We all know exercise is good for us. It lowers the risk of heart disease, cancer, and early death, and keeps the body and mind in shape. But when the air you breathe is polluted, a new study suggests that exercise might not provide the same benefits it normally would.

An international team, including researchers from University College London (UCL) analyzed health data from over 1.5 million adults, collected over more than a decade in countries including the UK, Taiwan, China, Denmark, and the United States.

The researchers focused on levels of fine particulate matter, specifically tiny particles known as PM_2.5. These fine particles are smaller than 2.5 micrometers; they can get stuck in the lungs and enter the bloodstream, where they can trigger inflammation and long-term damage.

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Researchers found that adults who exercised at least two and a half hours a week—moderate to vigorous activity like jogging or other sports—typically had a 30% lower risk of dying during the study period than less active people. But in areas where the yearly average PM_2.5 exceeded 25 μg/m³, the protective effects of exercise dropped to just 12–15%.

The protective effects of exercise weaken even further in more polluted regions. At PM_2.5 levels above 35 μg/m³, where about a third (36%) of the global population lives, exercise offered even less protection, particularly against the risk of death from cancer.

“Our findings emphasise that exercise remains beneficial even in polluted environments,” lead researcher Professor Po-Wen Ku said in a statement. “But improving air quality can significantly enhance these health gains.”

Co-author Professor Andrew Steptoe said, “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The study looked at the data from seven existing studies, including three previously unpublished datasets, combining both summary statistics and raw participant-level data. Researchers carefully accounted for a wide range of other factors, including income, education, smoking, and pre-existing chronic conditions.

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However, the team points out that their data mostly comes from high-income countries, so the impact could be greater in low-income regions, where PM_2.5 levels often exceed 50 μg/m³. They also mention the lack of indoor air quality data and limited information on participants’ diets as part of the study’s caveats.

“We don’t want to discourage people from exercising outdoors,” said Co-author Professor Paola Zaninotto. “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 study reminds us of one of the world’s most serious problems: air pollution. Staying active is not enough to protect your health if the air around you is toxic. Cleaner air and regular exercise go hand in hand, and tackling pollution is not just about the environment—it’s about our bodies too.

The study was published in BMC Medicine.