Higher levels of nitrogen dioxide on race days added minutes to finishing times at six major city marathons between 2010 and 2024, researchers at the University of Leicester said on Tuesday.
For every microgram-per-cubic-metre increase in nitrogen dioxide, finishing times were 1.43 minutes slower for men and 1.56 minutes slower for women, according to the study, which was supported by the British Heart Foundation.
The researchers analysed more than 2.7 million race performances at the Berlin, Boston, Chicago, London, New York City and Tokyo marathons, which together form most of the World Marathon Majors and host thousands of athletes each year.
From our analysis of more than two million marathon runners, we observed that levels of nitrogen dioxide were strongly associated with their finishing time
The effect was greater among non-elite runners. Recreational runners lost 2.12 minutes per additional microgram of nitrogen dioxide, compared with 0.34 minutes for elite runners, the study found.
On a high-pollution race day, when nitrogen dioxide reached 10.3 micrograms per cubic metre, compared with a low-pollution level of 5.2, finishing times were around seven to eight minutes longer.
The highest median race-day concentration of nitrogen dioxide was recorded in London at 10.9 micrograms per cubic metre, and the lowest in Berlin at 6.1.
The researchers also analysed fine particulate matter, known as PM2.5, but found no statistically significant association with finishing times.
Nitrogen dioxide comes mainly from burning fossil fuels in cars, trucks and factories, as well as from natural processes and chemical reactions in the air. It has been linked to impaired lung function, airway inflammation and adverse cardiovascular responses.
The authors said slower runners spend longer on the course and are more likely to train in the urban areas where marathons are held, accumulating chronic pollution exposure, while elite runners typically prepare in cleaner environments. James Donaldson, the study’s lead author, said the physiological adaptations from high-intensity training may give elite athletes some protection against air pollution.
Bryan Williams, chief scientific and medical officer at the British Heart Foundation, said nitrogen dioxide and PM2.5 levels on marathon day in London appeared to have fallen substantially since the 2010 event, which he called “great news for heart health”.
Williams said exercise could reduce the risk of heart and circulatory diseases by up to 35 per cent and that the findings should not put people off running.