Allergen Exposure As outlined above, the magnitude of allergen exposure appears to be related to the risk of sensitization. Allergen exposure is also related to the risk of developing asthma and the age at which asthma develops. Sporik and colleagues (1990) demonstrated that exposure to more than 10 µg of dust mite allergen (Der p I) per gram of dust was associated with a 4.8 relative risk of asthma by age 11.¹ These investigators also showed that the age at which the first episode of wheezing occurred was inversely related to the level of dust mite exposure assessed when the subject was 1 year old.

Allergen exposure has also been related to disease severity. Voorhorst and coworkers (1967) showed that allergic and atopic asthmatics with sensitivity to house dust mite had clinic visits that correlated well with mite growth curves in their homes, a finding that has been confirmed by others (Andersen and Korsgaard, 1986; Tovey et al., 1981a). Clark and colleagues (1976) found a reduction in peak flows ² of children with asthma that they associated with house dust when housecleaning increased particle concentrations in the air. O'Hallaren et al., (1991) provided evidence that exposure to the aeroallergen Alternaria is a risk factor for sudden respiratory arrest in asthmatics. The O'Hallaren team found that alternaria skin test reactivity was associated with a 200-fold risk of respiratory arrest, the timing of which corresponded uniformly with the alternaria season in that region.

It has been shown that prolonged allergen avoidance reduces the severity of asthma as indicated by tests of bronchial hyperreactivity (Platts-Mills et al., 1982). For example, asthmatic children who stayed in a sanitorium in Davos, Switzerland, for 1 year showed progressive reductions in bronchial hyperreactivity. It has also been demonstrated that patients with seasonal asthma will lose their bronchial reactivity to histamine after the pollen season.

Hospital emergency room studies have demonstrated an increased risk of allergen sensitization among asthmatics compared with patients being treated for other conditions. The content of allergen in reservoir dust samples from the homes of asthmatics compared with patients with other conditions is also reported to be larger. These studies are covered in detail in Chapter 3.

Geographic location Rates of asthma in other populations around the world appear to be highly variable (Barbee et al., 1985; Gregg, 1983). Charpin and coworkers (1988b) found no urban–rural difference in asthma