A number of epidemiological studies indicate that the prevalance of allergic airway diseases has been increasing over recent decades, especially in western industrialised countries, for reasons which are not yet completely understood (1, 2, 3). Changes in life style or an increase in indoor allergen exposure due to higher indoor temperature and humidity have been suggested as potential determinants, although evidence for both hypotheses is indirect (4). During the last years we have been following a birth cohort born in 1990 in order to understand the influence of the major genetic and environmental determinants, which are modulating the development of allergic sensitisation and the incidence of atopic symptoms. Sensitisation to indoor allergens has been demonstrated to be one of the major risk factors for the development of asthma in childhood (5, 6, 7, 8). Several cross-sectional studies in older children indicate that specific sensitisation to house dust mites is related to dust mite allergen concentrations in mattress dust (9, 10). Exposure threshold levels for several indoor allergens have been proposed, but individuals vary widely in their susceptibility to levels of exposure, and no absolute value has been identified which could generally ensure minimum risk.