Why You Should Check the Air Quality Index Before Exercising Outdoors

It’s easy to forget that not too long ago, walking through many U.S. cities was a hazy, smoggy affair. Once the Clean Air Act of 1970 tightened industrial regulations, dramatic air quality improvements followed, with pollutant levels dropping by nearly 80 percent in just a few decades. However, the recent rise in wildfires as a result of  extreme fire weather (extremely dry and windy conditions) and increased pollutants in the air from wildfire smoke is reversing some of this hard-earned progress.

As we gear up for another wildfire season, the following tips for monitoring air pollution should be on every outdoor athlete’s radar. 

Here’s an overview of how to read your local air quality index score, and how to use it to decide when it’s safe to train outside.

What Is the Air Quality Index and How Is It Measured?

The Air Quality Index, or AQI, is an air quality alert system maintained by the U.S. Environmental Protection Agency (EPA). It uses data from nationwide monitoring stations to compile local air pollution “scores,” ranging from zero to 301+. The higher the number, the more polluted the air.

The EPA maintains air quality standards for five criteria pollutants that are harmful to human health: particulate matter (PM2.5 and PM10), ground-level ozone (a harmful gas that forms when chemicals from cars and power plants react with sunlight), sulfur dioxide, nitrogen dioxide, and carbon monoxide. Particulate matter is a mix of varying sizes of several pollutants, including aerosols, dried fragments, liquid droplets, and even tiny pieces of the Earth’s crust. The numbers 2.5 and 10 refer to the size of the particles; PM2.5 is smaller than a strand of human hair, and PM10 is larger than a strand.

“Whichever pollutant is at the highest level relative to its own toxicity is one that drives your AQI score. So as long as you’re very high in a single pollutant, that’s all it takes to have a high AQI score,” says Sebastian Rowland, a senior scientist at PSE Healthy Energy, a climate research institute, who studies the intersection of climate and health.

Some pollutants are invisible to the naked eye, he adds, so it’s worth checking the AQI even on seemingly clear days. You can check on your local AQI by using apps like Apple Weather and IQAir or heading to AirNow.gov. The AQI spectrum is split into six colors, each signaling a different degree of risk. High AQIs are often found in cities, areas with coal-burning power plants, busy highways, and places with residual wildfire smoke.

What Do the Different AQI Colors Mean?

According to the EPA, an AQI below 100 (green or yellow) is safe for most people. A score from 100 to 200 (orange or red) can be unhealthy, especially for sensitive groups like children, older adults, and those with underlying heart or lung diseases. Anything over 200 (purple or maroon) is considered hazardous for everybody.

Here’s a breakdown of what each air quality range means:

How Does AQI Impact Health and Performance?

Poor air quality can affect outdoor exercise performance in several ways.

It Gets Harder to Breathe

When you go on a run or bike ride on a higher AQI day, you may notice that your chest tightens, your breath becomes more labored, or your workout feels more difficult, Dr. Michael J. Stephen, a pulmonary physician at the University of Pennsylvania, says. You may also start wheezing or coughing in response to the irritants.

Stephen says that the more intense the exercise, the higher your breathing rate and the more pollutants you’ll be exposed to, which can potentially worsen symptoms. He cites research showing that light, zone 2 cardio can increase the total number of particles deposited in the airways by roughly four times, while high-intensity workouts can lead to an up to 10-fold increase.

Bad Air Quality Can Make You Slower

As these particles build up in the body, they can also slow you down, as shown by a 2023 Scientific Reports study on 334 collegiate male track athletes preparing for a five-kilometer race. Rowland, a co-author on the study, explains: “When athletes were exposed to higher levels of fine particulate matter or higher levels of ozone during the 21-day training period prior to the race, they tended to run slower at the race itself.” Specifically, an increase in PM2.5 exposure was associated with a 12.8-second increase in race times.

Nicholas DeFelice, another co-author on that 2023 study and an associate professor at Mount Sinai’s Icahn School of Medicine, adds: “A small amount of particular matter creates inflammation within the body, and that is what, in theory, can lead to a slight decline in your running performance.”

Long-Term Health Risks of Air Pollution

Even relatively “good” levels of pollution on the AQI scale can impact health and performance in subtle, often undetectable ways.

“A ‘good’ [AQI] score is kind of a misnomer,” Rowland says. “Time and again, studies have shown that lower levels of air pollution, even below the current EPA-recommended thresholds, are still associated with higher risk of mortality.”

This is due to the way pollutants like particulate matter can, over time, increase oxidative stress (cell damage), blood pressure, and inflammation in the body, potentially contributing to cardiovascular issues and neurodegenerative diseases.

That said, there’s a paradox here. Even though outdoor exercise increases our exposure to air pollution, the act of exercising itself strengthens cardiovascular machinery, potentially helping make us more resilient to stressors like pollutants.

If you’re an outdoor exerciser, the name of the game is finding a workout routine that balances the risks of air pollution with the inherent benefits of movement.

How to Make Sure Your Outdoor Workout Is Safe

Given all of these considerations, it’s smart to check your AQI like you would the weather, and factor it into your training.

If the AQI score falls in the green or yellow zones, you should be all clear to head out as planned. “For an average person, getting outside and enjoying the physical and psychological aspects of exercise is very, very beneficial. If you have good lung health and the AQI is less than 100, I think that the benefits far outweigh any risks,” Stephen says, caveating that those who have underlying sensitivities will need to be more careful.

And since even low-AQI days still carry some degree of risk, here are a few tips to minimize pollutant exposure during workouts.

Stay Away From Busy Roads

“You want to choose a workout path where you’re going to minimize the amount of exposure. That can, I believe, make a big difference in terms of benefits versus risks,” says Stephen.

Workout When the AQI Is at Its Lowest

Air quality is dynamic and changes throughout the day. Research shows that pollution tends to be lowest in the late afternoon (from 3 P.M. to 5 P.M.), as carbon dioxide emissions often peak during morning rush hour and in the early to late evening, when there are more cars on the road.

Take Your Workout Indoors or Lower the Intensity

If the AQI in your area is between 0 and 150, you might want to consider pushing your outdoor workout to a time in the day where the AQI is lower or dialing back the intensity of your workout. But sensitive groups (people under 18, over 65, or with existing health conditions) should exercise indoors if the air quality index is 101 or higher, according to the American Lung Association. If the AQI surpasses 151, no one should exercise outside.

Want more Outside health stories? Sign up for the Bodywork newsletter.