A study by Monash University researchers has revealed that more than 99% of global land area and population are exposed to high levels of air pollution, exceeding the safety levels recommended by the World Health Organization (WHO).

In a world first study of daily ambient fine particulate matter (PM2.5) across the globe, the researchers found that only 0.18% of the global land area and 0.001% of the global population were exposed to PM2.5 concentrations lower than the 2021 WHO guideline limit (annual average of 5 μg/m³) in 2019.

Led by Professor Yuming Guo from the Monash University School of Public Health and Preventive Medicine, this study, which has been published in the prestigious journal, Lancet Planetary Health, provides a map of how PM2.5 levels have changed across the globe over the past decades. The research team utilised traditional air quality monitoring observations, satellite-based meteorological and air pollution detectors, and statistical and machine learning methods to more accurately assess PM2.5 concentrations globally.

“In this study, 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 of approximately 10km × 10km for global grid cells in 2000-2019, focusing on areas above 15 μg/m³, which is considered the safe limit by WHO (the threshold is still arguable),” Professor Guo says.

One significant finding from the study was the reduced daily PM2.5 levels in Europe and North America over the two decades to 2019. On the other hand, PM2.5 levels have increased in Southern Asia, Australia, New Zealand, Latin America and the Caribbean, with more than 70% of the days seeing levels exceeding the safety limit.

Key findings from the study:

  • Despite a slight decrease in high PM2.5 exposed days globally, by 2019 more than 70% of days still had PM2.5 concentrations higher than 15 μg/m³
  • In southern Asia and eastern Asia, more than 90% of days had daily PM2.5 concentrations higher than 15 μg/m³
  • Australia and New Zealand had a marked increase in the number of days with high PM2.5 concentrations in 2019
  • Globally, the annual average PM2.5 from 2000 to 2019 was 32.8 µg/m³
  • The highest PM2.5 concentrations were distributed in the regions of Eastern Asia (50.0 µg/m³) and Southern Asia (37.2 µg/m³), followed by northern Africa (30.1 µg/m³)
  • Australia and New Zealand (8.5 μg/m³), other regions in Oceania (12.6 μg/m³), and southern America (15.6 μg/m³) had the lowest annual PM2.5 concentrations

The study also revealed the different seasonal patterns of unsafe PM2.5 concentrations, with Northeast China and North India showing high PM2.5 during the winter months (December, January, and February), while eastern areas in northern America had high PM2.5 in the summer months (June, July, and August). South America recorded relatively high PM2.5 air pollution in August and September while sub-Saharan Africa had high PM2.5 from June to September.

“The study provides a deep understanding of the current state of outdoor air pollution and its impacts on human health. With this information, policymakers, public health officials, and researchers can better assess the short-term and long-term health effects of air pollution and develop air pollution mitigation strategies,” Professor Guo says.

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