Prolonged exposure to polluted air can significantly reduce the positive effects of regular physical activity, according to new research conducted by an international team of scientists, including experts from University College London (UCL).

Key Research Findings

The study, published in the journal BMC Medicine, analyzed data from more than 1.5 million adults across various countries, including the UK, Taiwan, China, Denmark, and the USA. Participants were observed for over ten years. The researchers found that people living in areas with high levels of air pollution experienced a noticeably smaller reduction in mortality risk during the observation period. This applied to both overall mortality and deaths from cancer and cardiovascular diseases, although some benefits of physical activity still remained.

The Impact of Fine Particles PM2.5

The researchers focused on the concentration of fine particulate matter—PM2.5, which are particles less than 2.5 micrometers in diameter. Due to their small size, these particles can penetrate deep into the lungs and even enter the bloodstream. The analysis showed that the benefits of physical exercise dropped sharply when the average annual PM2.5 level reached 25 micrograms per cubic meter (μg/m³) or higher. Nearly half (46%) of the world’s population lives in regions where pollution meets or exceeds this threshold.

Expert Opinions

Professor Po-Wen Ku from National Chung Hsing University (Taiwan) noted:
“Our results highlight that physical activity remains beneficial even in polluted environments. However, improving air quality can significantly enhance these positive effects.”

Professor Andrew Steptoe from the Department of Behavioural Science and Health at UCL added:
“Our study shows that toxic air to some extent blocks the benefits of physical exercise, though it does not eliminate them entirely. These findings further confirm the harm that fine particles cause to our health. We believe that clean air and physical activity are equally important for healthy aging, so more efforts are needed to reduce harmful pollution levels.”

Research Methodology

For the analysis, the team combined data from seven existing studies, three of which had not been previously published. Both aggregated data and individual participant records were used for three of the studies.

Across all studies, adults who engaged in moderate or vigorous physical activity for at least two and a half hours per week had a 30% lower risk of death during the observation period compared to those who did not reach this level of activity. However, for very active people living in areas where PM2.5 levels exceeded 25 μg/m³, the protective effect dropped to 12–15%.

When pollution levels rose above 35 μg/m³, the benefits of physical exercise weakened even further, especially regarding cancer-related mortality. In such conditions, the protective effect almost disappeared. About 36% of the world’s population lives in places where the average annual PM2.5 level exceeds 35 μg/m³.

The Situation in the UK

Participants from the UK were exposed to an average PM2.5 level of 10 μg/m³, which is below the threshold at which the benefits of exercise are significantly reduced. However, pollution levels in British cities fluctuate greatly and often exceed the critical threshold of 25 μg/m³ during winter.

Practical Recommendations

Professor Paola Zaninotto from the Department of Epidemiology and Public Health at UCL emphasized:
“We do not want to discourage people from exercising outdoors. Checking air quality, choosing cleaner routes, or reducing workout intensity on days with high pollution can help maximize health benefits.”

Study Limitations

The authors noted several limitations: most of the data came from high-income countries, so the findings may not fully apply to regions with lower incomes, where pollution levels are often much higher and can sometimes exceed 50 μg/m³. There was also a lack of data on indoor air quality and participants’ diets.

Despite these gaps, the study accounted for many other factors, such as income, education, smoking, and the presence of chronic diseases.

The project involved researchers from the UK, USA, Australia, and Asia. For the analysis of UK data, the UK Biobank database was used.

The study was funded by the National Science and Technology Council of Taiwan, Taichung Veterans General Hospital, and National Chung Hsing University.

Moderate and vigorous physical activity, such as jogging or sports, is usually defined as activity that causes shortness of breath and sweating.