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A study volunteer bikes with a face mask on, for an experiment comparing biking in filtered air versus polluted air.

Ever wondered what all those fumes are doing to your lungs while bike or walk in heavy traffic? I certainly have, especially when biking up a hill behind a bus. (Argh, not again!) So I had to check out this Globe and Mail story about a recent study of just my situation—biking in traffic exhaust.

A team of exercise physiologists at the University of British Columbia and the University of Fraser Valley, both in Canada, asked healthy volunteers to come to their lab, sit on stationary bicycles, and pedal away while wearing face masks that pumped at them either filtered air or air containing diesel exhaust. The researchers measured the volunteers’ oxygen use, heart rate and other physiological responses.

What they found seemed strange. People’s bodies reacted to diesel exhaust when biking at a low intensity, but when they were pedaling hard, their physiological reactions were indistinguishable whether they were breathing polluted or filtered air. Overall, the physiological reactions were slight.

I asked the study’s lead scientist, Michael Koehle of the University of British Columbia, about their results. His team was surprised, too. “We were expecting to find more effects from the pollution and bigger effects in high intensity pollution than in low intensity pollution,” he says. After all, when you exercise harder, you breathe harder, which means you’re taking in more pollution. You’re also more likely to breathe through your mouth, bypassing the natural filtering mechanisms inside your nose.

“Our results indicate that riding it easier in high pollution situations will not be protective,” Koehle says.

The study wasn’t set up to answer why the researchers saw an effect with low-intensity, but not high-intensity, biking, but they have some ideas. One explanation might be that the body makes so many changes during intense exercise that those overwhelm whatever small effects pollution might trigger.

This doesn’t mean air pollution is safe. Many studies have established that those exposed to air pollution over years have higher rates of heart and lung diseases than similar people who don’t have the same exposure. This study just didn’t happen to see effects in healthy people biking for half an hour while huffing in pollution.

For healthy cyclists with no pollution-aggravated conditions, such as asthma, Koehle has helpful, moderate advice. When possible, ride when there’s less traffic. Early morning is good. Also, choose routes away from traffic. Even a little bit away helps. “Every meter away from traffic makes a difference,” he says. “For example they have separated the bike lines from car lanes by a lane of parked cars here in Vancouver, and this will lead to a significant reduction in pollution dose.”

In spite of his lab’s findings, he doesn’t think cycling more intensely would really help. “At this point I would not recommend only riding hard,” he says, “but definitely our results indicate that riding it easier in high pollution situations will not be protective.”

What if you must ride in traffic, say on a daily commute? It’s difficult to determine whether the benefits of exercise outweigh the risks of doing so in polluted areas, but scientists are starting to tackle the question, as Outside magazine reported in 2012. Overall, preliminary studies suggest exercise’s pros greatly outweigh pollution’s cons.