When the air is really polluted, exercising can increase a runner's level of exertion and aggravate certain bronchial conditions, says sports medicine physician Jeff Tanji. He advises runners to try a slower pace if they find their level of exertion increasing. Those who begin wheezing or develop a dry, burning cough may be experiencing exercise-induced brochospasm, or exercise-induced asthma. If this happens, people should see their physicians, Tanji says. Hot summer days carry another risk for runners -- heat sickness. "Follow the rule of 150," Tanji says. "When temperature and humidity combined exceed 150, you're at greater risk for heat sickness. Exercise when it's cooler."