Norwegian cross-country skier Marit Bjorgen, who won the bronze medal in the women's 10-kilometer race this week, has something in common with a surprising number of people – she has exercise-induced asthma. As far as Olympians go, CNN.com reports, half of cross-country skiers and a quarter of aspiring Olympic athletes in general have exercise-induced asthma, according to a 2000 study of 170 athletes conducted by the United States Olympic Committee.

But exercise-induced asthma isn't triggered only by such extreme athleticism. Any exercise in cold weather can trigger an asthma attack. As Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City puts it, "Any [exercise] outdoors on a mountaintop is a culprit, but just jogging in Central Park will do it as well.

Why does the combination of cold air and exercise trigger attacks? "Both cold air and exercise in and of themselves can trigger asthma symptoms, but doing them together is more likely to trigger an exacerbation," explains Dr. Thomas M. Leath, an assistant professor of pediatrics at Texas A&M Health Science Center College of Medicine.

Dr. Horovitz adds, "The evaporative loss and cooling of the airways triggers the process of bronchial constriction. One sport you don't see exercise-induced asthma in so much is swimming. The humidity at water level is such that there isn't as much evaporation as in running or winter sports."

Exercise-induced asthma is characterized by bronchospams, which cause the victim to wheeze and have chest tightness, among other symptoms. The symptoms usually begin several minutes after exercise begins, but may start as late as after the exercise routine is completed. Though it's clear that brochospasms are responsible for the onset of exercise-induced asthma, it isn't certain why they occur. Some believe that increased blood flow caused by exertion forces the blood vessels in the airways to expand, leading to airway constriction. Others think that cold air activates inflammatory cells in the airways.

Fortunately, as many famous athletes have demonstrated, exercise-induced asthma doesn't have to slow you down. Bronchodilators, administered through inhalers, are effective at fighting symptoms by relaxing the muscles that line the airways to the lungs. Aside from inhalers, warming up for a few minutes before getting into full exercise mode, breathing through your nose, or wearing a scarf or mask around your mouth can go a long way in preventing symptoms.

For more information about asthma and cold weather, see our article Coping With Asthma During Winter.