Long-term exposure to air pollution can reduce the health benefits of regular exercise, according to a study by an international team including UCL (University College London) researchers.
The study, published in the journal BMC Medicine, analysed data from 1.5 million adults tracked for more than 10 years across countries including the UK, Taiwan, China, Denmark, and the United States.
The researchers analysed levels of fine particulate matter – tiny particles known as PM2.5s, with a diameter of less than 2.5 micrometres— because they are small and can get stuck in the lungs and enter the bloodstream.
The team found that people living in areas where the annual average PM2.5 level was 25 μg/m³ or higher lost most of the benefits of regular exercise. Worryingly, almost half of the world’s population lives in these conditions.
“Our findings emphasise that exercise remains beneficial even in polluted environments. However, improving air quality can greatly enhance these health gains,” said Professor Po-Wen Ku of National Chung Hsing University, Taiwan.
“Our study shows that toxic air can, to some extent, block the benefits of exercise, although not eliminate them. The findings are further evidence of the damage that fine particle pollution can do to our health. We believe clean air and physical activity are both important for healthy ageing, and so we encourage greater efforts to curb health-harming pollution levels,” added Professor Andrew Steptoe, from UCL’s Department of Behavioural Science & Health.
The team found that people who did at least 2.5 hours of moderate or vigorous exercise per week had a 30% lower risk of dying during the study period than those who did not meet this threshold. However, for those living in an area with high fine particle pollution (above 25 μg/m³), this risk reduction is halved to 12-15%.
At higher levels of fine particle pollution (above 35 μg/m³), the benefits of exercise reduced even more, particularly the risk of dying from cancer, where the benefits were no longer robust. About a third of the world’s population (36%) lives in areas whose yearly average PM2.5 levels exceed 35 μg/m³.
“We don’t want to discourage people from exercising outdoors. Checking air quality, choosing cleaner routes, or easing off intensity on polluted days can help you get the most health benefits from your exercise,” said Professor Paola Zaninotto, from UCL’s Department of Epidemiology & Public Health.
Ku, PW., Steptoe, A., Hamer, M. et al. Does ambient PM2.5 reduce the protective association of leisure-time physical activity with mortality? A systematic review, meta-analysis, and individual-level pooled analysis of cohort studies involving 1.5 million adults. BMC Med 23, 647 (2025). https://doi.org/10.1186/s12916-025-04496-y