Sunlight Steam cleaning Cleaning

Public Health Significance of Urban Pests 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 While it is true that freezing will control HDMs, we have found T. Wilkinson, unpublished observations, 2002 that 100 control of HDM populations can only be observed after 72 hours, although 98.3 mortality can be achieved after 24 hours.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

In some countries, rugs and bedding are commonly aired outdoors. Tovey Woolcock 1994 found that exposing carpets to direct sunlight killed all mites within three hours, by creating microclimates hostile to HDMs – with peaks in temperature of 55ºC and relative humidity as low as 24 within the carpets. Allergens, however, were not affec- ted significantly. The effectiveness of these experiments was in part due to the hot sunny weather in which they were conducted. Tovey 1992 reported that the ambient condi- tions during the day were 30ºC and 60 relative humidity. While extremely effective, the use of this technique will generally be confined to the summer months, which coin- cide with the period of maximum HDM growth, and will not be as applicable in some cooler climates. Some products and services are available to consumers that incorporate an ultraviolet light, but there is no scientific information available to ascertain the effectiveness of this technique. However, it is unlikely to be effective, since the ultraviolet light will not focus House dust mites 108 ficiency of vacuuming, but also that HDMs are not restricted to the mattress surface, where vacuuming is likely to be most effective.

3.3.4.6. Steam cleaning

Steam can be used as an alternative to insecticides. It can be extremely effective at control- ling HDMs and their allergens and can be used frequently, provided the room is subse- quently well ventilated. Steam does not, however, have residual activity. In laboratory experiments, Colloff, Taylor Merrett, 1995 found that steam cleaning controlled 100 of HDMs in samples of carpeting. Also, no subsequent development of HDM populations occurred, indicating that the cleaning also controlled the HDM eggs, which are normally more resistant to extremes of temperature than the mites themselves. Under field condi- tions, steam cleaning also resulted in a mean reduction in the measurable Der p 1 concentration of 86.7. The mean temperature measured in the carpets during treat- ment was 103.4ºC, decreasing to the background temperature in 20 minutes. After treat- ment, the humidity was found to remain at saturation point for 25 minutes before falling to background levels after 140 minutes. It should be noted that treatment of an entire room is likely to cause humidity levels to become elevated for a more prolonged period of time and that it is essential to ventilate both during and after treatment, to minimize both mite survival and the potential for the growth of mould.

3.3.4.7. Carpet cleaning