A new modeling study led by researchers from George Washington Universit (GWU) finds that 86% of people living in cities worldwide (~2.5 billion people) are exposed to annual average levels of fine particulate matter exceeding the World Health Organization (WHO) guideline from 2005, leading to 1.8 million excess deaths in cities globally in 2019.

The WHO 2005 guideline for PM_2.5 exposure was 10 micrograms (μg) per cubic meter. This has since been updated to the WHO 2021 guideline for PM_2.5 exposure of 5 μg/m³.

A second study, also led by the GWU researchers, finds that nearly 2 million cases of asthma in children are linked to traffic-related nitrogen dioxide air pollution, with two in three occurring in cities.

Both open-access studies are published in The Lancet Planetary Health journal and highlight the ongoing need for strategies to improve air pollution and reduce exposure to harmful emissions, particularly among children and the elderly.

1.8 million excess deaths attributable to PM_2.5 in 2019. In the first study, researchers looked at PM_2.5. Inhalation is known to increase the risk of premature death from conditions such as cardiovascular disease, respiratory disease, lung cancer, and lower respiratory infection.

Despite more than half (55%) of the world’s population now living in cities, to date there has been little research into how PM_2.5 disease burdens compare across urban areas globally, with most assessments analyzing PM_2.5 in megacities only. This new study examines PM_2.5 concentrations and associated mortality trends in more than 13,000 cities globally between 2000-2019.

Change in population-weighted PM_2.5 concentrations and PM_2.5-attributable mortality rates between 2000 and 2019 for the top 250 most populated urban areas based on 2019 WorldPop estimates.
(A) Percentage change in population-weighted PM_2.5 concentrations. (B) Absolute differences in population-weighted concentrations of PM_2.5. (C) Percentage change in PM_2.5-attributable mortality per 100 000 population. (D) Absolute differences in PM_2.5-attributable mortality per 100 000 population. Southerland et al.

Average population-weighted PM_2.5 concentration across all urban areas globally was 35 micrograms per cubic meter in 2019, with no change from 2000. This is equivalent to seven times the 2021 WHO guideline for annual average PM_2.5 (five micrograms per cubic meter) [2]. The authors estimate that 61 in every 100,000 deaths in urban areas was attributable to PM_2.5 in 2019.

Although global urban average PM_2.5 concentrations were consistent over this period, there were large variations by region. Urban areas in South-East Asia (including India) saw the largest regional increases, with a 27% increase in average population-weighted PM_2.5 concentration between 2000-2019. South-East Asian cities also saw the largest increase in PM_2.5-attributable mortality rates over this period, increasing by 33% from 63 to 84 in 100,000 people.

The research found that decreasing PM_2.5 concentrations in urban areas over the two decades (for example, African, European, and North and South American cities experienced 18%, 21%, and 29% decreases in PM_2.5 concentrations) did not correspond to the same level of decreases in PM_2.5-attributable mortality rates on their own, demonstrating that other demographic factors, for example, an ageing population and poor general health, are influential drivers of pollution-related mortality burdens.

The majority of the world’s urban population still live in areas with unhealthy levels of PM_2.5. Avoiding the large public health burden caused by air pollution will require strategies that not only reduce emissions but also improve overall public health to reduce vulnerability.

—lead author Veronica Southerland, George Washington University

The authors acknowledge several limitations with the study. Some uncertainties in the values were unable to be fully accounted for, such as the use of country-wide baseline disease rates in calculations of mortality, which may differ to those specifically in urban areas. This study also only assessed impact of PM_2.5 on mortality, without accounting for other health burdens caused by PM_2.5 such as low birth weight, premature birth, and cognitive impairment.

Two-thirds of pediatric asthma cases linked to air pollution in cities. In the second study, researchers looked at NO₂ (nitrogen dioxide gas), an air pollutant mainly emitted by vehicles, powerplants, industrial manufacturing, and agriculture. Previous research has shown transport-related air pollution, for which NO₂ serves as a marker, to be associated with both asthma exacerbation and new onset asthma in children. However, to date there have been no studies specifically looking at trends in the burden of transport-related NO₂ pollution on pediatric asthma incidence in urban areas.

In this research, global NO₂ concentrations were calculated with a 1 km resolution by combining satellite data with datasets on different types of land use, like roads and green space. The NO₂ concentrations were applied to population and baseline asthma rates to estimate pediatric asthma incidence attributable to NO₂ between 2000-2019 in 13,189 urban areas worldwide.

The study revealed that in 2019 there were 1.85 million new pediatric asthma cases associated with NO₂—8.5% of all new pediatric asthma cases reported that year. Approximately two in three of these pediatric asthma cases attributable to NO₂ occurred in the 13,189 urban areas covered in the study. In urban areas, NO₂ was responsible for 16% of all new pediatric asthma cases in 2019.

In both 2000 and 2019, 1.2 million pediatric asthma cases in urban areas could be attributed to NO₂ pollution, however the rate per 100,000 children decreased by 11% from 176 to 156 per 100,000 children as urban population grew by 14%.

Our results demonstrate the important influence of combustion-related air pollution on children’s health in cities globally. In places that have effective air quality management programs, NO₂ concentrations have been trending downward for decades, with benefits for children’s respiratory health. Even with these improvements, current NO₂ levels contribute substantially to pediatric asthma incidence, highlighting that mitigating air pollution should be a critical element of children’s public health strategies.

—Dr Susan Anenberg, George Washington University, co-first author on the NO₂ study and corresponding author of both studies

The authors acknowledge some limitations with this study. Baseline pediatric asthma rates may have been underestimated in low- to middle-income countries, leading to an underestimation in NO₂ attributable asthma impacts. Similarly, national pediatric asthma rates were used due to a lack of data on urban rates, and asthma prevalence varies within countries. Finally, it is currently unknown whether pediatric asthma incidence is associated with NO₂, the traffic-related air pollution mixture, or the broader combustion-related air pollution mixture. The results could be affected by exposure misclassification, which would lead to asthma impacts being underestimated.

The first study was funded by grants from NASA and the Wellcome Trust. It was conducted by researchers from George Washington University, the University of British Columbia, Washington University in St. Louis, the University of Washington, Dalhousie University, and the University of California.

The second study was funded by Health Effects Institute and NASA. It was conducted by researchers from George Washington University, Argonne National Laboratory, the University of Washington, the University of British Columbia, Oregon State University, and NASA Goddard Space Flight Center.

Resources

• Veronica A Southerland, Michael Brauer, Arash Mohegh, Melanie S Hammer, Aaron van Donkelaar, Randall V Martin, Joshua S Apte, Susan C Anenberg (2022) “Global urban temporal trends in fine particulate matter (PM_2.5) and attributable health burdens: estimates from global datasets,” The Lancet Planetary Health doi: 10.1016/S2542-5196(21)00350-8

• Susan C Anenberg, Arash Mohegh, Daniel L Goldberg, Gaige H Kerr, Michael Brauer, Katrin Burkart, Perry Hystad, Andrew Larkin, Sarah Wozniak, Lok Lamsal (2022) “Long-term trends in urban NO₂ concentrations and associated paediatric asthma incidence: estimates from global datasets,” The Lancet Planetary Health doi: 10.1016/S2542-5196(21)00255-2