City air pollution is linked to slower marathon times

A marathon already asks a lot from the body. Runners breathe harder for hours, their hearts work faster, and they have to deal with heat, humidity, wind, and whatever the city air contains that day.

That last part may matter more than people realize.

A large analysis of major marathons found that higher levels of nitrogen dioxide, a common traffic-related air pollutant, were linked with slower finishing times, especially among recreational runners.

Why city air matters during exercise

When people exercise hard, they breathe much more air than they do at rest. That means pollutants in the air can also reach deeper into the lungs during a long race.

Dr. James Donaldson, a postdoctoral researcher in health and the environment at the University of Leicester, described the two pollutants the team examined.

“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,” Dr. Donaldson said.

“Both pollutants have been linked to adverse health effects, particularly on the heart and lungs. From our analysis of more than two million marathon runners we observed that levels of nitrogen dioxide were strongly associated with their finishing time.”

“Each microgram per cubic meter (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.”

Millions of marathon performances compared

The researchers analyzed 2,756,553 finishing times from the World Marathon Majors in Berlin, Boston, Chicago, London, New York City, and Tokyo.

The races took place between 2010 and 2024 and covered 75 separate marathon events.

They compared finishing times with race-day levels of nitrogen dioxide, or NO2, and fine particulate matter, or PM2.5. These extremely small particles can remain suspended in the air.

The pollution values came from the Copernicus Atmosphere Monitoring Service. Everyone running the same marathon was given the same estimated pollution level, based on air near the race starting area.

That means the researchers did not know exactly how much pollution each runner breathed. They also did not track air quality along every mile of the 26.2-mile route.

Nitrogen dioxide had the clearest link

NO2 stood out much more clearly than PM2.5.

For every increase of 1 microgram per cubic meter of NO2, men’s finishing times were about 1.43 minutes slower and women’s times were about 1.56 minutes slower.

Another comparison makes the difference easier to understand.

Moving from a relatively low-NO2 race day to a relatively high-NO2 race day was linked with about 7.3 extra minutes for men and 7.9 extra minutes for women.

The researchers also looked at the full spread of finishing times, rather than only the average. On higher-NO2 days, the whole group tended to shift toward slower times.

PM2.5 did not show the same clear relationship. In the main analysis, the researchers could not confidently connect higher PM2.5 levels with slower marathon performance.

Recreational runners showed bigger effects

One of the most interesting patterns appeared when the researchers separated runners by performance level.

Their “elite” group meant the fastest 10 percent of runners in each race, so it did not necessarily mean professional athletes.

The fastest group showed a much smaller link between NO2 and finishing time. Recreational runners showed an effect around six times larger.

Runners ages 18 to 29 also showed a larger association than runners ages 60 and older.

The paper does not show that younger adults are generally more vulnerable to pollution, because older marathon runners may be an unusually fit group compared with other people their age.

Why slower runners struggle

Dr. Donaldson discussed why slower runners could show a stronger pattern.

“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,” Dr. Donaldson said.

“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.”

Dr. Donaldson added that recreational runners may have less training, so pollution could put more strain on them during a marathon and affect their performance more.

He said this is only one possible explanation because the researchers did not measure what was happening inside the runners’ bodies.

Previous evidence suggests air pollution can affect the lungs, blood vessels, and the body’s inflammatory response.

That gives researchers possible reasons why polluted air could make hard exercise more difficult.

Professor Bryan Williams, Chief Scientific and Medical Officer at the British Heart Foundation, put the results into that wider health picture.

“This large study suggests air pollution, specifically nitrogen dioxide, has a negative impact on how well people can exercise,” Professor Williams added.

“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.”

The key limitation is that the analysis shows a relationship, not proof that NO2 directly caused runners to slow down.

What the study can’t prove

The researchers adjusted their analysis for heat, humidity, wind, and changes over time. They also repeated the analysis in several different ways, and the NO2 pattern remained.

Still, several things could not be measured.

The pollution estimate came from the area around the race start, runners were not followed with personal air sensors, and the researchers could not include every other pollutant in the air, including ozone.

They also could not identify runners who took part in more than one marathon.

So although there were more than 2.7 million performances, that does not necessarily mean there were 2.7 million different people.

This is not a reason to stop running

The main message is not that outdoor exercise is dangerous or that people should avoid running.

Instead, the findings suggest that air quality may be another environmental factor worth considering alongside heat and humidity.

“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 is great news for heart health,” Professor Williams noted.

“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 percent.”

“While interesting, the findings of this study definitely shouldn’t put people off running.”

Professor Williams said the results should be viewed as another reason to pay attention to cleaner air, especially during long periods of outdoor exercise.

For most people, that is the part that matters beyond marathons.

People run, walk, cycle, and exercise in cities every day, and this work suggests the quality of the air around them could influence how hard that activity feels and how well they perform.

The study is published in the journal Sports Medicine – Open.

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