Bad air biggest hurdle for marathon runners

Every marathon runner has had a bad day. Your legs start to feel heavy. You feel out of breath. Your goal time slips away.

It's easy to blame nerves, or high humidity, or too much (or not enough) spaghetti the day before. But according to a recent study, there could be a less obvious culprit for a sluggish pace: air pollution.

Researchers at Brown University analyzed a dataset of millions of marathon finish times from major US marathons over 17 years. They found that higher air levels of fine particulate matter called PM2.5 - the term for pollutants smaller than 2.5 micrometers in diameter - were, on average, directly associated with slower finish times.

This kind of particulate matter is often what drives air quality warnings. For example, in New York state, an average PM2.5 concentration of 35 micrograms per cubic meter can trigger an air quality health advisory.

The marathon researchers used data from nine races, including those in New York City, Boston and Los Angeles, between 2003 and 2019. They found that, for every increase of 1 microgram per cubic meter of these tiny particles, men's average finish times were 32 seconds slower and women's were 25 seconds slower. That means on even a moderately polluted day, times could be slower by minutes.

"What's notable is that we're looking at people who are all incredibly healthy," said Joseph Braun, a professor of epidemiology at Brown University's School of Public Health. "But even among really healthy people, air pollution is having an important, albeit subtle, effect on your physiology."

Particulate pollution comes mainly from burning fossil fuels, for example from power plants that burn coal, oil or natural gas to generate electricity. Exhaust from gasoline or diesel vehicles also contributes, as do forest fires, waste incineration or burning wood for cooking or heating.

Even short-term exposure to PM2.5 can be harmful. The tiny particulates can reach deep into the lungs and enter the bloodstream, causing inflammation, coughing or tightening in the chest. For people with underlying health conditions, it can exacerbate asthma or bronchitis and potentially trigger strokes or heart attacks.

While air quality has improved in many parts of the United States, due to progress in regulating pollution, concern has grown over short-term spikes in pollution, particularly from wildfire smoke across the western United States and Canada. The Trump administration has also moved to roll back many regulations, including limits on pollution from tailpipes and smokestacks.

To map pollution, the researchers used machine-learning models to estimate PM2.5 levels along the marathon routes on a given day, based on readings from monitoring stations.

Heat and humidity are other big concerns for runners. An analysis of 221 marathons by Climate Central, a nonprofit research group, found that cool temperatures that help runners perform their best are becoming less common as the planet warms.