Athletes are more vulnerable to the health effects of poor air quality. During physical activity, air is inhaled more deeply carrying pollutants farther into the lungs. People who exercise also inhale more volume per minute than the average sedentary person.

For example - walking slowly, a healthy man would take in about three times more air than while not moving. The same man would take in six times more air while running.

People who exercise outdoors tend to do so near roads and often during, or just after, peak traffic hours when the level of pollution is the highest. They also tend to go outdoors on hot and sunny days when conditions are prime for air pollution, especially from ozone.