variability in responses among a human population. When a Lowest Observed Adverse Effect Level (LOAEL) is determined, a UF of 10 is applied. Additional uncertainty in the data is accounted for by use of a modifying factor, varying from 1 to 10, to further reduce the NOAEL (Shoaf, 1991).
The relationship between dose (or exposure) and response is used to characterize the risk within a population. For aeroallergens, this relationship might be expressed as the number of new cases expected at a given exposure level or the distribution of expected change in the severity of symptoms with changing exposure levels.
When dust mites were first reported in the literature in the 1970s, it was suggested that levels of more than 500 mites/g of dust in a house were likely to produce symptoms of asthma in allergic individuals. During the 1980s, data accumulated demonstrating a dose-response relationship between reservoir concentrations of dust mite allergens at home and both sensitization and asthma (Table 6-2). Results of this kind led to the proposal of threshold levels for reservoir concentrations. Two separate thresholds have been considered: more than 2 µg of Der p I/g of dust has been associated with increased prevalence of sensitization, whereas 10 µg increases the risk of symptomatic or acute asthma. In an 11-year prospective study of 68 children, Sporik and colleagues (1990) found that exposure1 to high levels of allergen at age 1 increased the risk of asthma and was inversely related to the age of onset of asthma in atopic children. In subsequent studies of children hospitalized in the south of England, it was found that about 80 percent were both sensitized and exposed to high levels of relevant allergen at home (Sporik et al., in press). These results suggest that in areas in which all houses contain high levels of mite allergen, sensitivity to mites is a major risk factor, not only for wheezing but for hospitalization of children with asthma.
Evidence to support a quantitative relationship between exposure to mites and asthma has also been found in studies from Denmark, Australia, Germany, and France (Table 6-2). Two studies in particular strongly imply that high levels of domestic exposure can increase the prevalence of asthma.