Control of the House-Dust Mite, Dermatophagoides Pteronyssinus (Trouessart) (Acarina, Pyroglyphidae) and the Management of Associated Allergens on Tufted Wool Carpet

Abstract

Carpet is a reservoir for house-dust mites and their faecal pellets, both of which contain protease enzymes known to be important triggers for asthma. Eliminating mites from floorcoverings (e.g. by replacing with hard flooring or applying acaricides) may lead to a reduction in the concentration of allergenic species, but it is unlikely to result in their complete removal. Irrespective of construction or presence of mites, floorcoverings will act as sinks for allergen produced elsewhere in the house (e.g. in bedding). However, the effect of floorcovering type/construction on the propensity for such material to build up, and its ease of removal, is poorly understood. In addition to comparing the performance of hard and soft floorcoverings, the validity of three approaches to reduce allergen levels in soft floorcoverings was investigated; (1) Modification of carpet structure to provide a habitat less favourable for house-dust mite proliferation, allergen accumulation, and disturbance, (2) Removal of mite allergen from carpets using different cleaning methods, and (3) Chemical intervention (incorporated at the time of carpet manufacture) to control house-dust mite numbers. Modification of the carpet construction to produce a microenvironment (relative humidity, temperature and light intensity) less conducive to mite survival was shown not to be a feasible method of controlling mite numbers. Differences produced by varying carpet parameters, such as pile height, stitch density, yarn twist, colour, fibre and underlay type, were only slight, while varying external stimuli, such as light level, also had little effect on mite behaviour/survival. In contrast, laboratory studies showed many of these same parameters (in particular, degree of wear, pile height, and density) had a marked influence on the tendency for carpets to accumulate particulate matter, or to release allergen during cleaning. Carpet structures that favour low build-up rates (low pile height, unworn, low pile density, loop pile) were also identified, as were the areas where allergen accumulates within different types of carpet (primarily near the base of the pile). However, while a field comparison revealed lower levels of allergen and mites on hard flooring, few differences were found between carpet types. An investigation of different cleaning regimes (wet vs. dry) found both resulted in similarly high allergen removal rates from unworn carpet, but that these decreased markedly with carpet wear. Investigation of a range of existing chemical finishes applied conventionally to wool carpets showed none to be effective at controlling house-dust mites; however, the delivery mechanism for acaricides was also shown to be a major factor in determining efficacy. By varying this, a number of new techniques were developed to restrict acaricides to the fibre surface (where they are likely to be more effective), yet preserve fastness properties. Industrial and field trials of these techniques were carried out. From the experiments and their findings, a set of recommendations for floorcovering selection and maintenance have emerged that will assist mitesensitised individuals to create low allergen environments. Keywords: House-dust mite (Dermatophagoides pteronyssinus, allergen, Der p I, Der f I, removal, vacuuming, cleaning, microclimate, carpet, permethrin, wool, aeroallergens.