There are countless factors a runner can prepare for on race day, but air quality isn’t one of them. While air pollution undeniably affects ecosystems and daily life, its specific impact on marathon performance has been less clear—until now. Researchers from the Department of Epidemiology at Brown University in Providence, R.I., recently conducted a study to determine just how much air quality influences male and female marathon finish times.
The study, published in the Journal of Sports Medicine, analyzed data from more than 1.5 million male runners and a million female runners at nine big-city U.S. marathons between 2003 and 2019. The researchers looked at marathon finish times and air quality data collected during the races, and investigated how fine-particle pollution influenced performance.
The study focused on fine particulate matter, also known as PM2.5—particles smaller than 2.5 micrometres in diameter, typically produced by sources like vehicle emissions, industrial processes and forest fires. Unlike larger particles, PM2.5 has the ability to penetrate deep into the lungs and even enter the bloodstream. The Canadian Cancer Society has linked PM2.5 exposure to heart disease, lung cancer and respiratory issues. Marathon runners are particularly vulnerable to inhaling pollutants, since they require an increased volume of air during physical activity.
The researchers measured particulate matter levels at every mile along the marathon routes, providing insights into how pollution varied throughout the race. The findings were clear: for every one microgram per cubic meter (µg/m³) increase in PM2.5, male runners finished 32 seconds slower on average, while female runners took 25 seconds longer to cross the finish line. Faster finishers appeared to be more affected by air pollution than back-of-packers, likely due to increased respiratory rates and higher air intake.
So, if you consider the spread between the lowest and highest fine-particule pollution measurements on a typical marathon course, if the spread is, say, 3 micrograms, that could potentially slow you down by around a minute and a half, compared to a marathon run in clean air. To a first-time marathoner, that may not seem like much, but if you’re trying to run a personal best or qualify for the Boston Marathon, for example, those seconds or minutes become very meaningful.
It’s always a good idea to check the Air Quality Index (AQI), just as you would check the weather, before heading outdoors to exercise. AQI levels between zero and 50 are ideal for running, while moderate levels (51–100) are generally safe for most people but may pose risks for individuals with chronic health conditions. AQI levels above 100 are not recommended for outdoor physical activity, and can even be hazardous to health.
