House dust mites belong as mites to the arachnids. Worldwide there are about 150 species, which are about 0.1 to 0.5 mm large and have eight legs. House dust mites feed on fallen skin scales, of which humans lose up to 1.5 g per day. However, these scales must first be digested by the fungus aspergillus repens before they can be used by the mites. Since the mites also carry this fungus on their body, it is constantly distributed during their movements. The mites live in beds, carpets and upholstered furniture. Their faeces contain allergenic components, which are distributed as fine dust (particle size about 35 µm), are inhaled and can cause allergies, such as house dust allergies. House dust mites have ideal living conditions at room temperatures of 25 °C and a relative humidity of 70 %. At a relative humidity of less than 50 % they die.
The highest concentration of house dust mites is usually found in pillows, because plenty of skin flakes as food and a lot of warmth and moisture are provided by the head of the sleeping person. By breathing alone, humans excrete 250-400 ml of water per night. A mite-free pillow is therefore not available. Even cleaned pillows contain some 10,000 of the 0.3 mm mites. Pillows that have not been washed for years (if only the cover is washed) contain up to 400,000 mites. Since mites only live for a good six weeks, the living and dead mites in such a cushion that has not been washed for years account for 10% of its total weight. One house dust mite produces about 20 excrement balls per day. In its approximately 6-week life, the weight of the excrement balls adds up to 200 times the mite's own weight. A teaspoonful of bedroom dust contains on average almost 1000 mites and 250,000 tiny excrement balls. Due to their lightness and shape, these remain in the cushions less than dead mites, but are mainly poured into the air.
The components of the faeces contain the allergenic parts. Typical symptoms of a house dust mite allergy are asthma, rhinitis, watery eyes - sometimes also hives and eczema. Especially babies and small children are therefore particularly dependent on effective protection: it is not possible to banish dust mites even with the greatest possible cleanliness. You can make it uncomfortable for them by making the bedroom cold and folding back the comforter completely, and washing everything (including blankets and pillows and mattress cover) weekly, but only encasings are really effective and practical. Encasings are allergy covers for mattresses, pillows and blankets. These covers keep away the allergy triggers of the house dust mite and thus make an important contribution to allergy prophylaxis.