With the Olympics set to take place this summer in London—one of the sootiest cities in Europe—the dangers of air pollution have sprinted to the public’s attention. Fine particles in air can aggravate respiratory and cardiovascular conditions and increase the risk of death. Over 4,000 Londoners die prematurely each year due to long-term exposure to air pollution. Exercising when the air quality is low only increases the amount of polluted air inhaled, especially in runners and cyclists who breathe in much larger amounts of air. Breathing through the mouth—as they often tend to do—also bypasses the natural air filtering mechanism of the nose. Based upon this, it people started assuming that athletes would be affected more when exercising in polluted areas. However, a recent study involving mice shows that it may not be that simple. Air pollution increases inflammation and stress in the lungs, which puts you at risk of dying. Aerobic exercise, however, has been shown to reduce inflammation and stress, which made researchers wonder whether the benefits of exercise outweigh the risk of exercising in polluted air. After studying mice for five weeks, the researchers found that mice who exercised in polluted air showed much less pollution damage than their furry friends who just sat around in the polluted air. Aerobic exercise protected the mice from inflammation caused by air pollution. Most of us are not mice, so the results may not hold as true for people. If you have underlying heart or respiratory conditions, you should definitely check with your doctor about exercising outside on polluted days. For the rest of us, though, running on a polluted day may actually be better than laying low.