Marathon runners already have plenty to think about: pacing, hydration, the dreaded wall. Now a new study suggests they may need to pay attention to one more thing: the air they’re breathing. Researchers at the University of Leicester have found that higher levels of air pollution on race day are linked to significantly slower marathon finishing times, and that recreational runners are hit harder than the elites. The findings, published in Sports Medicine Open, come from an analysis of over 2,000,000 finishing times across six of the world’s biggest marathons.
A Massive Dataset
The research, led by Dr James Donaldson and funded by the National Institute for Health and Care Research (NIHR) Biomedical Research Centre Leicester, looked at races held between 2010 and 2024 at the six World Marathon Majors: Berlin, Boston, Chicago, London, New York City, and Tokyo. The team cross-referenced net finishing times with air quality readings on race day for two major pollutants: nitrogen dioxide (NO₂) and fine particulate matter (PM₂.₅).
Dr Donaldson explained what these pollutants are: “Nitrogen dioxide is a pollutant mainly produced by road traffic and combustion processes, while fine particulate matter consists of tiny airborne particles that can be generated by vehicle emissions, industry, heating and other sources. Both pollutants have been linked to adverse health effects, particularly on the heart and lungs.”
What the Numbers Showed
Nitrogen dioxide stood out clearly. “From our analysis of more than two million marathon runners we observed that levels of nitrogen dioxide were strongly associated with their finishing time,” said Dr Donaldson. “Each microgram per cubic metre (1 µg/m³) increase in nitrogen dioxide was associated with a 1.43 minute slower finishing time in men and 1.56 minute slower finish time in women.”
To put that in perspective, on a high-pollution race day with NO₂ at around 10.3 µg/m³ compared with a low-pollution day of 5.2 µg/m³, finishing times were around seven to eight minutes longer. Interestingly, fine particulate matter (PM₂.₅) didn’t show a statistically significant link to performance in this study.
Not Everyone Was Affected Equally
Some of the most striking findings were about who was most affected. The negative effect of nitrogen dioxide was about six times larger for recreational runners than for elite athletes. Younger runners aged 18 to 29 also showed bigger effects than older runners.
Dr Donaldson offered some possible explanations: “There are many potential explanations for these patterns. Firstly, slower runners remain on the marathon course for longer, so they may accumulate greater pollutant dose during race day. Secondly, the physiological adaptations that result from the high-intensity training undertaken by elite athletes may offer some protection against the harmful effects of air pollution.”
He added: “Recreational runners may have less training behind them, so the added strain of pollution during a marathon could have a greater effect on their performance.”
In other words, the slower you are, the longer you’re out there breathing in pollutants, and the less your body may be conditioned to cope with the extra stress.
Should Runners Be Worried?
The pollution levels on race day across the six marathons were generally fairly low, close to or below the World Health Organization’s 2021 air quality guidelines. Even so, the effects showed up clearly in the data.
Professor Bryan Williams, Chief Scientific and Medical Officer at the British Heart Foundation, which supported the study, put the findings in perspective: “This large study suggests air pollution, specifically nitrogen dioxide, has a negative impact on how well people can exercise. While the study is observational and can’t prove cause and effect, it’s not surprising that air pollution could have this effect. We know that breathing in polluted air can trigger inflammation, damage blood vessels, and affect breathing.”
He was quick to reassure runners not to hang up their trainers: “Despite the known harms of air pollution, it is important to note that levels of nitrogen dioxide and PM2.5 on marathon day in London appear to have fallen substantially since the 2010 event, which a great news for heart health. As well as this, the fact remains that exercise is still very beneficial for your cardiovascular health – it can reduce the risk of heart and circulatory diseases by up to 35 per cent. While interesting, the findings of this study definitely shouldn’t put people off running.”
A New Factor for Race Planners
The researchers suggest that air quality and race-related traffic policies should sit alongside temperature and humidity when planning big city endurance events, and when advising runners, particularly amateurs.
The team is also developing the P-STEP app (Personalised Space Technology Exercise Platform), with support from the European Space Agency. It uses real-time, location-specific air quality data to help people plan when and where to exercise outdoors, hopefully helping runners of all levels breathe easier and finish stronger.
Donaldson, J. et al. (2026). Air Pollution Impairs Marathon Performance: A Cross-Sectional Analysis of 2.7 Million Finishers Across Six World Marathon Majors. Sports Medicine – Open. DOI: 10.1186/s40798-026-01099-6