- Posted November 6, 2025
Check Your City: Air Pollution Linked to Slower Marathon Times
Does the city you run in make a difference? Researchers say yes, it does.
When marathon runners hit the wall or fall behind their goal pace, they often blame fatigue, weather or nutrition. However, a study from Brown University published in Sports Medicine suggests a less obvious, environmental culprit: air pollution.
Researchers analyzed a dataset of 2.6 million marathon finish times from major U.S. races, including those in Boston, New York City and Los Angeles, spanning 17 years and matched it to estimated pollution levels from weather stations.
They found a direct link between slower average finish times and higher concentrations of fine particulate matter known as PM2.5.
The data also showed that the fastest runners were more affected by this effect.
PM2.5 refers to tiny pollutants smaller than 2.5 micrometers in diameter. These particles are often the focus of air-quality health advisories, because they can travel deep into the lungs and enter the bloodstream, causing inflammation and chest constriction, reports The New York Times.
The study revealed a measurable slowdown tied to PM2.5 levels. For every increase of one microgram per cubic meter increase in these tiny particles, the average finish time for runners dropped.
Men were 32 seconds slower for every one microgram/cubic meter increase.
Women were 25 seconds slower for every one microgram/cubic meter increase.
In other words, on a day with even moderately elevated pollution, a runner's time could be slower by several minutes.
The Los Angeles Marathon generally had the highest estimated median pollution levels and the slowest median finish times among the races studied.
While this could owe to other factors like warmer weather and a hillier course, the overall pattern of slower finishes in more polluted races held true across all cities, even when comparing different years within the same marathon city.
Boston had the fastest average finish time and one of the cleanest air levels among the cities, along with Minneapolis/St. Paul and New York City.
What makes this finding particularly notable is that it affects even the fittest individuals.
“What’s notable is that we’re looking at people who are all incredibly healthy,” Joseph Braun, a professor of epidemiology at Brown, told The New York Times. “But even among really healthy people, air pollution is having an important, albeit subtle, effect on your physiology.”
Surprisingly, the slowdown was more pronounced for faster-than-average runners. Researchers suspect this may be because elite and competitive marathoners breathe in more air — and do so more rapidly — inhaling a larger dose of the pollution over the 26.2-mile course.
PM2.5 primarily comes from the burning of fossil fuels — such as from power plants, gasoline or diesel vehicles — as well as from forest fires and wood burning.
While air quality has improved in many U.S. regions, short-term spikes from sources like wildfire smoke have become a growing concern, The Times said.
More information
The U.S. Environmental Protection Agency (EPA) provides current air quality standards and health information about particulate matter.
SOURCE: The New York Times, Nov. 1, 2025
