Dermatophagoides mites have supracoxal glands that secrete a concentrated solution of sodium and potassium chloride onto the mouthparts. The solution is hygroscopic: it pulls water vapour out of the air, and the mite then ingests it. The exchange only works above a critical equilibrium humidity, generally in the region of 55 to 75 per cent relative humidity depending on species and temperature.
Below that threshold the mite loses water faster than it can gain it and eventually desiccates. This is why dust mite populations collapse in cold dry climates and at high altitude, and why sustained indoor humidity control is the most reliably effective mitigation. Bedding is the prime habitat because it combines shed skin, warmth and moisture from a sleeping body.
The allergy is not caused by the mite itself but by proteins in its faeces and body fragments, particularly digestive enzymes such as Der p 1. Whether reducing mite allergen levels in homes actually reduces asthma symptoms has produced mixed trial results and remains contested.