Increased exposure to house-dust mites entails a greatly increased frequency of sensitization and increased frequency of mite asthma in the population. The available case-control studies demonstrate large variations worldwide in sensitization and disease risk dependent on the actual level of population exposure. In areas with comparatively low population exposure to house-dust mites, there is a strong association between exposure and effect, while other areas with a more or less uniformly high population exposure do not demonstrate significant differences between patients and controls because the unexposed groups are too small. As to the existence of a hygienic maximum threshold exposure level, current data all point to a single value of 100 mites/g of dust, corresponding to 2 μg allergen/g of dust. This applies to the risk of sensitization and risk of disease, and when programs of prevention imply exposure below this value, convincing clinical improvement is always observed. Today, the magnitude of the health problem related to population exposure to house-dust mites in many areas is comparable to the effects of active tobacco smoking and traffic accidents.