ALLERGY: WHAT ARE HOUSE DUST MITES?
House dust mites live on the debris of human environments, and on other small living organisms. They do not cause harm directly to humans, apart from being a potential allergen. The species of mite particularly associated with allergy in the UK is called Dermatophagoides pteronyssimus. Dermatophagoides means ’skin-eating’ and, in common with other mites, house dust mites feed especially on human skin scales. Humans shed on average up to one gram of skin scales a day -enough to feed many mites for months and these fall and collect around where humans live. House dust mites also feed on animal skin scales, and on micro-organisms such as moulds, bacteria and viruses.
Having house dust mites in your environment is not a sign of dirty or insanitary conditions, nor of slovenly or poor housekeeping. They need a particular ecology to survive and human environments provide the best conditions for them. House dust mites thrive where food supply is plentiful, and where the environment is moist, warm and dark. They like ideally a moisture level of 80 per cent relative humidity and a temperature of about 25°C (77°F). For humidity year round, the UK is ideal for them; and for temperature, many warm, dark places indoors such as unaired beds, duvets, chairs and carpets, are also well suited.
They are present all year round and hence are responsible for many cases of perennial rhinitis or other year-round symptoms. Their presence can increase when the weather is very damp and, like mould allergy (>MOULDS), allergy to house dust mites often gets worse in damp weather.
They can be found in very high densities where the environment is favourable to them. Up to thousands have been measured in one gram of surface dust. It is their droppings – their faecal pellets – that cause most problems with allergic reactions, although some people are allergic to debris of the mites themselves. The faecal pellets remain even when the mites themselves move on or die, so dust, bedding or pieces of furniture can continue to cause problems even if you kill the mites.
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