellite-derived data with ground-based monitor-derived data, chemical transport models, and emission inventories have improved global estimates and long-term trends of PM2.5 at finer scales. In addition to conventional monitoring, filter-based PM2.5 source apportionment has often been applied to principal component analysis, positive matrix factorization, and advanced exposure models to prioritize policies for regulation and to identify the sources of air pollution. The emerging application of low-cost air pollution monitors may provide further data to monitor exposure to air pollutants at the level of individuals, which could be especially valuable for those who tend to spend much time indoors, such as schoolchildren, pregnant women, and persons with preexisting diseases.
Traffic-related air pollution (TRAP) accounts for nearly 60% of daily exposure to ultrafine particles in urban populations. Ultrafine particles are associated with severe health adversities as they can enter deep into the lung, translocate into the systemic circulation, and subsequently reach the brain. Growing evidence suggests that UFPs have a high potential health impact because of their large surface area to mass ratio, which causes them to adsorb more toxic components, such as organic carbon, nitrates, sulfates, trace metals, and VOCs, than PM2.5. To support TRAP exposure reduction efforts, intensive monitoring of ultrafine particle sources is required. Population-level estimation of air pollution exposure requires a more in-depth understanding of indoor sources and outdoor pollutants that can penetrate the indoor environment. Multicenter studies in low- and middle-income countries, such as the Prospective Urban and Rural Epidemiology-AIR study, are underway to develop exposure assessment tools using models and quantitative household air pollution.
Substantial challenges remain in characterizing the effects of air pollution on children and pregnant women.
Health Effects of Air Pollution
An induction in the levels of air pollutants, including PM and ozone, has been found to be associated with increased risks of mortality, morbidity, and hospitalization. A range of clinical consequences of air pollution exposure have been observed in people with metabolic diseases (diabetes and obesity), neurological disorders (dementia), and kidney and immune diseases. The global burden of death attributed to ambient PM2.5 exposure in 2021 was dominated by cardiopulmonary diseases in both females and males, with the greatest impact among those ≥80 years of age. Epidemiological evidence also indicates that >50% of all PM2.5-associated deaths are triggered by clinically overt cardiovascular diseases, including myocardial infarction, stroke, heart failure, and sudden death. Cardiopulmonary diseases have been identified as the predominant cause of air pollution-related mortality worldwide, with the highest impact among older adults aged 80 years and above.
Recent evidence indicates that small-diameter PM can exert adverse health effects even at very low concentrations. Ozone, the most prevalent secondary pollutant, has been found to have a causal association with respiratory diseases. It is also plausible that a spectrum of pediatric disorders associated with air pollution may lead to underappreciated consequences and may manifest as heightened risks of NCDs throughout the life span. Higher ventilation rates and underdeveloped immunity make young children more susceptible to air pollution-related health adversities. Maternal physiological changes during pregnancy can also make both mother and fetus vulnerable to air pollution. A recent review of 32 million births in the United States showed associations between ambient PM2.5 and O3 with increased risks of adverse birth outcomes, including preterm birth and stillbirth.
Interventions to Improve Air Quality
The best possible intervention to improve air quality is to reduce ambient levels of air pollutants and risk communication to the government and stakeholders. Smokestack controls on power plants and large industries, tailpipe controls on motor vehicles, promotion of public transport and nonmotorized transport, electrification of homes and businesses, and promotion of good practices in agriculture and farming are some of the government-implemented policies to improve air quality.
The best possible way to reduce air pollution exposure is to stay indoors or reduce outdoor activity during high-pollution days. However, it is not feasible, or even advisable, for people living in low- and middle-income countries, highlighting the impact of socioeconomic inequity on the accessibility to clean air. These observations highlight the urgent need to develop and implement evidence-based air quality policies for protecting public health, especially in low- and middle-income countries. From the perspective of air quality management, developing evidence-based risk communication indices to provide timely air quality and health information can bridge knowledge gaps across sectors and improve the effectiveness of interventions at the government and public levels.
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