England [UK], November 29 : A new study led by researchers at University College London (UCL) shows that chronic exposure to toxic air can significantly diminish the health benefits of regular physical activity.
The study analysed data from more than 1.5 million adults tracked for more than a decade in countries including the UK, Taiwan, China, Denmark and the United States.
The team found that the protective effect of regular exercise on people's risk of dying over a specific period - from any cause and from cancer and heart disease specifically - appeared to be reduced, but not eliminated, for those who lived in high pollution areas.
The researchers looked at levels of fine particulate matter - tiny particles known as PM2.5s with a diameter of less than 2.5 micrometres across. These particles are so small they can get stuck in the lungs and enter the bloodstream.
The health benefits of exercise significantly weakened where the yearly average level of PM2.5s was 25 micrograms per cubic metre (mg/m³) or higher, the team found. Nearly half (46%) of the world's population live in areas exceeding this threshold.
The lead researcher, Professor Po-Wen Ku of National Chung Hsing University, Taiwan, 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's Department of Behavioural Science & Health, 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."
For the study, the research team looked at data from seven existing studies, including three that were unpublished, combining the summary statistics from each study into one overall analysis. For three of these studies, they re-analysed the raw data at the level of individual participants.
Combining the data from seven studies, they found that people who did at least two and a half hours of moderate or vigorous exercise* a week had a 30% lower risk of dying during the study period than those who did not meet this exercise threshold.
However, if people in this very physically active group lived in an area with high fine particle pollution (above 25 mg/m³), this reduction in risk halved to 12-15 per cent.
At higher levels of fine particle pollution, above 35 mg/m³, the benefits of exercise weakened further, particularly for risk of death from cancer, where the benefits were no longer robust. About a third of the world's population (36 per cent) live in areas whose yearly average PM2.5 levels exceed 35 mg/m³.
For study participants in the UK, the average yearly PM2.5 levels were lower than these thresholds, at 10 mg/m³. However, levels of fine particle pollution vary a lot and spikes in pollution in UK cities do exceed 25 mg/m³, the critical threshold identified in the study, mainly during the winter months.
Co-author Professor Paola Zaninotto, from UCL's Department of Epidemiology & Public Health, 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."
In their section on limitations, the authors noted that the study was mostly conducted in high-income countries, so the findings might not apply to low-income countries where fine particle pollution is higher, often exceeding 50 mg/m³. Other limitations included a lack of data on indoor air quality as well as participants' diets.
However, a wide variety of other factors were accounted for, including income and education levels, health behaviours such as smoking, and the presence or not of existing chronic diseases.
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