prevalence in various populations. The results indicate a range from a low of 20 percent to a high of 51 percent in different studies. It is estimated that about 40 percent of people in the United States have been immunologically sensitized as indicated by skin test reactivity to a panel of currently available allergens. The recent International Consensus Report (NHLBI, 1992) claims that about 30 percent of all populations are skin test positive.

Early childhood is a common time for sensitization. Barbee and others (1987) showed that 22 percent of a cohort of children less than 5 years old had skin test reactivity to one or more allergens. During an 8-year follow-up, the prevalence increased to 44 percent. Gergen and coworkers showed that 18 percent of Caucasian and 28 percent of African American children showed skin test reactivity when tested between the ages of 6 and 11 (Gergen et al., 1987). In a cohort of genetically susceptible children, 20 percent developed skin test reactivity by age 5; another 20 percent became reactive between ages 5 and 11 (Sporik et al., 1990).

Skin test reactivity increases in prevalence until age 20–45 and then decreases (Figure 2-2; Barbee et al., 1976, 1987; Gergen et al., 1987). In most studies, skin test reactivity is similar for both sexes, although men have more total IgE (Barbee et al., 1976; Burrows et al., 1980; Klink et al., 1990). Although immunologic sensitization is generally stable over time, small seasonal variations have been observed (Peat et al., 1987). Prevalence rates of sensitization also increase somewhat with the use of a greater number of allergens during testing (Barbee et al., 1987).

Table 2-3 and Figure 2-3 show estimates of the prevalence of sensitization in certain population samples to specific allergens as determined by skin test reactivity. Among Australian schoolchildren, for example, sensitization to house dust mite was shown to be most common, followed by grass and weeds, animal danders, and molds (Peat et al., 1987). Similar rates of sensitization have recently been found in middle-class American children. The prevalence of cockroach allergy shown in Figure 2-3 may well be higher in selected populations, such as among residents in cities in which cockroach infestations are widespread. A study of patients visiting an emergency room in Virginia showed that 5 percent of nonasthmatics were sensitized to cockroaches, compared to 33 percent of asthmatics (Pollart et al., 1989). Fifty-four percent of patients referred to a Kansas City clinic for possible allergic disease were sensitized to cockroach allergen (Hulett and Dockhorn, 1979).

Risk Factors

In keeping with a public health approach to prevention and control of allergic disease and the need to improve the understanding of the etiology,

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