Exercise-induced asthma (EIA) is a condition that affects people during vigorous activity, particularly in cold and dry conditions. People with EIA are unusually sensitive to changes in air temperature and humidity, and can find their breathing severely impaired within five to 20 minutes of beginning activity.

Most asthmatics experience asthma symptoms during exercise. However, for some individuals, exercise is the sole trigger that causes asthma symptoms. Children are more likely to experience symptoms related to EIA than adults.

Exactly why EIA occurs is unknown. Some experts believe that rapid breathing during exercise prevents air from being properly warmed as it enters the airway. Normally, most of the air a person breathes in enters through the nose, where it is warmed and moistened to 80 to 90 percent humidity before proceeding to the lungs.

However, during exercise, air tends to enter through the mouth, where air is warmed to just 60 to 70 percent humidity. As the cooler air enters the bronchial tubes, it causes the airways to swell (bronchospasm) and secrete excess mucus. This, combined with a contraction of the airway muscles, narrows the passageway. In addition, EIA may be caused by water loss around the cells of the bronchial lining during rapid breathing. As the tissue dries out, mast cells release the chemicals that trigger inflammation and asthma symptoms.