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House dust mites

Insects have six legs, spiders eight and mites ten. House dust mites belong to a sub-group called pteroglyphid, which for millions of years have evolved to occupy an ecological niche in animals nests, possibly why it they are sometimes called bed mites. Adult mites are about 200 microns (two tenths of a millimetre) long – just too small to be seen by the naked eye. Mites live on human skin scales, like it warm and need moisture in the air to survive. Which is why they particularly enjoy our beds. But they also live in carpets and other soft furnishings throughout the house. There may be several million live mites in a pillow at any one time – even more in a bed.

People are not allergic to the mites themselves, but to allergens they produce.

A house dust mite yesterdayThe increase in our exposure to mite products in recent years is related to a change in things like the use of fitted carpets, but also to central heating and energy-efficient buildings. Indoor humidity levels become higher as we reduce the exchange of heated indoor air with cold, dry outdoor air. Asthma and house dust mite allergies, which were rare in cold Scandinavia, became an epidemic after the oil crisis forced houses to be made “tight”. Not only did the drafts in old Victorian houses blow away any dust, but the open fireplaces sucked the damp air up and away.

Killing mites

Killing mites does nothing to protect you from the allergens that may have been building up in mattresses or carpets for years. It may still seem a good idea to prevent new allergens being produced. Problem is how? Reducing indoor humidity will certainly help, but in the damp British climate, getting levels below those that prevent mite growth is impractical.

There are three problems with mite-killing chemicals. Firstly, mites rapidly develop resistance to them. Secondly, their human safety, particularly in the beds of pregnant women and small children is uncertain. Thirdly, people become allergic to some of them (particularly pyrethrin and pyrethroids).

Freezing mites very rapidly with liquid nitrogen makes them explode. But this is hardly a practical solution at home. The rate of cooling of toys and pillows put in a domestic freezer is so slow, that the air becomes very dry before the temperature drops. Under these circumstances the mites form a desiccation-resistant protonymphal stage and their eggs may also survive.

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