Mattresses as a winter refuge for house-dust mite populations

House-dust mites are a major source of allergens. Beds are often heavily infested. It is conceivable that, during the winter period in temperate climates, the ambient indoor air is too dry for their survival, but that mites can still survive in beds where the humidity is raised when the bed is occupied. To investigate whether house-dust mites can take advantage of daily episodes with elevated humidity to restore water balance, we exposed mites to spells of high humidity (90% relative humidity [RH]), lasting 0, 1.5, 3, 6, or 12 h, once every day, while the humidity was low (10% RH) during the rest of the time. Survival was markedly promoted when only 1.5 h of moist air per day was given. Three hours of moist air per day allowed the mites to reproduce. The same average humidity but at a constant level would have been lethal to the mites. Thus, average humidity can be misleading as an indicator of mite survival conditions. Measurements of relative humidity were made inside the foam core of a mattress and underneath the ticking. The rise of humidity occurring when the bed was occupied was often insufficient for the mites, because the temperature rose at the same time. At higher temperatures, mites required a higher relative humidity, abolishing the effect of the simultaneous rise in absolute humidity. Thus, it seems quite possible to regulate house-dust mite numbers also in beds by reducing the water content of the ambient air. The rules governing changes in temperature and humidity inside a bed may be simple enough to allow assessment of house-dust mite living conditions by measurements in the ambient air.