House dust mites are tiny creatures, about a quarter of a millimetre long. They live off human skin scales which have been partially digested by moulds and thrive in humid environments. Mites are found in bedding, carpets, soft furnishings and clothing.
In people allergic to dust mite, it is often not the mite itself but proteins in their droppings which cause the allergy. Each mite produces about 20 of these waste droppings every day and the droppings continue to cause allergic symptoms even after the mite has died.
House dust mite allergy is very common and is associated with asthma, eczema and perennial allergic rhinitis. A significant amount of exposure to house dust mite allergen happens in the bed, so taking precautions in the bedroom by using allergy-proof covers on bedding, washing it regularly can sometimes help, although clinical trials suggest that multiple measures need to be taken, possibly including the use of chemicals called acaricides, in order to see an effect. However, remember that dust mite allergen is found in all rooms of the house, on the floor and in soft furnishings, not just in the bedroom.
Measures to avoid house dust mite will lower, but do not totally remove, dust mite allergens. Often, this will be sufficient to significantly improve symptoms, but sometimes, the reduction may simply not be enough to result in a noticeable difference. There is no way to predict whether someone will benefit from avoidance measures, except by trying them. Remember that it is better to properly carry out several allergen avoidance measures in order to see an improvement in symptoms. Just doing one or two things half-heartedly may not make any difference.