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“Exercise is good,” explained Michael Koehle, head of the University of British Columbia’s Environmental Physiology Lab. “On the other hand, air pollution is bad.”

Seems like a pretty basic message. That’s precisely the point Koehle was trying to make as he kicked off his talk at a recent International Triathlon Union–sponsored in Edmonton. He’s used to giving talks about controversial topics like platelet-rich plasma therapy, where, after carefully outlining the gaps and contradictions in the evidence, all the questions afterward are along the lines of “How much does it cost, and where can I get it?” So, as Koehle prepared to dig into the intricacies of the current literature on exercising in polluted air, he wanted to make sure we all had a firm grasp of the fundamentals.

In a perfect world, of course, we would all be riding on remote desert highways or car-free mountain passes and running along Edenic forest trails. But that’s not reality for most of us. I live in a city of 4 million people, and the health toll incurred by all the cars, trucks, and smokestacks is well established. It’s not just respiratory conditions like asthma; hospital admissions for heart disease, as well as subsequent deaths, in sync with ambient levels of air pollution. This year’s wildfires have sullied the air up and down the West Coast, Koehle pointed out, so big cities aren’t the only danger zone.

The question for any outdoor exerciser: When do the benefits of physical activity get washed out, or even reversed, by the negative effects of air pollution? There’s no simple, universal answer. But research by Koehle and others offers some useful insights.

Pre-Exposure Matters

In 2012, Koehle and his student Luisa Giles the effects of breathing polluted air for an hour before a 20K cycling time trial. Pre-exercise exposure resulted in a higher heart rate and less-dilated airways, though it didn’t produce a significant decrease in performance. In some respects, Koehle says, the effects were bigger than they saw in subsequent studies with cyclists breathing pollution during the time trial. The takeaway? Be mindful of the air you’re breathing before a workout or competition. Commuting across town so you can work out indoors may not be a winning strategy.

For a similar reason, Koehle recommends that athletes with asthma take their medication consistently, “because pollution is 24 hours a day, not just during exercise.”

It’s OK to Hammer

When you exercise, you breathe more deeply, bypassing the nasal filters (that is, your nose hairs) and sending higher volumes of polluted air deeper into your lungs. Sounds bad—and sounds like a reason that if you must exercise in bad air, you might want to at least take it easy.

That’s the assumption Koehle and Giles started with when they tested cycling in polluted air at two different intensities: 30 percent or 60 percent of peak power from a VO2max test. The higher intensity was tough enough that most of the subjects could barely finish the 30-minute ride. But showed virtually no difference in the effects of pollution between the two rides. In fact, it was only in the slower ride that respiratory parameters like the amount of air breathed in and out were elevated by the air pollution. As a result, if you’re going to exercise in bad air, you may be better off sticking to shorter, more-intense workouts to minimize your overall exposure.

Location, Location

Avoidance is the best policy, so you should move your workout as far away—in time or space—as possible from the worst air. Pollution is generally lowest in the early morning and late evening; Koehle uses a French app called to track and predict local air quality. Even small distances and barriers like trees can make a big difference in air quality. That’s one of the main reasons the city of Vancouver, where Koehle is based, puts a row of parked cars between bike lanes and car traffic. The decline in pollution levels as you move away from a road is exponential.

For what it’s worth, heading indoors isn’t necessarily a panacea. New carpets, ski wax, , and even candles and in a yoga studio can all compromise air quality. You can minimize your exposure to these sources with a little planning, but it’s another reason not to shy away from heading outside.

Warm Up Your Lungs

One of Koehle’s was investigating the performance-boosting potential of asthma medications for the World Anti-Doping Agency. Do illicit puffers give non-asthmatic athletes an edge? Actually, Koehle says, “We couldn’t even prove that asthma medication improved performance in asthmatics!”

The problem, they eventually concluded, is that all the athletes were getting a good warm-up as part of the study protocol. It turns out warming up well helps to ward off the decline in lung function that people with asthma or exercise-induced bronchoconstriction (a common airway problem in athletes that is distinct from asthma) often experience in polluted air. An tested in research studies: 30-second sprints five to seven times at 80 to 90 percent of maximum intensity, with 1.5 to 2.5 minutes of rest.

Finding the Balance

When you combine exercise and pollution, the results obviously depend on the details of the dose. Overall, though, the picture is reasonably reassuring. A in 2015 followed 52,000 people for an average of 13 years, combining data on their exercise and lifestyle habits with a sophisticated street-by-street model of air pollution in Denmark, then analyzing the health outcomes. The results found that “exposure to high levels of traffic-related air pollution did not modify…[the] beneficial effects of physical activity on mortality.”

In fact, there’s some evidence that exercise may actually reverse some of pollution’s negative effects. After all, pollution is an inflammatory trigger, and exercise is an anti-inflammatory. In November, Brazilian researchers published a in which mice inhaled several types of air pollutants while running on a treadmill or not exercising. The pollution caused inflammation in the airways and throughout the body, but exercise inhibited both these effects.

Is that sufficient to conclude that you can ignore air pollution? Definitely not. Pay attention to air quality, and make whatever adjustments you can to avoid the worst pollution. But don’t let it trump your training plans. “Exercise is good, pollution is bad,” Koehle concluded in his conference talk, “but in the battle, exercise just seems to have more power than air pollution.”

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