For optimal marathon performance, check training plan, gear, nutrition, weather -- and air quality?

When the research team assessed the association between fine particulate matter in the air and marathon finish times, they found that greater race-day pollution is associated with slower average marathon finish times. Their findings were published in the journal Sports Medicine.

The difference seems small, said study author Elvira Fleury, who led the research while enrolled as a graduate student at Brown, but for marathon runners trying to achieve a personal record, every second matters.

"Think of all the effort, time and money that a professional runner like Eliud Kipchoge put into trying to break the world record and run a marathon in less than two hours," said Fleury, who earned a master of public health degree from Brown in 2024 and is now a doctoral student at Harvard University. "Runners at that level are thinking about their gear, their nutrition, their training, the course, even the weather. Our results show that those interested in optimizing athletic performance should consider the effect of air pollution, as well."

The study included 1,506,137 male marathon finishers and 1,058,674 female finishers, and measured results at nine major marathons across the United States, including races in Boston, Houston and Los Angeles, from 2003 to 2019. The researchers obtained finish times for all participants in these races from public marathon data.

They combined that information with data from a statistical model created by Allan Just, an associate professor of epidemiology and environment and society at Brown, which showed the amount of fine particulate matter in the air at different points along the marathon routes.

"This really sophisticated spatial-temporal model of particulate matter allowed us to plot pollution at every mile of every course," Fleury said. "Without a model like this, it wouldn't have been possible to look at so many different marathons in different states across different years."

The concentration of an air pollutant is measured in micrograms per cubic meter of air, or µg/m³. Analyzing all finish times, the researchers found that each 1 µg/m³ higher particulate matter on race day was associated with 32-second slower average finish times among men and 25-second slower average finish times among women. The effects were more pronounced in faster-than-median runners.

Fine particulate matter refers to airborne pollution particles smaller than 2.5 microns in diameter, which are generated by sources such as forest fires, agricultural waste burning and inefficient fuel combustion, as well as vehicle emissions, energy generation, industrial activities and fertilizer application.

Previous studies have shown that daily fluctuations in air pollution are associated with mortality, heart disease and lung disease, and chronic exposure to air pollution is associated with lung cancer as well as heart disease and premature mortality. The researchers posited in the study that pollution's effect on marathon performance could be attributed to increased blood pressure, constricted blood vessels, reduced lung function, respiratory discomfort or possibly even short-term cognitive issues.

The findings apply not just to marathon runners, but also to everyone living with air pollution, said study author Joseph Braun, a professor of epidemiology at Brown's School of Public Health.

"People who can complete a marathon are generally quite healthy, and we can assume they have honed their cardiorespiratory fitness," Braun said. "This study revealed a negative impact from air pollution, even at levels below current health-based standards, on these very healthy people. This means that air pollution can be a health risk not just for those who are elderly or susceptible -- it can negatively affect even the most healthy and well-trained among us."

Based on their findings, the researchers concluded that efforts to reduce pollutionemissions by regulating power generation, industry and transportation should continue, as they benefit overall health and may benefit athletic performance.