Q: I run by a busy highway with a lot of pollution. Is that bad for my lungs?

Air pollution is a health issue even when you’re not exercising, and a worse one when you are. The reason is pretty straightforward: During aerobic activity, such as running, you typically breathe in more air. The more air you breathe in and the more deeply you do so, the more pollutants reach your lungs. (Car and truck exhaust contains toxic substances like carbon monoxide and volatile organic compounds, or VOCs, which contribute to smog.) Also, normally people breathe through the nose, which acts as a filter to trap inhaled particles that may be harmful to other parts of the respiratory tract. But while running, you’re probably taking in a lot of air through your mouth, so those pollutants don’t get filtered out.

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Air pollution can potentially damage the airways in your lungs and even increase your risk of lung cancer and heart disease. It can also worsen lung conditions you might already have, like asthma. We don’t know for sure how much exercise in a polluted area it takes to pose a serious health problem—and I still recommend hitting the great outdoors for a sweat session, unless your doctor has instructed you not to.

Your best bet is to find a less polluted route (try staying at least 500 feet away from the road) and avoid jogging through construction zones or during rush hour. And if there’s ever an air pollution alert in your area, settle for the treadmill.

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Health’s medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine.