Justification for Dust Mite Exposure Control
The decision to initiate environmental controls to reduce dust mite exposure can be complex. Total prevention of exposure to mite allergenic material to prevent IgE sensitization to mite allergens in genetically susceptible individuals requires strict, continuous avoidance of mite exposure, which is practically all but impossible [199]. Furthermore, to curtail development of all cross-reacting specific IgE, avoidance of all arthropods would probably be required [200]. The majority of the world's population lives on seacoasts [201] or along rivers [202] and these areas typically have adequate humidity to support growth of dust mites and storage mites during all parts of the year.
Much research has been conducted to determine if it is possible to reduce development of mite-specific IgE-mediated sensitization (primary prevention). Several studies comparing dust mite sensitization rates in children from areas endemically low and areas endemically high in dust mite allergen indicated that the prevalence and degree of sensitization to dust mite was strongly associated with the amount of exposure to mite allergens [203, 204]. A prospective study of mite allergen avoidance in Manchester, UK, [205, 206] using a combination of interventions, decreased Der p 1 from mattresses by 97% to the nanogram range during pregnancy and 12 months after birth in the active group [205]. However, with all possible dust mite exposures at homes of friends and family, on public transportation and in public places and at schools and day care centers, primary prevention of dust mite sensitization by mite allergen avoidance may not be possible [207, 208, 209].
Secondary prevention, or the attempt to reduce the risk of asthma in dust mite sensitized children has also received much attention. The link between asthma and dust mite exposure is one of the most extensively studied relationships between environmental exposure and disease development [210, 211, 212, 213]. In all climates conducive to the growth of dust mites, mite exposure may be one of the factors contributing to the development of asthma [112, 214]. Secondary prevention has also been the goal for many children with allergic rhinitis who are at risk of the subsequent development of asthma. However, to date there is no evidence-based information as to whether mite avoidance may be effective as a secondary preventive measure to prevent/delay asthma development among mite-sensitized individuals, or those with allergic rhinitis.
The relation of dust mite allergen exposure and the worsening of allergic respiratory symptoms is well documented [215]. In one study of 311 subjects both sensitized and exposed to high levels of indoor allergen including dust mite allergen there was significantly lower FEV1% predicted values (mean, 83.7% vs 89.3%; mean difference, 5.6%; 95% CI, 0.6%-10.6%; P = .03), higher eNO values (geometric mean [GM], 12.8 vs 8.7 ppb; GM ratio, 0.7; 95% CI, 0.5-0.8; P = .001), and more severe airways reactivity (PD20 GM, 0.25 vs 0.73 mg; GM ratio, 2.9; 95% CI, 1.6-5.0; P < .001) as compared with subjects not sensitized and exposed [216]. Adults in a 4-year study who were both sensitized and exposed to high levels of dust mite allergens had increased bronchial hyper-responsiveness [217]. Many additional links between dust mite exposure and allergic disease are documented in the recent environmental practice parameter on dust mites [198]. A reduction in the symptoms experienced by those with atopic dermatitis has also been linked to house dust-mite allergen avoidance [218].
Facilitative factors and Allergen Reservoirs
Controlling factors that facilitate the growth and reproduction of dust mites has been an often sought goal in exposure control. The dependence of dust mites on the water content of the air has been extensively documented [219, 220]. Arid climates have an intrinsically low abundance of dust mites, and the most effective method of controlling dust mite exposure is to live in a very dry climate such as the high desert of New Mexico in the US or the Altiplano or Bolivian Plateau, in west-central South America [202]. Since this is not a practical solution, mimicking these conditions in the home environment as much as possible provides an opportunity to control mite population growth.
Humidity control should be the mainstay of any mite control efforts. The most important factor facilitating dust mite growth, reproduction and allergen production is the availability of water in the surrounding environment [220]. Mites absorb moisture directly from their surroundings under conditions of high moisture and lose water when moisture is low. The mite moisture equilibrium therefore is not directly relative humidity dependent. It is instead dependent of the moisture situation of the local microenvironment and the moisture retention ability of the mite's immediate surroundings such as carpet dust reservoirs or bedding. A simple measurement of relative humidity may not assure an environment free of dust mite activity. Microenvironments that exist in bedding, in carpet next to concrete or in pet lounging areas may provide adequate moisture for mite survival in climates not expected to have a mite presence. A mite surrounded by a hygroscopic microenvironment as moist bedding can survive much dryer conditions than would be expected. Of note, exposure to a moisture rich environment for only a short period can provide enough moisture for growth and metabolism [221].
