Peer-reviewed/Simulation or modelling/People
- A new modelling study finds that 86% of people living in cities worldwide (or 2. 5 billion people) are exposed to annual average levels of fine particulate matter exceeding the World Health Organisation (WHO) guideline from 2005.
- A second study 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.
- Together, the studies highlight the urgent need to improve air quality in cities and reduce exposure to pollution, particularly among children and the elderly.
Approximately 86% of people living in urban areas across the globe, or 2.5 billion people, are exposed to unhealthy particulate matter levels, leading to 1.8 million excess deaths in cities globally in 2019.
Additionally, nearly 2 million asthma cases among children worldwide were attributable to NO2 pollution in 2019, with two in three occurring in urban areas.
Both 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 PM2.5 in 2019
In the first study, researchers looked at PM2.5 (fine particulate matter with a diameter of 2.5 micrometres or less) - the leading environmental risk factor for disease. 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 over half (55%) of the world’s population living in cities, to date there has been little research into how PM2.5 disease burdens compare across urban areas globally, with most assessments analysing PM2.5 in megacities only. This new study examines PM2.5 concentrations and associated mortality trends in over 13,000 cities globally  between 2000-2019.
Average population-weighted PM2.5 concentration across all urban areas globally was 35 micrograms per cubic metre in 2019, with no change from 2000. This is equivalent to seven times the 2021 WHO guideline for annual average PM2.5 (five micrograms per cubic metre) . The authors estimate that 61 in every 100,000 deaths in urban areas was attributable to PM2.5 in 2019.
Although global urban average PM2.5 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 PM2.5 concentration between 2000-2019. South-East Asian cities also saw the largest increase in PM2.5-attributable mortality rates over this period, increasing by 33% from 63 to 84 in 100,000 people.
Globally, approximately 86% of urban inhabitants (2.5 billion people) lived in areas exceeding the WHO 2005 guideline for annual average PM2.5 exposure (10 micrograms per cubic metre)  in 2019 resulting in an excess of 1.8 million deaths.
The research found that decreasing PM2.5 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 PM2.5 concentrations) did not correspond to the same level of decreases in PM2.5-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 PM2.5,” says Veronica Southerland of George Washington University, USA, and lead author of the study. “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.” 
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 PM2.5 on mortality, without accounting for other health burdens caused by PM2.5 such as low birth weight, premature birth, and cognitive impairment.
Two thirds of paediatric 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 paediatric asthma incidence in urban areas.
In this research, global NO2 concentrations were calculated with a 1km 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 paediatric 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 paediatric asthma cases associated with NO2; 8.5% of all new paediatric asthma cases reported that year. Approximately two in three of these paediatric 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 paediatric asthma cases in 2019.
In both 2000 and 2019, 1.2 million paediatric 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,” says Dr Susan Anenberg of the George Washington University, USA, who is co-first author on the NO2 study and corresponding author of both studies. “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 paediatric asthma incidence, highlighting that mitigating air pollution should be a critical element of children’s public health strategies.” 
The authors acknowledge some limitations with this study. Baseline paediatric asthma rates may have been underestimated in low- to middle-income countries, leading to an underestimation in NO2 attributable asthma impacts. Similarly, national paediatric 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 paediatric 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.
NOTES TO EDITORS
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 Centre.
 City level data unavailable.
 The WHO 2005 guideline for PM2.5 exposure was 10 micrograms per cubic metre at the time of the analysis. This has since been updated to the WHO 2021 guideline for PM2.5 exposure of five micrograms per cubic metre.
 Quote direct from author and cannot be found in the text of the Article.
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Second Study: https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(21)00255-2/fulltext (full study information listed below)
Article Title: Long-term trends in urban NO2 concentrations and associated paediatric asthma incidence: estimates from global datasets
COI statement: GHK declares a consulting relationship with the Environmental Defense Fund. SCA declares consulting relationships with the Environmental Defense Fund and International Council on Clean Transportation; and unpaid advisory board membership for the American Geophysical Union, US Environmental Protection Agency, WHO, Clean Air Partners, and the National Academies of Sciences, outside the submitted work.
All other authors declare no competing interests.
All other fields are the same as for the first article.
The Lancet Planetary Health
Method of Research
Subject of Research
Global urban temporal trends in fine particulate matter (PM2·5) and attributable health burdens: estimates from global datasets
Article Publication Date
No conflicts of interest