Does the city where you run a marathon affect your finish time? According to new research, the answer is yes—and air quality plays a bigger role than many runners realize.
When marathoners struggle to maintain their goal pace or hit the dreaded wall, they typically point to fatigue, unfavorable weather, or nutritional issues. However, a Brown University study published in Sports Medicine identifies a less obvious environmental factor: air pollution.
Comprehensive Analysis of Race Data
Researchers examined an extensive dataset containing 2.6 million marathon finish times from major U.S. races, including events in Boston, New York City, and Los Angeles. The data spanned 17 years and was cross-referenced with estimated pollution levels recorded by weather stations.
The analysis uncovered a clear connection between slower average finish times and elevated concentrations of fine particulate matter known as PM2.5.
Notably, the fastest runners appeared most vulnerable to pollution’s effects.
Understanding PM2.5
PM2.5 describes tiny pollutants measuring smaller than 2.5 micrometers in diameter. These particles frequently feature in air-quality health warnings because they can penetrate deep into lung tissue and enter the bloodstream, triggering inflammation and chest tightness, according to The New York Times.
Quantifying the Impact
The study documented measurable performance declines linked to PM2.5 concentrations. For every one microgram per cubic meter increase in these microscopic particles, average finish times worsened:
- Male runners slowed by 32 seconds per one microgram/cubic meter increase
- Female runners slowed by 25 seconds per one microgram/cubic meter increase
Put simply, on days with even moderately elevated pollution levels, a runner’s finishing time could drop by several minutes.
City-by-City Differences
The Los Angeles Marathon typically recorded the highest estimated median pollution levels and the slowest median finish times among races studied. While other factors such as warmer temperatures and a more challenging course profile may contribute, the overall pattern of slower performances in more polluted conditions remained consistent across all cities—even when comparing different years within the same marathon location.
Boston posted the fastest average finish times and ranked among the cleanest air environments studied, alongside Minneapolis/St. Paul and New York City.
Even Elite Athletes Affected
What makes these findings particularly striking is their impact on exceptionally fit individuals.
Joseph Braun, a professor of epidemiology at Brown, told The New York Times: “What’s notable is that we’re looking at people who are all incredibly healthy. But even among really healthy people, air pollution is having an important, albeit subtle, effect on your physiology.”
Unexpectedly, the performance decline proved more significant for faster-than-average runners. Researchers theorize this may occur because elite and competitive marathoners breathe more frequently and deeply, inhaling larger quantities of pollutants over the 26.2-mile distance.
Sources of PM2.5
PM2.5 originates primarily from fossil fuel combustion—including emissions from power plants and gasoline or diesel vehicles—as well as from wildfires and wood burning.
Although air quality has improved across many U.S. regions, short-term spikes from sources like wildfire smoke have emerged as an increasing concern, The Times reported.
This research underscores that environmental conditions extend beyond temperature and humidity to include air quality as a meaningful factor in athletic performance, even among the healthiest populations.