A lot of runners use the extreme heat of summer to train for fast fall running. And, yes, it’s important to push yourself beyond your comfort zone, but there are cases where that may do more harm than good — like running in pollution.

If you live in the western part of the United States, the tail-end of summer is met with ashy, hazy sky from wildfires you likely can’t even see. As of this writing, there are 93 active large fires blazing on. And seeing a film of ash on your doormat might have you wondering, “Should I be running in this?”

But even on presumably clear days or in areas far away from fire — you might be wondering, what am I inhaling? We probed experts to help you take your health into your own hands. Here’s all about what’s up in the air — and what you can do about it.

The Dirt on Dirty Air

What, exactly, is hitting your lungs when you go for a run? “Air pollution is like a recipe that varies by location, weather, season, and time of day,” says Michael Koehle, the head of University of British Columbia’s Environmental Physiology Lab and a leading expert on exercise and air pollution. “The recipe includes a mixture of various gases, like carbon monoxide or smog, and particles like dust and pollen.” 

Diesel exhaust, for example, can trigger asthma. Other pollutants, like ozone (a key component of smog), can be harmful to the cardiovascular system and can even cause cancer. “The lungs are taking the air we breathe and transferring it on to our blood cells, which immediately goes to the heart to be pumped to the rest of the body,” says Jason Way, a naturopathic doctor in the San Francisco area. “Over a prolonged period of time, smog can decrease your ability to run at your full capacity.”

Wildfire smoke can cause some of the same issues as other pollutants because it is made up of particulate matter, carbon dioxide, hydrocarbons, and other chemicals and minerals. Although, according to the California Department of Public Health and the California Air Resources Board’s , the long-term health effects of cumulative exposure from multiple seasons hasn’t been studied enough to make definite health claims. 

“We do know that when the air quality is bad, people with respiratory disease such as asthma are more likely to use their medications and have symptoms that make them seek health care. It can result in increased visits to the emergency department and being hospitalized,” says Dr. Colleen Reid, assistant professor of geography at the University of Colorado, who researches the health effects of exposure to air pollution and wildfires. “There is also some evidence that people with heart disease may also be affected.”

Exercising outdoors in areas with increased air pollution is not advised, according to the World Health Organization, because the increased respiration also increases the amount of harmful particles coming into your lungs. 

Pollution can also undo some of the benefits of exercise. For example, there is emerging evidence that aerobic exercise can have a . However, some studies (like one cited by Oiselle Volee team member Taisa Kushner, Ph.D., in ) show that those benefits are inhibited when people exercise outdoors in polluted areas. 

And what about the masks we’ve been wearing to keep each other safe from the virus? Dr. Reid explains that while the cloth and surgical masks are effective in slowing the spread of the virus because they catch many of the droplets we exhale, they’re not very effective in protecting us from the air we breathe, “and therefore do not help protect the wearer from the tiny particles in the air from the wildfires.”

That’s not to say you must stick to indoor treadmill runs forever-more, especially if has left you without that option. These expert tips can help you determine the safety of your outdoor run.

4 Ways to Minimize Your Exposure to Air Pollution

Quality check.

provides air quality data in partnership with the Environmental Protection Agency, National Oceanic and Atmospheric Administration, National Parks Service, NASA, the Centers for Disease Control and Prevention, and tribal, state, and local agencies. Visit their site to check the outdoor air quality index (AQI) in your area before you head out. If the AQI measures more than 150 (100 if you have asthma or other breathing issues), hit the gym or be extra alert for any symptoms like shortness of breath, coughing, or throat irritation while you run.

Another similar option is the or . Dr. Reid explains that AQI may differ between apps and websites, depending on the monitoring systems and how they choose to calculate the exact AQI number. “But you can use the AQI category to give you a sense of whether the air quality is good for a run,” she says.  If it’s listed as “Good” or “Moderate,” it’s probably safe for a run, while “Unhealthy,” “Very Unhealthy,” or “Hazardous” are probably better to hold off on.

Time it right.

Koehle says it’s key to be aware of when you run outside to be sure you’re breathing in the best air. “Pollution levels are higher during rush hour and in the heat of the day, so it’s better to go out early or in the late evening,” he says. Ground level ozone is the result of a chemical reaction between pollutants and sunlight, which means that it will be at its worst if you try hitting the pavement in the middle of the afternoon when the sun is strongest.

Go green.

Some studies show that you may breathe in 100 times more pollutants running within a mile of the highway than you would in a more rural area. Avoid congested areas if you can, and run in a park or a quieter neighborhood with tree coverage whenever possible. Minimize your time near busy roads since pollution drops significantly once you’re as little as 200 yards away from the congestion. Or dedicate high pollution days to a fresh air trail run. 

Eat right.

Loading up on foods rich in can help your body eliminate toxins picked up from pollution, Way says. Try sticking to leafy greens, broccoli, tomatoes, bell peppers, oranges, berries, nuts, and seeds, and drink plenty of water.

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