When asthma first develops after age 20, women are much more likely to be affected than men are. In fact, 75 percent of adults hospitalized for asthma treatment are women; women also remain hospitalized longer for asthma than do men of the same age group.

Although less common than asthma in children, adult-onset asthma can also be triggered by allergies. Between 30 percent and 50 percent of all adult cases are associated with allergies, but often allergic exposures don't seem to be the most important, driving factors. This nonallergic adult-onset asthma is sometimes called "intrinsic."

In men, occupational exposure to chemicals and organic dust is responsible for an estimated 15 percent of asthma cases. Unfortunately, these cases may be misdiagnosed as chronic bronchitis or chronic obstructive pulmonary disease and not treated properly.

The fact that women are more prone to asthma suggests that female sex hormones play a role. Data from Harvard's ongoing Nurses Health Study, which has followed 121,700 female registered nurses since 1976, found that postmenopausal women who took estrogen as hormone replacement for 10 years or longer were 50 percent more likely to develop asthma than were women who never used estrogen.

New research also shows that women with asthma are most likely to have a severe attack immediately before or during their menstrual period. A study at the Medical College of Pennsylvania found that twice as many women sought emergency asthma treatment during the onset of their period or the first days of menstruation, compared with the middle of their menstrual cycle. The fewest number of women sought treatment toward the end of their period. Some researchers believe that adult-onset attacks are associated with the sharp drop in levels of estradiol, a type of estrogen that occurs in the days before menstruation.