It can be hard enough for an asthmatic to walk in cold, dry air without gasping or wheezing - and it can be tougher still to exercise. But doctors say asthmatics can stay active outdoors in winter if they take precautions.

The trick is to control the allergic reaction in the airway that sets off the attack, said Michael A. Kaliner, chief of allergic diseases at the National Institute of Allergies and Infectious Diseases in Bethesda, Md.Although experts don't know exactly how the reaction works, they know that exercise and cold, dry air are separate causes.

"If you exercise outside in warm weather, you can have trouble, and if you don't exercise in cold weather, you can still have trouble," said Dr. Henry J. Fishman, an asthma specialist in Washington, and an assistant clinical professor at Georgetown University Hospital and George Washington University Medical Center.

Combining both problems just makes things tougher, said Fishman: "Jogging in the cold is one of the hardest things an asthmatic can do."

Asthmatic exercisers may, however, benefit from a phenomenon known as the refractory period that can make the next workout easier, said Dr. Oded Bar-Or, an asthma researcher at McMaster University in Ontario.

"When you exercise and rest, the second time, you do not have an attack," he said. "The period in which you are at least partially protected is the refractory period."

The period generally is 45 minutes to an hour, but the intensity varies from individual to individual, and its cause is not known, said Bar-Or.

Because vigorous exercise makes a person breathe hard, exercising in the cold can make an asthmatic suck in more cold dry air than he would ordinarily, said Bar-Or. To cut down on the problem, keep the air you breathe as warm and humid as possible, he said.

For mild exercise, such as walking, you can simply breathe through your nose, he said. This can let cold, dry air mix - before it can set off an allergic reaction - with air that your body has already warmed and humidified, he said.

For more vigorous exercise, Bar-Or recommends putting a scarf over the mouth and nose. The space between the scarf and the face holds warm and humid exhaled air to be mixed with colder air, he said. A surgical mask may do the same thing.

Medication can also make it easier to exercise in the cold, Kaliner said. Some preparations can prevent the mast cells in the airways from releasing the histamines that trigger the asthma, he said. Others keep the airway open even though the histamines are released, he said.

Asthmatics can avoid the problem by working out indoors. Swimming is considered one of the best exercises for asthmatics.