Scientists make surprising discovery about unexpected factor slowing runners down around the US: 'Is this something that we have to pay more attention to?'

For marathoners, every second counts, whether they're hunting for a personal best, a Boston Marathon qualifying time, or a time barrier.

A new study finds that there is a new, hidden variable holding runners back from their goals: air pollution.

What's happening?

Scientific American reported on a study published in the Sports Medicine journal, which found that increases in air pollution slowed the finishing times of marathoners across nine U.S. races. Notably, this was true for air-quality measures below those deemed by the U.S. Environmental Protection Agency to be unsafe.

The focus was on fine particulate matter, which is known as PM2.5. These are extremely small particles that can emerge from anything from tailpipe pollution to forest fires. PM2.5 exposure is linked to health issues. 

The study aimed to examine the impact of finishing times in an aerobic, endurance event that exposes runners to PM2.5 for hours on end.

Researchers used a machine-learning model to gather results that better reflected real conditions than remote weather stations. It estimated air quality throughout a course, as precisely as to the mile.

The researchers found that after adjusting for other factors, a modest rise of one microgram per cubic meter in PM2.5 levels slowed male runners by 32 seconds and female runners by 25 seconds.

The slowdown was most pronounced in the group faster than the median runner but slower than the elites. 

Why is the effect of air pollution on marathon race times important?

Runners racing a bit slower than they might want doesn't seem like the biggest deal. However, this study suggests the impact of air pollution on exercise is worse than we think.

The researchers told Scientific American that the next logical step is to look at what is happening inside our bodies that inhibits performance.

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The study also introduced the idea that even at what would be considered "safe" levels of air pollution, there's a decrease in performance. That could be a major negative if that also means it is detrimental to exercisers' health in common conditions.

Matthew Ely, an expert in the field, explained the potential pitfalls to Scientific American, noting that "we all know that exercise is important to us."

"We also know that environmental pollutants are present in big cities," he continued. "Is this something that we have to pay more attention to?"

What's being done about air pollution?

The researchers suggested that the best way to reduce PM2.5 pollution was by regulating a number of industries.

This includes steps like cleaning up transportation with EV adoption and planet-friendly public transportation, keeping industrial polluters in check, and transitioning away from dirty energy to clean energy sources like wind and solar.

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