It's not clear how laughter brings on inflammation that narrows airways, but it may involve hyperventilating. Exercise was the only trigger more common than laughter in patients who reported laughter-induced asthma.

"It's as common as some of the most well-known asthma triggers, such as grasses, trees, pollen, fumes and odors -- and it's even more common than dust mites, allergy to animals and molds. It's a little-appreciated frequent trigger," said Dr. Stuart Garay, a professor of medicine at New York University Medical Center in New York.

Garay reported the results of his study before the American Thoracic Society's International Conference in San Diego.

Asthma is a chronic lung disease that affects more than 17 million Americans. Asthma occurs when a trigger causes inflammation of the airways, which in turn makes the passages of the lungs smaller and makes it more difficult to move air in and out.

The study of 235 patients with asthma found that 56 percent had experienced laughter-induced asthma. Most commonly, they started coughing or experiencing chest tightness within two minutes of laughing hard.

Asthma experts have long known that any of a number of strong emotions and stress -- anger, crying, yelling or laughing -- can act as triggers, but the extent it served as a trigger hadn't been quantified.

Asthmatics don't have to guffaw hard or for long to bring on an attack, either, Garay said. "It depends on the patient. For a majority of patients, mild laughter or even a chuckle will set off coughing. For others, laughing hard will bring on asthma symptoms."

It's not clear how laughter brings on inflammation that narrows airways, but Garay said it probably involves hyperventilating. He noted that exercise was the only trigger more common than laughter in patients who reported laughter-induced asthma.

Sixty-one percent of those patients reported exercise also caused asthma. Of those who said their asthma was never laughter-induced, only 35 percent said exercise was a trigger.

The researcher said asthma triggered by laughing doesn't seem to cause more asthma flare-ups that require emergency room visits or hospitalization.

"But patients did report that during times when their asthma is well-controlled, they can laugh for longer without getting asthma symptoms. That suggests that laughter-induced asthma may be a sign that a person's asthma isn't as well controlled as it could be," Garay said. "People with asthma should be allowed to laugh."