Two studies of asthma in children, using identical survey methods and objective assessments separated by a period of 10-15 years, have now been published; each shows an increase in asthmatic symptoms and airway hyper-responsiveness and in seasonal rhinitis.1 (p 1591).2 The increased prevalence of asthma has been matched by, and is probably a manifestation of, an increase in sensitisation among children to inhaled allergens, such as those present in house dust, cat fur, and grass pollen.3

Although genetic factors are important in determining both the propensity to atopic disease and the specificity of the response to protein epitopes, the short period during which the increases in asthma and other allergic diseases have occurred suggests that environmental influences have been mainly responsible. For example, secular trends in the Finnish armed forces show a 20-fold increase in asthma among 18 year old recruits during the past 30 years.4

Further evidence of important environmental influences comes from the recent observation that sensitisation to common aeroallergens is about …