February 28, 20254 min readHow Bad Air Quality Slows Down Marathon RunnersEven modest amounts of air pollution may affect athletic performance, a new study findsBy Claire Maldarelli edited by Tanya LewisParticipants run during the 2014 Beijing International Marathon in heavy smog. Even moderately poor air quality can affect finish times, a new study shows. Imaginechina Limited/Alamy Stock PhotoAsk any marathon runner a week before their big race what they are doing, and they’ll almost certainly be refreshing the weather app on their phone. That’s because humid conditions, freezing rain or even a day that is too sunny and hot can cause them to be slower during a race than they were in training. Runners spend months training for marathons, and it can be crushing if their hard-earned performance is affected by something that is completely out of their control, such as the weather. Now these athletes might have another factor to obsess over in the days leading up to their race: air quality.A new study published last December in Sports Medicine found an association between air pollution at higher levels—albeit below those set by current Environmental Protection Agency standards—and slower marathon finishing times. The results suggest that even modest amounts of air pollution may be affecting athletic performance.The researchers zeroed in on the levels of a type of air pollution known as fine particulate matter, or PM2.5. These are microscopic particles that accumulate in the air from a variety of pollutants such as forest fires, agricultural waste and car exhaust. The particles are so small that they can easily make their way deep into the lungs and can even enter the bloodstream. Previous research has long implicated these tiny air particles in various medical issues. Studies have found correlations between increased exposure to PM2.5, both for short and extended periods of time, and a variety of health issues, including cancer, cardiovascular disease and lung conditions.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Studies have also shown that exercising during times of poor air quality—for instance, when there is a high level of these fine particulates in the air—is detrimental to our health. But little is known about the effects this type of air pollution can have on performance in endurance events such as marathons, which require athletes to be outside exercising and breathing heavily for hours at a time.To better understand this, the researchers used a machine-learning model to estimate how much fine particulate matter was in the air at every mile marker on the course of nine marathons located across the U.S. between 2003 and 2019. This learning model combined data from various sources, including air sensors, with longitude and latitude coordinates, weather and topography. Using this model—rather than relying on monitoring stations, which can often be miles away from a marathon course—allowed the researchers to more precisely estimate what the air quality was like throughout each course, often down to the mile.They then compared these data with corresponding finishing times for the marathon events from more than 1.5 million finish times for male runners and slightly more than a million finish times for female runners. The researchers adjusted for other weather factors that could influence times, such as high heat or high humidity.The results showed that an increase of just one microgram per cubic meter in PM2.5 levels correlated with a 32-second-slower finishing time for male marathoners and a 25-second-slower finishing time for female ones. Further, these effects were the most profound on what the study called “faster than median” finishers—in other words, runners who were fast but not necessarily elite-level.Elvira Fleury, a doctoral student at the Harvard T. H. Chan School of Public Health and lead author of the study, says that marathoners should think of air pollution in a similar way to other weather conditions, such as heat and humidity, on race day. “When evaluating your performance, think about it the way you might think about heat. If it was really hot on the day of the marathon, you might tell yourself, ‘It’s okay that I ran a little slower because it was hot,’” says Fleury, who is an endurance athlete herself. “That’s what you can do with this data and say, ‘It was really polluted; maybe I ran a little slower than I perhaps could have.’”“These results highlight the need to look at how the ‘effective dose’ of a certain pollutant might change in exercise conditions and what that might mean for health,” says Matthew Ely, an assistant professor of human physiology at Providence College, whose work focuses on cardiovascular physiology and the influence of environmental factors on exercise performance. Ely wasn’t involved in the current study.While there hasn’t been much research on the connection between air quality and race performance in endurance events in general, this new study is in line with what is currently available. A small study published in 2023 in Scientific Reports tracked 334 male college athletes from 46 universities across the U.S. and measured their exposure to air pollution and ozone during a 21-day training period leading up to a five-kilometer (3.1-mile) championship race. The researchers found those exposed to higher levels of both PM2.5 and ozone during the training period had, on average, slower race finishing times.Beyond race times, the results open up new questions about exercise, air pollution and runners’ safety. For one, the researchers want to better understand what is happening on a physiological level when people run in poor air quality. Because this study only found a correlation between running performance and air quality, a fundamental next step for research, Fleury says, would be to understand what is going on in the body to cause worse performances. The researchers speculate that increases in fine particulate matter could be reducing running performance by constricting blood vessels, increasing inflammation, and impairing lung and brain function, but further research is needed to pin down the exact mechanisms at play.The findings also suggest that air quality may be having a more profound effect on exercise than previously thought. Importantly, the study found that even modest increases in fine particulate matter—at levels below current ambient air quality standards set by the EPA—resulted in reduced athletic performance.Most major marathons, such as those in New York City, Boston, Chicago, Tokyo and London, are in large urban areas, Ely notes. “We all know that exercise is important for us. We also know that environmental pollutants are present in big cities,” he says. “Is this something that we have to pay more attention to?”

Even modest amounts of air pollution may affect athletic performance, a new study finds