Housing characteristics and mite allergen levels: to humidity and beyond

Abstract

There is a dose–response relationship between mite allergen exposure and sensitization. Sensitization to dust mite allergens is an important risk factor for asthma [1]. However the importance of exposure in progression from sensitization to established asthma is far less clear [2]. Nevertheless, evidence is mounting that in mite-allergic asthmatic patients mite allergen exposure increases the severity of disease [3,4]. Therefore, the knowledge of the household characteristics which are associated with high mite allergen levels is important in a public health context. Mean levels of mite allergen (Der p 1 or Group 1 Dermatophagoides spp. allergen) in different areas of the world are very different. For example, the mean levels of Der p 1 in infants' mattresses in cold, dry Sweden is much lower (0.12 µg/g) [5] than Der p 1 levels in mattresses in warm, humid Sydney (22.5 µg/g) [6]. Mean allergen levels in different geographical areas tend to be influenced by the local climate. However, within any geographical area the variation in mite allergen levels between individual homes is greater (c.1000-fold) than the variation in mean mite allergen levels between different geographical regions (c. 200-fold). To date, there are 15 published studies from around the world that have investigated the relationship between household characteristics and mite allergen levels (summarized in Table 1) [7–21]. Five of the studies were too small for multivariate analyses [12–15,21] and five recruited only patients with asthma into the study [12–15,17]. Despite the fact that the data collected on household characteristics varied greatly between the studies and that building materials and techniques are very different in different parts of the world, some common themes have emerged. Indoor humidity is an important determinant of dust mite population growth. House dust mites live in an environment where there is no liquid water, and are dependent upon ambient humidity to absorb water from the atmosphere. Passively, water is gained by diffusion through the body. Actively, they extract water vapour from air via hygroscopic crystals in their supracoxal glands, located at the base of their first pair of legs [22]. When humidity is low, the salts crystallize and block the entrance to the glands, thus slowing the rate of dehydration. The most useful parameter to measure when considering the water balance of house dust mites is relative humidity (RH). The optimum RH for mite growth is 75–95%, at temperatures of 15–30 °C. RH has a major influence on the survival of mite colonies and therefore levels of mite allergens. Outdoor RH influences indoor RH, but other household factors can influence mite allergen levels. So, is there a clearly established link between indoor RH and other housing characteristics and mite allergen levels? Although laboratory and early field studies suggested there was a strong relationship between RH and mite allergen levels, this has not been confirmed by more recent large-scale studies when other factors have been considered in a multivariate analysis. In only 10 of the 15 listed studies were actual measurements of indoor RH taken, and some of these on only a single occasion. Of the 10 studies, one found no association between Group 1 mite allergen levels and RH in the multivariate analysis [17], two found no association but performed univariate analyses only [9,21], three found a linear association, but were under-powered to perform a multivariate analysis [10,12,13], one found a univariate association which was lost in the multivariate analysis [20], and three found a significant association between higher indoor RH and higher mite allergen levels in the multivariate analysis, but only for the mattress (living room floor was not sampled in one study) [11,18,19]. Indeed, in one study the authors go on to comment that the complete model of the significant independent predictors explained only 18% of the variance in mite allergen levels in the mattress [19]. A novel technique has been developed recently which allows measurements of RH to be made within the mite microhabitat; that is, where it matters, in the depth of the carpet or mattress [23]. This has revealed that the RH in the carpet may be higher than that in the room air, and that with different types of construction, the differences between room RH and floor RH will vary. This suggests that the RH in the room air does not necessarily reflect the RH in the microhabitat of the mite, in the depth of the carpet pile. The study published by Wickens et al. in the current issue of the journal is the first to use this technique [24]. The authors did not find a linear association between carpet RH and Der p 1 levels, and go on to suggest that this is because of the ubiquitously high RH throughout homes in Wellington, all of which had at least one recording of RH greater than 50% (i.e. that once local humidity levels are adequate, other factors play a part). So, which other factors are important? In the study by Wickens, floor insulation (or a room/garage below the living room) was associated with significantly lower Der p 1 levels in the living room carpet. Floor insulation was associated with lower Der p 1 levels in one other study in smooth floors with rugs (but not in smooth floors without rugs or carpeted floors) [10]. Of the eight other studies which have analysed the effect of a room/garage directly below the room being sampled, three found no effect [9,10,15] and five showed variably lower Der p 1 levels [7,11–13,16]. Visible damp was associated with higher total Der p 1 in nine of the 15 published studies [8,10,11,13–16,19,20] and also in the Wickens study [24]. In a previous study of household characteristics and mite allergen levels in New Zealand [18], carpeted floors had higher Der p 1 levels than smooth floors (GM Der p 1 26.9 µg/g from 459 homes vs. 2.6 µg/g, but in only 11 homes), but these were still higher than levels in carpeted floors in Europe (1.9 µg/g) [7]. Fitted carpets are popular in New Zealand, as they are in many temperate areas. In the current study the presence of 'insulation' was associated with a reduction in Der p 1 levels of less than two-fold (27.5 µg/g v 51.9 µg/g). Clearly, the best way to reduce Der p 1 levels in floors is to get rid of the carpets and improve the housing to reduce visible damp, which would seem to be far more effective than insulating floors. Some common themes have emerged from the world literature. Factors which tend to be associated with higher mite allergen levels are: increasing numbers of occupants, older homes, presence of damp in the home, presence of carpets vs. smooth floors, older carpets and older mattresses (Table 1). At the present time, the only way to guarantee lower mite allergen levels in modern homes in the western world is to remove the carpets and to encase the mattress and bedding [25].