Long-term exposure to toxic air can substantially weaken the health benefits of regular exercise, according to a major new study involving researchers from University College London (UCL) and several international institutions. The findings arrive at a critical moment for India, where deadly levels of fine particle pollution are now a year-round public health emergency.

The study, published in BMC Medicine, analysed more than 1.5 million adults tracked for over a decade across the UK, Taiwan, China, Denmark, and the United States. Researchers examined exposure to PM2.5—tiny dust-like particles less than 2.5 micrometres wide. Because they are so small, they can travel deep into the lungs and even enter the bloodstream.

The team found that regular exercise continued to protect people from early death, cancer, and heart disease. However, this protection weakened sharply in highly polluted environments. The effect was most visible when the annual average PM2.5 level crossed 25 micrograms per cubic metre (μg/m³). Nearly half of the world’s population lives above this threshold, and India remains among the most affected.

Professor Po-Wen Ku, the study’s lead researcher from National Chung Hsing University, said: “Our findings emphasise that exercise remains beneficial even in polluted environments. However, improving air quality can greatly enhance these health gains.”

Co-author Professor Andrew Steptoe from UCL added: “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.”

The analysis combined data from seven major studies, including re-analysed raw data from three. Overall, people who completed at least two and a half hours of moderate or vigorous weekly exercise reduced their risk of death by 30 per cent. But in highly polluted regions, this benefit fell to just 12–15 per cent. At extreme pollution levels above 35 μg/m³, the benefits weakened further, especially for cancer-related deaths.

For India—where PM2.5 levels routinely exceed 50 μg/m³ in major cities during winter—the implications are severe. While experts stress that people should continue exercising, they recommend monitoring air quality, choosing cleaner routes, and reducing intensity on smog-heavy days.

Professor Paola Zaninotto from UCL said: “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.”