microscopically. Thus, evaluation of airborne indoor allergens depends on sensitive immunoassays and requires a method for collecting particles. This can be done either with a filter or with a multistage impactor (Solomon and Matthews, 1988; see also the discussion in Chapter 6).

In 1981, Tovey and colleagues (1981b) showed that fecal particles were a major form in which the allergen Der p I becomes airborne and that very little or no (i.e., <1 ng per cubic meter of air) mite airborne allergen was detected in undisturbed rooms (de Blay et al., 1991b; Platts-Mills et al., 1986; Swanson et al., 1989; Yasueda et al., 1989). Furthermore, airborne mite allergen falls rapidly after disturbance. These results support the view that mite allergen is predominantly airborne on particles that are larger than or equal to 10 µm in diameter. The levels found in the air during disturbance depend critically on the form of the disturbance and vary from 5 to 200 ng of Der p I/m3. Assuming that airborne Der p I is carried predominantly on fecal particles, it is possible to estimate the number of particles that become airborne and to an estimate of the number of particles that could enter the lung, since the mean allergen content of the particles is known. Chapter 6 discusses methods of assessing exposure and risk.

Thresholds: The Relationship Between Exposure, Sensitization, and Disease

Voorhorst and his colleagues (1967) found that dust from the houses of symptomatic allergic patients generally had more than 500 mites/g of dust. During the 1980s, further data accumulated demonstrating a dose-response relationship between exposure to mite allergens (or mites) and both sensitization and asthma (Bernton et al., 1972; Kang et al., 1991; Pollart et al., 1989, 1991). From these results, it also appeared that there were levels of exposure (or thresholds) below which the risk of sensitization or asthma was much less. This finding notwithstanding, the 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.

Fewer data are available on the levels of exposure associated with sensitization or disease for allergens other than dust mite. However, there are data about the levels of cat allergen present in house dust. Dust from all houses with a cat contains at least 8 µg of Fel d I/g (the levels range as high as 1.5 mg of Fel d I/g). In houses without a cat, levels vary from less than 0.2 µg/g to 80 µg/g; it is thought that this allergen is transported into the houses on the clothes of inhabitants. Levels of cat allergen of less than 1 µg of Fel d I/g of dust appear not to give rise to sensitization or disease.

For cockroach allergen the rarity of sensitization among suburban patients suggests that the levels found in suburban houses (i.e., less than 1



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