Long-term exposure to toxic air can significantly weaken the health benefits typically gained from regular exercise, according to a major international study involving researchers from UCL (University College London). While physical activity remains beneficial even in polluted environments, the findings show that air pollution can substantially reduce its protective effects on long-term health.
Published in the journal BMC Medicine, the study analysed health data from more than 1.5 million adults who were followed for over a decade across several countries, including the UK, Taiwan, China, Denmark, and the United States. This large population allowed researchers to closely examine how air quality interacts with exercise to influence the risk of death from all causes, as well as from cancer and heart disease specifically.
The researchers found that regular physical activity lowers the risk of dying. However, this protective effect was noticeably reduced among people living in areas with high levels of fine particle pollution. In heavily polluted regions, exercise still offered health benefits, but these were weakened rather than eliminated, suggesting that air quality plays a decisive role in shaping overall outcomes.
The study focused on delicate particulate matter known as PM2.5, which consists of tiny airborne particles measuring less than 2.5 micrometres in diameter. These particles are small enough to penetrate deep into the lungs and enter the bloodstream, where they can contribute to inflammation, cardiovascular stress, and long-term disease. The researchers found that once average yearly PM2.5 levels reached 25 micrograms per cubic metre or higher, the health benefits of exercise declined significantly. Around 46% of the world’s population currently lives in areas above this threshold.
Lead author Professor Po-Wen Ku of National Chung Hsing University in Taiwan said the results show that exercise remains essential even in polluted environments. Still, cleaner air would significantly enhance its benefits. UCL co-author Professor Andrew Steptoe added that toxic air can partially block the benefits of physical activity and that the findings provide further evidence of the serious harm caused by fine particle pollution. He stressed that both clean air and regular exercise are essential for healthy ageing.
The research team combined evidence from seven existing studies, including three unpublished ones. For three of these, they re-analysed individual participant data to improve accuracy. Overall, they found that people who completed at least 2.5 hours of moderate or vigorous exercise per week had a 30% lower risk of death than those who were less active.
However, among highly active people living in areas with PM2.5 levels above 25 micrograms per cubic metre, this risk reduction fell to just 12–15%. At even higher pollution levels, above 35 micrograms per cubic metre, the protective effects weakened further, especially for cancer deaths. About 36% of the global population lives in areas exceeding this higher pollution threshold.
In the UK, average yearly PM2.5 levels were much lower, at around 10 micrograms per cubic metre. However, pollution levels can spike above 25 micrograms per cubic metre in cities, particularly during winter. UCL co-author Professor Paola Zaninotto said the findings should not discourage outdoor exercise but advised checking air quality, choosing cleaner routes, and reducing intensity on highly polluted days.
More information: Po-Wen Ku 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 Medicine. DOI: 10.1186/s12916-025-04496-y
Journal information: BMC Medicine Provided by University College London
