Almost nowhere on Earth is safe from air pollution — with only 0.18 percent of land not exposed to levels of ambient fine particulate matter (PM2.5) currently considered dangerous by the World Health Organisation (WHO). This is the conclusion of a study by researchers from Australia, which also found that 99.999 percent of the global population is exposed to unsafe levels of PM2.5. The findings, it is hoped, may encourage policymakers across the globe to draw up new regulations to help limit air pollution.

A lack of air pollution monitoring stations around the world has resulted in a deficit of data on local, national, regional and global PM2.5 exposure.

In their study, however, environmental health expert Professor Yuming Guo of the Monash University in Melbourne, Australia combined available air quality monitoring data and satellite-based meteorological and air pollution observations to map out how PM2.5 levels have changed across the globe over the last two decades.

Prof Guo explained: “We used an innovative machine learning approach to integrate multiple meteorological and geological information to estimate the global surface-level daily PM2.5 concentrations at a high spatial resolution […] in 2000–2019.”

The study, he added, focussed “on areas above 15 µg/m3 [micrograms per cubic metre], which was considered the safe limit by WHO (the threshold is still arguable)”.

Over the two decades in question, the study revealed that the annual PM2.5 concentration and the number of high PM2.5 exposure days decreased in both Europe and North America.

However, exposures were seen to increase in southern Asia, Australia, the Caribbean, Latin America and New Zealand.

Globally, the annual average PM2.5 level between 2000 and 2019 was 32.8 µg/m3

In fact, despite a slight overall decrease in high PM2.5 exposed days globally, by 2019 more than 70 percent of days still had PM2.5 concentrations higher than 15 µg/m3.

This level increased to more than 90 percent of days when considering just eastern Asia and southern Asia — which also saw the highest PM2.5 concentrations, at 50.0 and 37.2 µg/m3 respectively.

According to Prof Guo, the unsafe ambient fine particulate matter concentrations also showed seasonally varying distributions.

These, he said, “included Northeast China and North India during their winter months (December, January and February), whereas eastern areas in northern America had high PM2.5 in its summer months (June, July and August).

“We also recorded relatively high PM2.5 air pollution in August and September in South America and from June to September in sub-Saharan Africa.”

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The study, Prof. Guo said, is important because “it provides a deep understanding of the current state of outdoor air pollution and its impacts on human health.

“With this information, policy makers, public health officials and researchers can better assess the short-term and long-term health effects of air pollution.”

This, he concluded, could help to develop new air pollution mitigation strategies.

The full findings of the study were published in the journal The Lancet Planetary Health.

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