Long-term exposure to polluted air can significantly diminish the health benefits of exercise, according to a recent study by an international team of researchers, including those from University College London (UCL). The study, published in the journal BMC Medicine, analyzed data from over 1.5 million adults across multiple countries, including the UK, Taiwan, China, Denmark, and the United States, over a decade. The findings reveal that the protective effects of regular exercise on mortality risk from various causes, including cancer and heart disease, seem to be reduced but not entirely eliminated for individuals residing in high pollution areas. The research focused on fine particulate matter, known as PM2.5s, which are tiny particles with a diameter of less than 2.5 micrometres. These particles can penetrate deep into the lungs and bloodstream, posing significant health risks. The study found that the health benefits of exercise were significantly compromised when the yearly average level of PM2.5s exceeded 25 micrograms per cubic metre (μg/m³). Alarmingly, nearly half (46%) of the world’s population lives in areas where this threshold is surpassed. Po-Wen Ku, the lead researcher and professor at National Chung Hsing University in Taiwan, emphasized that exercise remains beneficial even in polluted environments, but improving air quality can significantly enhance these health gains. Co-author Professor Andrew Steptoe from UCL’s Department of Behavioural Science & Health added that toxic air can partially negate the benefits of exercise, although it doesn’t eliminate them entirely. This further highlights the detrimental impact of fine particle pollution on our health. The study involved a comprehensive analysis of data from seven existing studies, including three unpublished ones, combining summary statistics and re-analyzing raw data at the individual participant level. The research team found that individuals engaging in at least two and a half hours of moderate or vigorous exercise weekly had a 30% lower risk of mortality during the study period compared to those who didn’t meet this exercise threshold. However, for those in the most physically active group living in areas with high fine particle pollution (above 25 μg/m³), this risk reduction was halved to 12-15%. At even higher pollution levels (above 35 μg/m³), the benefits of exercise weakened further, particularly for cancer-related mortality. Approximately one-third of the world’s population (36%) resides in areas with PM2.5 levels exceeding 35 μg/m³. In the UK, the average yearly PM2.5 levels were lower than the thresholds, at 10 μg/m³, but pollution spikes in cities, especially during winter, can still reach the critical threshold of 25 μg/m³. Co-author Professor Paola Zaninotto from UCL’s Department of Epidemiology & Public Health advised that while outdoor exercise should not be discouraged, checking air quality, choosing cleaner routes, or adjusting intensity on polluted days can maximize the health benefits of exercise. The study’s limitations include its focus on high-income countries, which may not be applicable to low-income countries with higher fine particle pollution levels. Additionally, the study lacked data on indoor air quality and participants’ diets, but it did account for various factors such as income, education, smoking habits, and the presence of chronic diseases.