Regular physical activity is widely known to reduce the risk of early death, heart disease, and cancer — but long-term exposure to polluted air may blunt these benefits, according to a new review published on November 28 in BMC Medicine.

The study, led by Dr. Po-Wen Ku of University College London, examined whether fine particulate matter (PM2.5) — one of the most harmful air pollutants — reduces the protective effects of leisure-time physical activity (LTPA) on overall, cardiovascular, and cancer-related mortality.

Polluted Air Reduces the Protective Impact of Exercise

The review analyzed seven cohort studies involving 1.5 million people and more than 115,000 deaths. Researchers found that while exercise consistently reduced mortality risk, its protective power dropped in environments with higher PM2.5 levels.

Meeting the recommended weekly exercise level (7.5 to 15 MET-hours) cut the risk of all-cause mortality by 30% in areas with PM2.5 levels below 25 μg/m³. But in regions where PM2.5 exceeded that threshold, the benefit dropped to 12–15%.

Data pooled from three additional cohorts — covering 869,038 individuals and 45,080 deaths — reinforced this trend. Hazard ratios for mortality varied significantly based on pollution levels, from 0.30 at PM2.5 below 10 μg/m³ to 0.75 at concentrations between 35 and 50 μg/m³.

At PM2.5 levels above 25 μg/m³, the benefits of exercise were noticeably weakened for all causes of death. For cancer mortality, the protective effect of exercise disappeared altogether at pollution levels of 35 to 50 μg/m³.

Exercise Still Helps — But Clean Air Helps More

Despite the concerning findings, researchers emphasized that physical activity remains beneficial across all pollution levels.

“Our findings emphasize that exercise remains beneficial even in polluted environments,” Dr. Ku said. “However, improving air quality can greatly enhance these health gains.”

A Call for Policy Action

The review highlights a critical public health message: exercise should continue to be encouraged, but long-term solutions require cleaner air. The authors noted that reducing PM2.5 exposure could dramatically magnify the health benefits of an active lifestyle — especially in heavily polluted regions.