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Planning to tackle a marathon in the new year? While getting in your stretches and miles of practice, you may also want to prepare for air quality when training. According to new research, poor air quality could play a role in slower marathon finish times.

Researchers at Brown University School of Public Health have found a link between the amount of fine particulate matter, or pollutants measuring less than 2.5 microns in diameter, in the air on a race day and slower average finish times for marathon runners.

Fine particulate matter can come from combustion, such as vehicle exhaust, fossil fuel plants and wildfires, as well as natural sources, like dust and dirt, as explained by Natural Resources Defense Council (NRDC), which was not involved in the study.

In a study of 1,506,137 men and 1,058,674 women who finished nine major U.S. marathons, including the Boston Marathon, from 2003 through 2019, researchers evaluated the marathon finishing times and compared them with race-day fine particulate matter levels at various points throughout the race routes. Allan Just, an associate professor of epidemiology and environment and society at Brown University, developed statistical models to determine these PM2.5 levels.

"This really sophisticated spatial-temporal model of particulate matter allowed us to plot pollution at every mile of every course," Elvira Fleury, lead author of the study who earned a master of public health degree at Brown University and is now a doctoral student at Harvard University, said in a statement. "Without a model like this, it wouldn't have been possible to look at so many different marathons in different states across different years."

Researchers measured concentrations of PM2.5 in micrograms per cubic meter of air (µg/m3) and found that for every increase by 1 µg/m3 of fine particulate matter, race times slowed 32 seconds on average for men and 25 seconds for women. The team published their findings in the journal Sports Medicine.

While the findings revealed minor decreases in average finish times when particulate matter was higher, the researchers noted that even small slow-downs could affect racers, especially those trying to reach personal records.

"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," Fleury explained. "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."

This is not the first study to examine the negative impacts of air pollution on long-distance runners. A 2010 study found that each 10 µg/m3 increase of PM10, or fine particulate matter with a diameter of less than 10 microns, was associated with a marathon performance decrease of 1.4%.

For runners who train in areas with poor air quality, the longer exposure to fine particulate matter could also increase health risks. As NRDC reported, an increase of PM2.5 in the air by 10 µg/m3 can increase risk of heart disease-related death by 10%.

"If you run in a polluted city you can decrease your performance," Eliud Kipchoge, two-time Olympic champion and long-distance runner from Kenya, told BBC Sport Africa. "When you go to a polluted city, you really feel that your lungs are really compressed."

The United Nations Environment Programme (UNEP) and Stockholm Environment Institute (SEI) have teamed up to install air quality sensors in sports training facilities and stadiums in Cameroon, Ethiopia, Kenya, Senegal, Tanzania and Zambia to better protect athletes from long exposures to PM2.5, BBC Sport Africa reported. The sensors help coaches and athletes schedule trainings and organizers plan major sporting events around poor air quality times.

As a result of the findings from the latest study on how air pollution could affect athletes, the researchers are urging for continued regulations on emissions sources to reduce fine particulate matter in the atmosphere, not only for runners but for overall public health.