It's healthful to exercise and harmful to breathe polluted air, so how can you decide whether you are doing more harm than good? The worst time for pollution is when clouds cover the sky and automobiles fill the roads. Automobile exhaust fumes are the principal source of air pollution in most cities, and overlying clouds increase pollution. Usually the sun's rays heat the ground to warm air closest to the ground. Hot air rises, taking large amounts of pollutants skyward. On air inversion days, the clouds prevent the sun's rays from getting through to the ground, so the air near the ground is not heated, remains colder and doesn't rise, causing the air with its pollutants to remain close to the ground.
Air pollutants such as carbon monoxide, ozone, carbon dioxide, and sulfur dioxide can damage your lungs. When you exercise, you breathe more deeply and more frequently so that you breathe in more pollutants. However, you don't retain more pollution. Bicycle riders in rush hour downtown Washington traffic breathe in more carbon monoxide than car riders do, but have lower blood levels of carbon monoxide. So keep exercising, but if possible, try to avoid heavily trafficked streets, and exercise before the heavy morning traffic peak or at least two hours after the evening rush hour ends.