The Dirty Secret of Outdoor Exercise

Dangers of Exercising Outside

I love to be outside. So it makes sense to exercise outdoors -- and I do, several times a week, despite living in the gritty heart of Baltimore, which has the dubious honor of being part of the 14th most polluted metropolitan area in the country. I try my best to avoid the often hot, humid metropolis, crammed with bumper-to-bumper traffic. Instead I go to a nearby park and power walk my dog on wooded paths or jog around the reservoir. There I can pretend that I'm circling a lagoon on a Maine island, that the sound of the adjacent highway is really the rush of a mountain stream complete with a flock of honking Canada geese, and that the whoosh of passing cars is a gentle ocean breeze. I exercise my imagination and my body at the same time -- a holistic fitness routine that works.

Sort of. Sometimes.

It works until I am choked out of my fantasy by a layer of hazy gray smog stretching to the horizon before me and I find myself hunched over, gasping for one good deep breath of clean, cool air. That's when I have to admit: This pollution is killing my workout.

You don't have to be sweating in the heart of a big city to feel the effects of pollution. A host of research shows that often when you exercise outdoors, your lungs are drawing in gulps of car exhaust, soot, construction dust, diesel, factory fumes, and possibly microscopic bits of ozone and mercury, all of which are slowing you down and harming your health. What's more, college athletes who biked in a polluted environment (the lab-created equivalent of a busy highway) continued to feel the ramifications when riding again three days later, according to a 2008 study published in the Journal of Strength and Conditioning Research.

The distance they could pedal decreased by 5 percent, perhaps because inflammation in their lungs made it harder to breathe for days afterward, explains study author Kenneth Rundell, PhD, director of the Human Physiology Laboratory at Marywood University in Scranton, Pennsylvania.

Beyond the daily irritation, long-term contact with high levels of pollution can increase your chances of incurring heart disease and heart attack, high blood pressure, and asthma. It can even raise your risk for lung cancer as much as regular exposure to secondhand smoke, a study in the Journal of the American Medical Association found. And one study of runners showed that pollution can actually negate the positive effect exercise has on lungs. "Usually when you exercise, your lung function improves," says researcher George Thurston, ScD, a professor of environmental health science at NYU School of Medicine in New York City. "But as ozone levels rose, we saw that benefit shrink to the point where there was no improvement in lung capacity. In short, the benefits of exercising are absolutely diminished by ozone on high-pollution days."