Asthma is a condition in which the air passages in the lungs become narrow or blocked. Asthma can be caused by a wide variety of factors such as pollen, pollutants, and exercise. Exercise-induced asthma is brought on by breathing large volumes of air that dry and cool the respiratory system. This causes constriction and increased secretion of fluid in the airway. The problem is usually controlled through medication taken prior to the exercise. It is also recommended that a long warm-up period be completed before strenuous exercise. Because breathing warm, moist air reduces the chances of an attack, swimming is a good exercise for individuals with asthma. Runners and cyclists can reduce the drying of the respiratory passages by wearing a mask that traps some of the moisture. For more information on exercising with asthma, contact a health care professional.