Epidemiology of house-dust mites

Available epidemiologic data on the occurrence of house-dust mites in dwell ings demonstrates a clear association between increased indoor air humidity and the increased occurrence of house-dust mites in house dust. Furthermor e, in temperate climates, there is a threshold level of indoor air humidity of 7 g/kg (45% relative humidity at usual indoor air temperatures). Indoor air humidities below this level for extended periods will eradicate house- dust mites from dwellings. A reduction in inhabitant exposure to house-dust mites is implemented by reduction of indoor air humidity by controlled mec hanical ventilation. Individual ventilation levels are estimated from the a ctual size of house, number of inhabitants, and average outdoor air humidit y in winter. In contrast, more humid areas of the world with average outdoo r humidities above 6-7 g/kg in winter will support uniformly large populati ons of house-dust mites, and reductions in indoor air humidity will have a comparatively minor effect on the occurrence of house-dust mites. Present-d ay building of energy-efficient houses with increased sealing of the buildi ng envelope, paralleled by a similar renovation of older houses, has increa sed indoor air humidity and is probably the cause of the almost fourfold in crease in the occurrence of house-dust mites in Danish dwellings.