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107
HDMs. However, the mites were soaked in the solutions for a considerable period of time, which does not necessarily reflect real life conditions, where mites may only be
exposed to the disinfectants for a short period of time or else come into contact with them after the water has evaporated.
3.3.4.2. Washing
HDM allergens are extremely soluble in water, enabling them to be removed from items during washing. Watanabe and colleagues 1995 found that washing blankets with a hot
55ºC soap solution reduced Der p 1 levels by an average of 97 and Der p 2 levels by an average of 91. McDonald Tovey 1992 found washing bedding at 55ºC killed all
mites present, while reducing the temperature to 50ºC killed only half of them. Most was- hing machines have washing cycles at 40ºC, 60ºC and 90ºC; temperatures of 60ºC or more
are therefore recommended. If the bedding has to be washed at low temperatures, it is possible to add special products to the wash to kill the mites McDonald Tovey, 1993;
Bischoff et al., 1998. In addition to bedding, clothing can also be a source of allergens Tovey, Mahmic McDonald, 1995.
3.3.4.3. Dusting
Dusting is an effective method of removing house dust and therefore allergens; damp dusting is particularly effective. No specific studies have been reported, although Tovey
1992 found that a vigorous cleaning regime in addition to a number of other measures generally is important in the success of clinical trials.
3.3.4.4. Dry cleaning
Kniest, Liebenberg Bischoff 1989 reported that all HDMs were killed by dry clea- ning in perchloroethylene. Watanabe and colleagues 1995 found that dry cleaning of
blankets with perchloroethylene reduced levels of Der p 1 by a mean of 69 and Der p 2 by an average of 54. These levels of allergen reduction are significantly less than
would be achieved through washing at 55ºC.
3.3.4.5. Vacuuming
Householders primarily use vacuuming to remove dirt and dust from carpets and other floor coverings, but in this way they are also able to reduce the quantity of allergen within
the home. However, this can take a substantial period of time de Boer, 1990b and is best achieved by regular vacuuming Munir, Einarsson Dreborg, 1993. The concentration
of airborne allergens has been found to increase significantly after vacuuming with a stan- dard vacuum cleaner, thus increasing the risk of exposure to these allergens. This can be
minimized when vacuum cleaners are fitted with high efficiency filters Kalra et al., 1990.
Vacuuming an area has been shown to decrease the number of mites present Hill, 1998, although it does not remove all of them. Bischoff, Fischer Liebenberg 1992 reported
that vacuuming a 1 m
2
area of carpet for two minutes may only remove 10 of the HDM population. Hay 1995 vacuumed the surface area of a spring mattress and recorded a
population density of 3–46 living mites per m
2
, three orders of magnitude lower than the estimate of 8,200 m
2
26,800 living mites per m
2
obtained by extracting mites from a core taken from the upper 1.5 cm of the same mattress. This demonstrates not only the inef-
House dust mites
106
Finally, the evidence on the effect of air conditioning is conflicting. Some studies have found a beneficial effect on mite numbers and allergen levels Lintner Brame, 1993;
van Strien et al., 2004, while others have not Chan-Yeung et al., 1995; Chew et al., 1998. When considering the suitability of air-conditioning as a possible solution for controlling
mites, its energy requirements should be taken into account.
Smaller scale mechanical systems that modify environmental conditions, such as porta- ble dehumidifiers and bed heaters, are discussed in sections 3.3.5 and 3.3.8.
3.3.3.4. N on-mechanical passive ventilation systems
The most important non-mechanical passive ventilation system is passive stack ventila- tion PSV which, like a traditional chimney, makes use of the stack effect – that is,
buoyant warm air rising to the cold outside. The system consists of a 10–15 cm-diameter tube typically polyvinyl chloride installed in the ceiling of kitchens and bathrooms and
extending to vents in the roof ridge. An advantage of PSV systems is that the stack effect works best in winter, when the temperature difference between the inside and outside is
greatest, which is also when the need to continuously remove warm moist air from kit- chens and bathrooms is greatest. Following a pilot study with a test house fitted with
both MVHR and PSV systems, used alternately, Palin and colleagues 1993 reported that the use of PSV was as energy efficient as the use of MVHR in achieving the same condi-
tions of comfort. PSV can also be used in conjunction with so-called supply air windows, which preheat the incoming air, using heat that would otherwise be lost through the win-
dow glazing McEvoy Southall, 2002.
Architects and engineers have devised a variety of effective ways to achieve good venti- lation at low or reasonable energy cost and without sacrificing thermal comfort.
Householders are thereby encouraged to ventilate sufficiently well in the crucial winter period to keep HDM populations under control. Appropriate technology can undoub-
tedly play a helpful role, but perhaps the greatest need is to put more emphasis on edu- cating the public about how to use the buildings they live in most efficiently. To this end,
it is to be hoped that computer modelling will in due course increase our knowledge of how buildings are best used to achieve a healthy environment.
3.3.4. Cleaning
Although rigorous household cleaning, with the exception of steam cleaning, will not eli- minate or significantly reduce HDM populations, it will remove allergens. Many clea-
ning methods will not remove dirt and allergens from deep within a carpet or mattress, although they will remove allergens from the surface where it is most likely to come in
contact with people. Cleaning will also reduce the amount of food human skin scales available to HDMs, thus potentially reducing the size of future HDM populations.
3.3.4.1. Home disinfectants
Cleaning of hard surfaces is generally conducted using some form of disinfectant diluted with water. In laboratory experiments, Schober and colleagues 1987 found that a num-
ber of different household disinfectants, some containing acaricides, were able to kill
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109
3.3.5.3. Electric blankets
De Boer van der Geest 1990 observed reductions in HDM populations of between 19 and 84 over a 10-week period in heated areas of a mattress. Areas of the mattress
that had been heated by electric blankets had significantly fewer mites than unheated areas. This indicates that electric blankets are able to suppress, but not completely elimi-
nate, H DM populations. Using a combination of electric blankets and vacuuming, Mosbech, Korsgaard Lind, 1988 also observed significant reductions in allergens.
They suggested electric blankets are best used to help prevent mattresses from becoming infested, rather than to combat pre-existing mattress infestations.
3.3.5.4. Bed heaters
A number of devices are currently being developed to introduce hot air, at varying tem- peratures, into a mattress. As they are able to heat up mattresses, they have the potential,
over several days, to control H DMs by reducing the humidity within the mattress. However, no work on their effectiveness has been published to date.
3.3.5.5. Freezing
Laboratory and field studies indicate that the use of liquid nitrogen as a freezing agent, combined with vacuuming, is effective at reducing HDM populations when compared
with vacuum cleaning alone Colloff, 1986. Freezing toys in a conventional freezer can also kill the mites Nagakura et al., 1996. While it is true that freezing will control
HDMs, we have found T. Wilkinson, unpublished observations, 2002 that 100 control of HDM populations can only be observed after 72 hours, although 98.3 mortality can
be achieved after 24 hours. It appears that some eggs, which later hatch, survive the free- zing process. Freezing does not, however, remove allergens from treated items. It should
therefore be combined with washing, in the case of smaller items, such as toys and pillows; it should also be combined with such actions as vacuuming, in the case of such large items
as mattresses. At present, the use of freezing to treat carpets seems impractical. Although freezing leaves behind no residues, this does mean that it has no residual activity see also
subsection 3.1.3.3.
3.3.5.6. Sunlight
