Is it safe to exercise outdoors when the air quality is bad?

As smoke from wildfires in Canada migrates south and blankets the northern United States with air the color and aroma of char, many people are probably wondering: Should I exercise outside in these conditions?

The answer, experts say, depends on your health, fitness and age. Here are answers to common questions about working out in smoke-filled air.

What air quality range is safe for exercise?

Check the current air quality index where you live. Airnow.gov provides hourly air quality information, by Zip code. The results are available as a number (such as 75) or color-coded scale. Air quality in the Orange range is considered "unhealthy for sensitive groups," which includes people with asthma or other respiratory conditions, young children and the elderly.

If you're not among those groups, most experts say it's reasonable to exercise outside while the air quality index is Orange. But don't be surprised if your clothes and hair smell of smoke after spending time outdoors.

"The health benefits of physical activity generally outweigh the health risks of air pollution" at this level, "in healthy individuals," according to a position statement this year from the Canadian Academy of Sport and Exercise Medicine and other groups.

If the smoke intensifies, though, and the air quality index rises into the Red (unhealthy), Purple (very unhealthy) or Maroon (hazardous) zones, you may want to drastically shorten or skip outdoor exercise.

"I'd probably still do my exercise outside," when air quality is in the Orange zone, said Michael Koehle, director of the Environmental Physiology Laboratory at the University of British Columbia, senior author of the new Canadian position statement and an avid outdoors person.

"If it is in the Red zone, then I'd avoid long workouts" and probably move any exercise indoors, he said.

Will a mask help?

Wildfire smoke and haze are composed of many different gases and particles, including extremely small microparticles. Normally, "a lot of pollutants can get filtered out by the nose," said James Hull, a sports pulmonologist with the Institute of Sport, Exercise and Health in London. But "if you are vigorously exercising, then you have to breathe through an open mouth to allow sufficient air in," and wind up taking in large amounts of the pollutants.

Masks, such as the cloth or surgical masks many of us may have stockpiled during the pandemic, are too loose to filter out any but the largest particles.

Tighter-fitting N95 masks filter more of the particles, but "it is very hard to do vigorous exercise in a mask," such as an N95, "that is sufficiently protective," Hull said.

In addition, "some exercisers may be falsely reassured by a mask that inadequately protects them," he said, and continue working out longer than is advisable.

How long can I exercise when the air quality is bad?

Assuming the air quality is no worse than Orange level, an outdoor workout of about 30 minutes to an hour is probably "reasonable," Koehle said.

"If you develop chest tightness or discomfort, then discontinue your exercise," Hull said.

Does intensity of exercise matter?

Surprisingly, "there is some evidence that shorter high-intensity exercise bouts are less harmful than longer low-intensity ones," Hull said.

For a 2014 study, Koehle and his colleagues tested the lung function of cyclists while they alternately rode slowly or intensely in polluted air. Unexpectedly, the riders experienced more difficulties breathing while riding gently than when they pedaled all-out.

The reasons are not altogether clear, but the practical implications are, Koehle said. If smoke hangs in the air, keep your workouts short and sharp, rather than long and slow. The fitness benefits will be about the same, but your "air-pollution exposure" will be much less, he said.

Does morning or evening matter during air quality alerts?

Avoid exercising before dawn or late at night.

"Particulate concentrations are generally worse at night," Koehle said, when smoke settles closer to the ground. These concentrations begin to lessen after the sun rises and the smoke begins to lift somewhat, "but pre-dawn levels are still high."

So, it may be healthier to sleep in a little longer and schedule your usual pre-dawn ride or run for after the sun is up. Check local conditions before you head out, however, since they change frequently and depend on wind and other variables, as well as time of day.

Is indoor exercise better when outdoor air is bad?

Indoor exercise requires its own precautions. Particulate levels inside, such as in a gym or your living room, tend to be about 50 percent lower than outside, according to experts, but only if the windows and doors are tightly closed. Make sure the air conditioner is on and in recirculation mode. An air purifier with a HEPA filter is also advisable.

"When ambient air pollution concentrations are high, it may be favorable to relocate an exercise bout indoors," the Canadian position statement says.

Should soccer practices and outdoor youth activities be canceled?

The Environmental Protection Agency has produced a handout to help school administrators, coaches and other adults decide when children should be outside during air quality alerts.

"It's OK to be active outside" during Orange-level alerts, the guide says, "especially for short activities such as recess and physical education."

But children with asthma should keep their medications handy, in case the smoke triggers an episode.

The calculations become trickier if air quality is in the Red (unhealthy) zone.

"For all outdoor activities, take more breaks," the guide suggests, and consider moving games or practices indoors or rescheduling them. If the air quality moves into the Purple (very unhealthy) zone or worse, head for the gym, with the windows and doors tightly closed, or just skip practice that day.