Long-term exposure to polluted air can significantly reduce the health benefits of regular exercise, according to a new study involving researchers from University College London (UCL). The research, published in BMC Medicine, tracked more than 1.5 million adults over a decade across countries such as the UK, Taiwan, China, Denmark, and the United States.
The study focused on fine particulate matter known as PM2.5s—tiny particles less than 2.5 micrometres wide that can lodge in the lungs and enter the bloodstream. Researchers found that while regular exercise lowers the risk of death from any cause, cancer, and heart disease, these protective effects are reduced for people living in areas with high levels of PM2.5 pollution.
According to the findings, when yearly average PM2.5 levels reached or exceeded 25 micrograms per cubic metre (μg/m³), the health benefits of exercise weakened considerably. Nearly half of the global population lives in areas above this threshold.
Co-author Professor Andrew Steptoe from UCL's Department of Behavioural Science & Health stated: "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."
The research team combined data from seven studies—including three unpublished ones—and re-analysed individual-level data for three studies. They determined that individuals who exercised at least two and a half hours per week had a 30% lower risk of dying during the study period compared to those who did not meet this threshold. However, for those living in areas with PM2.5 levels above 25 μg/m³, this reduction dropped to between 12% and 15%.
At even higher pollution levels—above 35 μg/m³—the benefits were further diminished, especially regarding cancer mortality risk where no significant benefit was observed. About one-third of people worldwide live in regions where annual average PM2.5 exceeds this level.
In the UK segment of the study using UK Biobank data, average annual PM2.5 was lower at 10 μg/m³; however, urban spikes sometimes surpassed critical thresholds during winter months.
Co-author Professor Paola Zaninotto from UCL's Department of Epidemiology & Public Health commented: "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."
The authors noted limitations such as most data coming from high-income countries and lacking information on indoor air quality or participants' diets. They did account for factors like income, education level, smoking habits, and chronic diseases.
Researchers involved came from institutions in the UK, United States, Australia, and Asia; funding was provided by Taiwan's National Science and Technology Council as well as Taichung Veterans General Hospital (TCVGH) and National Chung Hsing University.
