prevalence rates. Sly found no difference in urban versus rural asthma mortality rates and no variation by geographic region (Sly, 1988). K. B. Weiss and colleagues (1992a), however, found that inner-city asthma was more prevalent than asthma in suburban areas. Moreover, in a comparison of asthmatics being treated at two Delaware hospitals, cat allergy and cat allergen exposure were more common among patients who were treated in a suburban Wilmington hospital, and cockroach allergy and allergen exposure were more common among patients being treated in an urban Wilmington hospital (Gelber et al., in press). This suggests that similarities in urban and rural asthma prevalence rates may mask significant differences in relevant allergen exposures and sensitivities.
Age Asthma prevalence rates are highest in earliest childhood, declining to a low at around age 20 and then slowly increasing with age (Barbee et al., 1985; Mak et al., 1982). Between 5 and 14 percent of children will have a respiratory illness with wheezing at some time (Barbee et al., 1985). All wheezing in children is not asthma, however; an infectious disease, bronchiolitis, is also associated with wheezing.
Hospitalization rates for asthma vary with age. NCHS data (1992) indicate that from birth to age 14, 169,000 hospitalizations occurred due to asthma (30.8 hospitalizations per 10,000 population); from age 15 to 44, 86,000 hospitalizations occurred (19.1 per 10,000 population); from age 45 to 64, 86,000 occurred (18.2 per 10,000); and for age 65 and older, 102,000 occurred (32.4 per 10,000 population).
Gender Most studies report a greater prevalence of asthma among boys than among girls, usually by a ratio of 1.5–2 to 1. However, studies in Sweden, Finland, and Arizona have not demonstrated this predominance in males. The increased prevalence of asthma among boys does not appear to be related to genetic or familial factors (Sibbald et al., 1980), but it has been associated with males' greater predilection for infections, especially wheezing lower respiratory infections (Gregg, 1983). S. Weiss and colleagues (1992) report apparent sex differences in the relationship between asthma and lung function development, with males more likely to have asthma but with females experiencing a greater deficit in pulmonary function.
In adults over age 40, new cases of asthma are more likely to occur among women. If the asthma is associated with airflow obstruction (at the time of diagnosis or later), incidence is greater among adult males than among adult females (0.44 percent per year versus 0.39 percent per year; Lebowitz, 1989). Increased rates later in life reflect overlap with "wheezy bronchitis," which is most commonly due to smoking.