Dust Mite Habitat.

In order to thrive, dust mites require food (flakes of human skin), moisture (in the presence of some species of mould such as Aspergillus Penicilloides) and a dark warm habitat. Dust Mites need a narrow range of high temperature and humidity conditions to thrive. Dust mites populations show optimal growth at conditions between 18ºC to 25ºC and 60% to 70% relative humidity, conditions of temperature and humidity rarely found in carpet in homes in the UK. Furthermore, when the relative humidity falls below 50% the dust mites desiccate and die.

The dust mites feed on the dead scales of skin, which the human body constantly sheds at a rate of 5g per day. The scales are too dry and horny when they flake off the human body for the mites to eat and they need to condition in a moist atmosphere for a few days to absorb water from the air, and to promote the growth of miniature surface moulds, before they are suitable as mite food. Since water is not readily available in carpet the mites depend upon the absorbed moisture for their entire water needs. Since the dust mites require such an extreme range of conditions to survive it is common to find carpet completely free of mites or with very low concentrations. Hysterical claims that, as many as 100,000 dust mites ?may? be found in a square metre of carpet have no scientific validity.

Bedding

Bedding on the other hand offers ideal conditions for dust mites to live and thrive which are not dependent upon the ambient relative humidity of the bedroom. We spend approximately one third of our lives in bed during which time the body loses approximately one litre of water each night through perspiration and breathing. During this period the body continues to shed skin scales which are concentrated in a relatively confined area and so the bed provides a dark, warm, humid and food rich environment for the dust mite with long periods of intimate contact between bedding nose and throat.

In a Dutch study it was found that the level of dust mite allergen in dust taken from mattresses was more than 78 times greater than in dust taken from wall to wall carpet in school rooms, almost 3 times greater than dust taken from carpet in living rooms and 1½ times greater than in dust taken from bedroom carpet. This illustrates the importance of beds and bedding as the main environment for dust mite to thrive and produce allergen.

Surveys of households in Germany have shown that whilst almost 30% of all houses were mite free, 50-60% of mattresses, blankets and upholstery fabrics contained mites but only 16% of wall to wall carpets in living rooms and 45% of bedroom carpets contained them. Most researchers agree that the main centre for dust mites to breed is in bedding and following agitation during bed making the airborne dust mites and their allergen then settle on other surfaces including the bedroom floor and soft furnishings. Migration of the dust mites to other areas of the home then slowly takes place through air changes, agitation and vibration. In schools (where there are no beds) the allergen level in dust taken from carpet is very low and Professeur du Blay of the University of Strasbourg has concluded from a study that there is negligible allergen in office carpet because of the absence of beds. In a study, which examined the health of 98 asthmatic children in Holland, it was found that those children who were exposed to wall to wall carpets suffered no poorer health than children exposed to uncarpeted floors.

Dust Mite Allergen

Live dust mites do not themselves cause allergic reactions but their faecal pellets are the cause of sensitisation and can trigger allergic reactions in some people. These faecal pellets are extremely small measuring about 10-15 microns (1 micron is 1/1000th of a millimetre). When these are held trapped in the pile of carpet they do not pose a hazard to sensitised persons. It is only when they are airborne that allergen particles pose a health problem by being available for inhalation. Airborne allergen settles on all surfaces but whilst hard surfaces, including floors, allow the allergens to become easily airborne through the slightest draught or vibration carpet retains the allergen deep in the pile. It has been found, for example, that the velocity of air over a carpet surface is required to be ten times greater than that over a smooth surface if the same number of particles is to be released into the air.

There is no direct link between the number of mites in a carpet and the amount of allergic faeces present. Production of allergen is very slow and it takes a long time for a significant build up to occur. In a residential environment, wall to wall carpet prevents the allergic material from being released into the atmosphere by holding the fine allergen particles in the pile and improves the quality of life of allergic persons. This has been confirmed by research, which showed that measurements of the level of dust mite allergen in the room air above the surface of carpets, even those containing high levels of dust mite allergen, failed to detect the presence of allergen. These measurements were made over an extended period at only 24? above the carpet surface. This study, quotes other research (Air Quality Sciences, An Indoor Air Quality Study of Alafia Elementary School, (1994-95) Atlanta, Georgia, U.S.A.) which also found no detectable dust mite allergen above carpet.

In a further study which compared the flooring in fourteen Swedish town halls, it was found that whilst more dust could be collected from carpeted floors than uncarpeted floors no greater amount of dust could be found in the indoor air over carpeted floors. This again confirms the inherent property of carpet to hold dust, including allergen, in the pile preventing it from becoming airborne.

Dust mite expert, Dr John Maunder, Director of the Medical Entomology Centre at Cambridge University is certain that carpet cannot be blamed for allergic reactions. ?Only in poorly ventilated humid dwellings can large populations of mites build up in carpets. In dry well ventilated places either no mites exist or they are in such low numbers that no significant production of allergen occurs.?