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Every four years, exercise-induced asthma draws special attention among scientists and the media, in lock step with the Winter Olympics. That’s because when the world’s premier cold-weather sports athletes convene in Vancouver for the 2010 Winter Olympics next month, they will share, as a group, not only exceptional fitness but also a disproportionate tendency to wheeze.

Exercise-induced asthma has been diagnosed in as many as half of all elite cross-country skiers and almost as many world-class ice skaters and hockey players. It’s far more common in winter athletes than in those who compete in the summer, although nearly 17 percent of Olympic-level distance runners have been given the same diagnosis.

Even among the unexalted ranks of recreational athletes, it’s a condition that strikes far more often in the winter and also among the most committed. “In my experience, recreational athletes” who have symptoms of exercise-induced asthma but no asthma symptoms otherwise, “tend to be the ones who train” for the most hours, says Dr. Christopher Randolph, a clinical professor at Yale University who sees patients at the Center for Allergy, Asthma and Immunology.

All of which raises the intriguing issue, especially pertinent with the Olympics so close, of whether training strenuously outside in the cold for hour after hour might somehow be bad for your respiratory system. It’s an area of “active study,” Dr. Randolph adds.

Exercise-induced asthma is not quite the same condition as asthma. And in fact, these days, according to Dr. Randolph, the “preferred term” in the scientific community for exercise-induced asthma is exercise-induced bronchoconstriction, or E.I.B. “It’s a reversible, obstructive airway disease” that typically begins about five minutes after you stop exercising — particularly if your exercise was intense, “at between 85 and 95 percent of maximum heart rate,” Dr. Randolph says.

About 90 percent of people with asthma also suffer from E.I.B. and, in children, wheezing after exercise is often the first clue that they have asthma. But people can also suffer from E.I.B. without underlying asthma.

Not so long ago, most researchers held that exposure to cold air was what triggered the bronchial narrowing, with blood vessels in the throat constricting to conserve warmth, just as they do in ungloved fingers; when the vessels later reinflated, they were thought to ignite spasms in the bronchial tubes, like tingling in warming fingers. “And that probably does happen,” Dr. Randolph says.

But most researchers no longer consider it to be the primary cause. Instead, says Dr. Randolph, most experts think that the problem is not with the coldness of the air but with the “dryness.” Lungs need water-saturated air. If the air entering your bronchial tubes is dry, as it usually is in winter, the cells lining your airway release their own moisture to humidify it.

“Think of a sponge being squeezed,” Dr. Randolph says. The squeezing and loss of moisture prompt certain cells within the bronchial tube to release allergic chemicals that initiate an inflammatory process, slowly closing your throat. Thankfully, the process is “self-limiting,” Dr. Randolph says. “No one dies” of exercise-induced bronchoconstriction. (Although some athletes have died from uncontrolled asthma attacks, a different issue.)

A recent study of sweat rates in people with exercise-induced respiratory problems has bolstered this dehydrated-bronchial-tubes theory. The work was inspired by genetic studies in mice whose genes for aquaporin had been turned off. Aquaporin is a protein in cell membranes that moves water in and out of the cells. Mice engineered without this cellular plumbing system developed a multitude of moisture-related problems. They didn’t sweat. They didn’t salivate much. And they had the mouse equivalent of asthma after running on a wheel.

It was this last effect that interested Dr. Warren Lockette, a medical officer at the Naval Medical Center in San Diego. He recruited 22 Marines with laboratory-confirmed exercise-induced asthma and 34 control subjects, then tested how much the various Marines sweated, salivated and grew teary (which even Marines will do, with the right provocation).

What he found was that those who perspired, spit and cried the least were also the most prone to exercise-induced asthma. “There was obviously a common thread here,” Dr. Lockette told me, “and it involved moisture.” His expectation is that further studies will show that aquaporin genes play some role in people’s vulnerability to E.I.B.

But pinpointing genetic susceptibilities to E.I.B. doesn’t answer the question of why it strikes so disproportionately among the world’s best athletes, especially those in winter sports. “That’s one of those really intriguing questions,” Dr. Randolph says, and it may, if a new theory gaining currency is borne out, change how some Olympians and even more-everyday athletes train.

This theory posits that E.I.B. is, in some fashion, a sports injury. “What we think is happening,” Dr. Randolph says, is that elite endurance athletes, especially those training more than 20 hours a week, actually “injure their airways” by breathing so much and so hard. “They take in up to 200 liters of air per minute,” he says, in comparison to perhaps five or six liters per minute at rest, all of which must be humidified.

The resulting inflammation within their bronchial tubes becomes chronic over time, and each subsequent workout more easily triggers a new E.I.B. episode. Cross-country skiers, runners, cyclists and other athletes who train in the wintertime may not have been born with breathing problems, but their repeated episodes of hyperventilating in cold air induce the malady.

What can be done? At the moment, most asthmatic Olympians rely on a short list of allowable drugs to get them through their training and competitions. But in the long term, “we think they may need to back off from some training,” Dr. Randolph says. He instructs serious athletes who suffer from E.I.B. “to drop below 20 hours a week of training in their primary sport and cross-train at a lower intensity the rest of the time,” he says. “They don’t want to hear that, though.”

The few studies of retired elite athletes with E.I.B. suggest that their breathing problems lessen or disappear when their training slackens; but so, of course, does any chance of competing. “We don’t know yet how to balance the need to train to win” with the need to reduce E.I.B., Dr. Randolph says.

In the meantime, and especially for non-Olympians, if you wheeze after exercising outside in the winter, visit a doctor. Gasping and tightness in your chest after exercise can be caused by other conditions besides E.I.B., Dr. Lockette says, including cardiac disease. A lung-function test is part of a formal diagnosis of E.I.B. If you do have the condition, drug treatments are available, Dr. Randolph says. But start with easier fixes.

“Breathe through your nose,” he says. The mucus there is far moister than that in your mouth. Wear a face mask, which recirculates the moisture you exhale into the air you draw in. Drinking more water, while generally wise, hasn’t been shown to combat E.I.B. But warming up gradually before exercising in earnest does seem to help, Dr. Randolph says.

A warm-up of least 5 to 15 minutes, at an intensity of 60 percent or less of your maximum heart rate, seems to fool the inflammation-stimulating cells in your bronchial tube, leading to a “refractory period” of an hour or so, during which you can usually complete a workout without setting off bronchospasms. And if none of that works, “indoor gyms can be a good option,” Dr. Randolph says, “as long as they don’t dehumidify the air.”