Habitat modification Carpets Physical control methods

Public Health Significance of Urban Pests 111

3.3.6.4. Barrier fabrics

Barrier bedding materials can be a major benefit in combating HDMs. They can act as a physical barrier to the mites themselves; on a new mattress, the barrier prevents mites from entering and therefore colonizing it. Also, the barrier stops mite faecal material already present in an old mattress from escaping into the atmosphere, where it can be inhaled. Fabrics with a pore size of 10 µm or less can effectively block the faecal pellets Vaughan et al., 1999. In addition, these fabrics prevent the dead skin cells upon which the mites feed from penetrating the mattress. Thus, the mites and their allergens are effectively contained within the mattress, and the mites are cut off from a continuing food supply. A number of trials of barrier bedding have highlighted the clinical benefits of this type of intervention. As a result of its use, atopic infants who received barrier bedding did not become sensitized to HDM allergens Nishioka, Yasueda Saito, 1998. Also, airway hyperresponsiveness improved significantly after six months with barrier bedding in place van der Heide et al., 1997, and symptom scores in patients with barrier bedding improved after a year. Barrier fabrics should be fitted not only to mattresses, but also to pillows and duvets. It is also essential that barrier fabrics be properly constructed. An effective fabric can lose a significant amount of its efficacy as a result of poor quality zippers and stitching, which can let through allergens. It is also recommended that zippers be covered with flaps and that the barrier provides 100 cover. Moreover, it is important to select a breathable fabric, to prevent the risk of mould growing within the mattress and to minimize dis- comfort from sweating; in general, woven fabrics are longer lasting than other types. For barrier fabrics to be effective, it is also essential for them to be properly cleaned and for the rest of the bedding to be washed regularly, so that it remains free of allergens.

3.3.7. Pesticides

Acaricides or insecticides can be used to control HDMs. They are typically applied as surface treatments or impregnated into fibres and fabrics used in the construction of mat- tresses, soft furnishings and carpets. Acaricides can kill mites, providing they are applied correctly, although they generally have very little effect on HDM allergen levels. For controlling HDMs, benzyl benzoate is the most commonly used acaricide, although an increasing number of products use pyrethroids, such as permethrin. In vitro studies normally show acaricidal products to be highly effective, causing rapid HDM death Hart, Guerin Nolard, 1992; Hyden et al., 1992. However, Colloff and colleagues 1992 reported that the high level of mortality observed in the laboratory from the use of aca- ricides cannot be simply reproduced in the home. Also, Tovey Marks 1999 highligh- ted the importance of ensuring that acaricides made available to the public are not only subjected to in vitro studies, but that they also have been properly field tested in domes- tic environments. Moreover, De Boer 1998 questioned the ability of acaricides to pene- trate deeply into upholstered furniture and mattresses, thus reducing their potential effi- cacy. In such a situation, it may be more effective to make bedding and upholstery with House dust mites 110 on one spot for a prolonged period of time and a considerable proportion of the HDMs will be shaded from the light by their environment.

3.3.6. Physical control methods

3.3.6.1. Habitat modification

HDMs inhabit a diverse range of habitats within the home, and by removing or modi- fying these habitats it is possible to reduce the potential for mite population development. Habitat modification can also make rooms easier to clean, thus aiding the removal of HDM allergens.

3.3.6.2. Carpets

Physically, carpets provide an excellent habitat for HDMs, particularly near the edges of beds, chairs and sofas where they are showered with skin scales Colloff, 1998. Platts- Mills and colleagues 1996 suggested that fitted carpets, together with increased indoor temperatures and decreased ventilation, are among the housing-related changes that have increased the prevalence and severity of asthma. Carpets are one of the major habitats of HDMs and can contain the largest reservoir of mite allergens in the house Tovey, 1992. However, carpets favour a somewhat different hygrothermal environment than the rest of the room. Van Bronswijk 1981 cites a 1966 study by Leupen Varekamp that shows that they tend to be cooler and damper. This is particularly the case where carpets are laid directly onto a concrete and screed ground floor. Hygrothermally, carpets are dis- tinct from beds and upholstery in that they do not generally benefit from proximity to human warmth and moisture. Studies have shown that long or loose-pile carpets tend to harbour more mites and allergens than short-pile carpets or hard floors Arlian, 1989, for example. Carpets in homes are typically replaced with hard flooring, such as wood, tile and lami- nate. When properly fitted, these hard surfaces are inhospitable to HDMs and, by remo- ving one of their major habitats, it is possible to reduce the number of HDMs and there- fore the amount of allergen produced. Hard floors are also more readily cleaned than soft floors. Mulla and colleagues 1975 found that vacuum cleaning removed nearly all of the mites and allergens from hard floors. Because dust may become more easily air- borne from a hard floor than from a carpet; it is essential that hard floors be cleaned on a regular basis.

3.3.6.3. Soft furnishings