Almost nine out of ten (86%) people living in urban areas across the globe, or 2.5 billion people, are exposed to unhealthy particulate matter levels, leading to the 1.8 million excess deaths in cities globally in 2019, in a study that examined urban air pollution and related mortality trends in over 13,000 cities globally. 

The first of two studies, both published today in The Lancet Planetary Health journal, found that 86% of urban inhabitants lived in areas that exceeded the 2005 World Health Organization (WHO) guidelines for annual PM2.5 exposure of 10 micrograms per cubic meter (μg/m³); in 2021 that safe air quality guideline was tightened even further to 5 μg/m³ of PM2.5.  

“The majority of the world’s urban population still live in areas with unhealthy levels of PM2.5,” said lead author of the study Veronica Southerland, of US-based George Washington University. 

Exposure to PM2.5 (fine particulate matter with a diameter of 2.5 micrometers or less) is a leading environmental risk factor for disease. 

Inhalation is known to increase the risk of premature death from health conditions such as cardiovascular disease, respiratory disease, lung cancer, and lower respiratory infection. 

Additionally, the second study, focusing on transport-related NO2 (nitrogen dioxide gas) pollution and paediatric asthma incidence, found that nearly two in three urban childhood asthma cases in 2019 were attributable to NO2 pollution, with nearly 2 million asthma cases found among children worldwide.  

Both studies highlight the urgent need to improve air quality in cities and reduce exposure to pollution, particularly among children and the elderly. 

PM2.5 concentration seven times WHO 2021 guidelines in first study 

The first study focused on PM2.5 concentrations and associated mortality trends in over 13,000 cities globally between 2000 – 2019. 

Average population-weighted PM2.5 concentration in 2019 across all urban areas globally was 35 micrograms per cubic meter – equivalent to seven times the new 2021 WHO guideline for annual average PM2.5 of five micrograms per cubic meter. 

Study authors estimate that 61 deaths per 100,000 inhabitants in urban areas were attributable to PM2.5 in 2019. 

A 2021 report by Greenpeace Southeast Asia and IQAir had also found the link between urban air pollution and attributed mortality, with exposure to PM2.5 responsible for the deaths of 160,000 in the world’s five biggest cities.  

Southeast Asia sees largest increase in PM2.5 concentration and attributable deaths 

While global average PM2.5 concentrations remained consistent over the 19-year period, there were large variations by regions.

Urban areas in Southeast Asia (including India) saw the largest regional increases, with a 27% increase in average population-weighted PM2.5 concentration between 2000 – 2019. Southeast Asian cities also saw the largest increase in PM2.5-attributable mortality during this period – 33% – from 63 to 84 deaths annually per 100,000 people. 

However, higher or lower PM2.5 concentrations in some urban areas of Asia, Africa and elsewhere did not always yield correspondingly lower PM2.5-attributable death rates. That is because demographic factors, such as the age of the population, as well as baseline health conditions, also have a strong impact on pollution-related deaths. 

For instance, Guangzhou, China, had a decrease in population-weighted PM_2·5 of 14% during the 19 year period, from 37 μg/m³ to 32 μg/m³. Howver, PM_2·5-attributable mortality rates increased by 10%. In contrast, Luanda, Angola, saw a 38%  increase in PM_2·5 population-weighted concentrations from 13 μg/m³ to 18 μg/m³. But attributable mortality decreased by 16%.

At the same time, decreases in PM2.5 concentrations had a comparatively larger impact on reducing PM2.5 attributable mortality in urban areas where pollution levels were already low, such as the Americas and Europe, the researchers found.

For instance, in European cities, a 20% reduction in contractions, from an average of 20 μg/m³ to 16 μg/m³ over the course of the 19 year period, resulted in a 23% decrease in PM2.5 attributable deaths despite an ageing population, the study found. That is because proportionally speaking, decreases in air pollution at relatively lower levels yield comparatively greater reductions in mortality.   

“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,” stated Sutherland. 

Second study finds two out of three new pediatric asthma cases in cities due to NO2 

The second study analyzed global concentrations of NO2 with population and baseline asthma rates in order to estimate the paediatric asthma incidence attributable to NO2 between 2000 – 2019 in 13,129 cities.

NO2 is an air pollutant mainly emitted by vehicles, power plants, industrial manufacturing, and agriculture.

Previous research has shown transport-related air pollution is associated with both asthma exacerbation and new onset of asthma in children. 

The study revealed that 1.85 million, or 8.5% of all new paediatric asthma cases reported in 2019 were attributed to NO2.  Two out of  three of these new cases occurred in the 13,129 urban areas covered in the study.  

In urban areas, NO2 was responsible for 16% of all new child asthma cases in 2019. 

“Our results demonstrate the important influence of combustion-related air pollution on children’s health in cities globally,” says Susan Anenberg of US-based George Washington University, who is co-first author on the NO2 study and corresponding author of both studies.

Anenberg pointed out that despite the downward trend of NO2 concentrations over several decades – the result of effective air quality management programs, NO2 continues to contribute to child asthma. 

“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.”

Image Credits: Mohammad Hossein Taaghi, The Lancet, Planetary Health , Flickr, The Lancet, Planetary Health .