The environmental influence on childhood asthma.

The pathogenesis of allergy depends on the interaction between the time and amount of allergen exposure and the presence of nonspecific "adjuvant" factors in genetically susceptible individuals. There seems to be a period in early life during which the individual is particularly susceptible to sensitization, and there are variations in susceptibility over time. Allergens are almost ubiquitous, but the relative importance of the individual allergens varies between regions. In many temperate regions, house-dust mites used to be absent but are now more common. This may be due to modern methods of building houses. Differences in the prevalence of a particular allergy cannot explain variations in the prevalence of allergy in general. Various environmental factors that may enhance sensitization include tobacco smoke, NO2, SO2, ozone, and diesel particles. Passive smoking is by far the best established risk factor, particularly in early childhood. The indoor environment probably plays a larger role than outdoor air pollution in the development of allergic disease. The mother is not only a source of genetic information, but also an "environmental factor", as there is a close immunologic interaction between the mother and her offspring, mediated through the placenta and the breast milk, which may affect the likelihood of allergic disease. The concepts of "lifestyle" and "environment" should be expanded to include, for example, dietary changes, the microbial environment, and extensive traveling, as all the currently suspected risk factors taken together can only explain a small proportion of the geographic differences in and increasing prevalence of allergy. The future search for significant environmental factors should be interdisciplinary and be directed toward areas that have not yet been explored, thus giving "lifestyle" a broader interpretation.