Air pollution measurably slows marathon runners, according to a study of 2.7 million race performances reported on 30 September. Led by James Donaldson of the University of Leicester, the research analysed results from the Berlin, Boston, Chicago, London, New York City and Tokyo marathons between 2010 and 2024 and matched them against air quality data for each race day. The headline finding is that levels of nitrogen dioxide were strongly associated with finishing time.
The size of the effect depends on who is running. For every one microgram per cubic metre increase in nitrogen dioxide, men's finishing times rose by 1.43 minutes and women's by 1.56 minutes. Recreational runners bore the brunt, with an average penalty of 2.12 minutes per unit, compared with just 0.34 minutes for elite athletes. When the researchers compared high-pollution race days, at around 10.3 micrograms per cubic metre, with low-pollution days at roughly 5.2, runners finished approximately seven to eight minutes slower.
Not every pollutant behaved the same way. Fine particulate matter, PM2.5, showed no meaningful association with finishing times in this dataset, which is a notable result given how heavily it features in public discussion of air quality and exercise. The study's focus on nitrogen dioxide, a pollutant closely tied to road traffic and combustion, points toward urban transport emissions as the more relevant lever for race-day performance.
The six cities sit at different points on the scale. London recorded the highest average nitrogen dioxide concentration at 10.9 micrograms per cubic metre, while Berlin was the cleanest at 6.1. That spread helps frame why course and city matter for runners chasing personal bests, although it sits alongside better-known variables such as temperature, wind and pacing. The finding also fits a wider pattern in recent research, including a Japanese study of 1,938 runners that showed heat reshuffles which runners lose the most time.
The authors and outside experts were careful about the interpretation. Bryan Williams of the British Heart Foundation emphasised that exercise remains very beneficial for cardiovascular health, and nothing in the data suggests runners should avoid racing in cities. For organisers, the practical read is about route design, start times and traffic management; for runners, it is a reminder that a slower-than-expected result on a busy urban course may owe something to the air as well as the legs.