It’s not just an urban legend. Exercising in dirty city air can negate some of the positive effects of your workout.
According to a study published in the British medical journal The Lancet, people with compromised pulmonary systems had the positive effects of exercise on their hearts and lungs entirely canceled out when they went for a two-hour walk on a busy street, versus rambling in a park.
Healthy folks won’t have as severe a reaction (their lungs are better equipped to filter out toxins), but researchers say even they should minimize exercising on crowded roadways, which can curtail cardiorespiratory benefits. To reap the benefits of exercise minus the pollution, work out in the early morning or after dusk, avoid running along busy highways or at rush hour, and pay attention to air quality alert days in your area—if it’s bad, hit the treadmill.