Tuberculin positive children.

Dr IH Feather and colleagues have reviewed the central role of house dust mites in the aetiology of allergic asthma (January 1993;43:5-9). In my personal series of 744 consecutive asthmatic children aged 5-15 years compiled over the years 1985-91, positive skin prick test reactions to Dermatophagoides pteronyssinus (ALK, Copenhagen) were nevertheless found in only 180 subjects (24%) and more often among those from rural domiciles, large families, cramped dwellings, and blue collar social classes (table). The mite population in homes is dependent on the supply of food-that is, human skin scales-and air humidity. Since there is generally no shortage of the former it is the humidity that is the decisive factor for mite survival. In many regions the circumstances are ideal for mite growth, and bedrooms in particular are universally infested with mites. In Scandinavia, however, the relative humidity indoors is low during the long cold season from September to May when the houses are heated continuously, and conditions are therefore unsuitable for mites. Modern energy saving with heat insulation and poor ventilation have caused