has estimated that 10 million people in the United States currently have asthma (NHLBI, 1991). Figure 2-6 shows how the definition of asthma can affect estimates of prevalence in epidemiological studies. Additional work is needed to clarify the relationship between asthma and the various definitions of allergy phenotype.
About half of the cases of childhood asthma develop before age 5, and most of the rest develop before adolescence. Incident cases are uncommon between ages 20 and 40, with a gradual increase in incidence beginning in the fifth decade of life. Overall, cases of asthma have increased in the past few decades. During the 1920s and 1930s, incident cases were uncommon; a gradual increase began in the 1940s, however, and has continued up to the present. The overall annual incidence of asthma in a Tucson study was 0.46 percent (Lebowitz, 1989).
Asthma is generally treated in outpatient settings. In 1985, of the 640 million total ambulatory care visits estimated by the National Ambulatory Medical Care Survey (NHLBI, 1991), 6.5 million (1 percent) had asthma as a first-listed diagnosis. From 1970 to 1987, hospital discharge rates for asthma increased nearly threefold. African Americans were more than twice as likely as Caucasians to be hospitalized.
Most recently K. B. Weiss and colleagues (1992b) estimated that 1.8 million people required emergency room services for asthma in 1985, and 48 percent of the visits involved children under age 18. There were an estimated 1.5 million hospital outpatient visits to treat asthma. K. B. Weiss and colleagues (1992b) also estimated the cost of illness related to asthma in 1990 to be $6.2 billion.
Another measure of impact is absenteeism. A WHO/EURO report (1990) reported that a substantial portion of absenteeism from work or school is associated with infectious and allergic episodes caused by exposure to indoor air. Similarly, a 1979 report (NIAID, 1979) stated that asthma was among the leading causes of physician visits, hospitalizations, and workdays lost.
Figures 2-7, 2-8, and 2-9 show recent trends over time in asthma prevalence, mortality, and hospitalization rates, respectively. The prevalence rate of asthma in the first half of the century was about 1-2 percent in Caucasian populations in industrialized countries (Gregg, 1989). The first estimate of the prevalence rate of asthma in the United States—in 1928—was less than 1 percent in both children and adults; a 1930 survey reported a prevalence