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Old Curiosity Shop: House Dust Mite


House dust mites have been recognised as a major source of allergen. Because we have wall-to-wall carpets, soft furnishings, and central heating in most homes, the density of house dust mites ( Dermatophagoides pteronyssinus ) in our homes has increased dramatically. Over 80% of young adults and children with asthma are hypersensitive to its allergens.

The house dust mite is common

The concentration of the main allergen protein, Der p1, can outlast the mite itself, because it is found in high concentrations in mite faeces. In 1991, 40 houses in south Manchester were studied for concentrations of the Der p1 allergen [1], in dust collected from bedroom and living room carpet, and from mattresses. Concentrations varied widely between houses, without much variation with season. Concentrations in dust taken from mattresses and carpets were in the range of about 0.5 to 200 µg/gram of dust.

Getting rid of the mites

A number of methods has been tried, including using acaricides (mite-killing chemicals) and liquid nitrogen (the mites are killed by freezing). The problem is that these methods may kill the mites, but the Der p1 allergen that has collected is still there.

Another approach is steam cleaning. A classic paper [2] did experimental work in the laboratory, and in the field. In the laboratory, carpet squares were seeded with live mites. Some were cleaned immediately with a domestic steam cleaner, but whether cleaned or not all the carpet squares were incubated in conditions of temperature and humidity, and with food that the mites find congenial.

Over a period of 114 days the number of live mites was sampled from the squares. You do this by heating the bottom of the square and catching mites on sticky tape as they climb up to get away from the heat. Results were that only one square originally steam cleaned showed any growth, while the mites thrived in the uncleaned squares.

Steam cleaning in the home

The field trial was carried out on a ground floor tenement in Glasgow which was 100 years old, but with central heating. Carpet areas in bedrooms, living room, kitchen and hall were delineated with masking tape. The carpets were about 15 years old. Dust samples were taken from each area, and then one area steam cleaned while another was left untreated. When carpets had dried, another dust sample was taken from each area.

There was no difference in the concentration of Der p1 in dust samples of untreated carpets, but in those which had been steam cleaned there was an average 87% fall in the concentration of Der p1, from 3.3 µg/gram of dust to 0.44 µg/g. But some areas had low concentrations of allergen to begin with, and there were some quite dramatic falls in the areas with the highest concentration.

So if you've got carpets, and problems with allergy in a family member, then it is worth remembering that steam cleaning removes the allergen as well as killing the mites. Technical details about steam cleaning are to be found in the paper.

References:

  1. 1 S Kalra, P Crank, J Hepworth, CA Pickering, AA Woodcock. Absence of seasonal variation in concentrations of the house dust mite allergen Der p1 in South Manchester homes. Thorax 1992 47: 928-31.
  2. 2 MJ Coloff, C Taylor, TG Merrett. The use of domestic steam cleaning for the control of house dust mites. Clinical and Experimental Allergy 1995 25: 1061-6.



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