Although directly linked to water content of the air in the calculation of relative humidity, temperature is also a factor in dust mite survival. Conditions at the extreme ends of the temperature spectrum, either to cold or to hot can impact mite survival although elevated temperature conditions tend to be more lethal than freezing. Mites and their eggs survive poorly when exposed to hot water and clothes dryers but survive during short periods of freezing conditions. The exposure to direct sunlight is an often forgotten factor in the destruction of dust mites [222].
It is not enough to address mite factors facilitating mite population growth. Reservoirs of mite allergen must also be eliminated. House dust mites can be found in any area of the home, however they are most often associated with certain indoor environments including the bedroom carpet, mattresses and bedding, frequently occupied upholstered furniture and in pet lounging areas [223, 224]. Recent investigations have questioned the traditional concepts of the location of dust mite reservoirs indicating that significant exposure can occur in public transportation conveyances and associated with work environments as well as clothing [207].
Climate Factors
Although residents of cold and arid climates are less likely to be exposed to house dust mites, the large majority of the world population is exposed to house dust mites. Nearly half of the people in the world live within 200 km of the coast where humidity levels are typically higher. The rate of population growth in coastal areas is accelerating. In China alone over 400 million live in coastal cities. Dust mite exposures and the allergic problems related to those exposures are likely to increase [201].
Although many climates are naturally conducive to mite growth and allergen production, the artificial control of indoor climates is increasing. Even though it is energy intensive, the use of forced air heating and air conditioning is growing around the world and especially in more affluent economies. Dust mite allergen exposure control is therefore a viable option for large numbers of persons. In many areas seasonal heating requirements result in very dry indoor environments and subsequently dust mite exposure is a seasonal phenomenon. Low humidity conditions can also be obtained through use of air conditioning and dehumidification. Yet, in many areas of the world ambient humidity levels are high enough that producing low humidity levels sufficient to preclude dust mite growth is not practically achievable. The recent Cochrane study on dehumidification alone indicates that evidence of clinical benefits of dehumidification using mechanical ventilation with dehumidifiers is scanty [225]. Indeed, the meta-analysis of multiple dust mite control studies would lead the reader to believe that there is nothing that can be physically done to control dust mites and improve health. Yet, this conclusion is disputed by many experts in the field of allergy [226]. Furthermore, the nature of single source exposure control studies may preclude successful clinical improvement because allergen sensitization is typically to multiple agents.
A significant amount of work has been done on removal of mites and mite allergens through cleaning. It goes without saying that efforts to control mite infestations of the skin and remove mite infestations from clothing are essential in the maintenance of overall health [227]. Humans have been living with dust mites for generations and they might even be described as among our "old friends" [228]. But no physician would advocate for wearing mite infested clothing or sleeping in mite infested bedding. Mite sensitization is likely to occur in genetically susceptible individuals, therefore efforts to reduce instances of elevated mite exposure and thus reduce allergic symptoms are only prudent [229].
Since mite allergens are located in known areas of a typical house [229, 230] removing mite allergen reservoirs is a very effective way to reduce mite allergen exposure. Efforts to remove carpets, drapes, upholstered furniture and any other fabric covered objects from the living environment can effectively reduce mite allergen exposure. The extent to which these items are removed will ultimately be a matter of personal preference. Since mite allergens are known to be heavy and not aerodynamically suited for airborne disbursal [34] and high humidity microenvironments are known to exist in bedding it is logical to focus dust mite reduction efforts on bedding. Efforts to enclose mattresses, box springs and pillows in mite-impermeable covers are known to be very effective [231]. However, it is important to mention that the efficacy of allergen avoidance in patients with already established rhinitis or asthma is a matter of debate [232, 233, 234, 235].
Washing bedding in hot water and even with bleach and drying bedding in very hot conditions or even in direct sunlight are known to reduce both the presence of mite allergen and the mites themselves [236, 237]. Washing bedding and clothing removes mite allergens and kills mites. Most of the killing is through drowning, although washing in hotter water kills more mites. The temperature used to wash bedding has become an issue. Elevated temperatures are more energy intensive and hotter water is a scalding hazard. Experts agree that washing is better than not washing and washing with water that is 48° Celsius provides optimum mite killing and home safety [199].
Heat treatment can be effective in killing mites and their eggs. Treatment of cloth at 95° Celsius killed all mites present [238]. However, treatment at 40 °C under dry and wet conditions allowed approximately 80% of all mite eggs to survive. Under dry heat at 50 °C, the thermal death point of dust mite eggs occurred at 5 h and at 60 °C death occurred almost instantaneously [239]. Presumably the eggs survive heat better than the mites themselves. Homes treated with heat-steam over a period of months showed a sustained reduction of Der p 1 and Der p 2 compared to sham treated homes [240] However, mite allergens have been demonstrated to be stable even at elevated temperatures [241].
Although the practice has fallen into senescence in the modern world of appliances, there was a time when frequently placing bedding in direct sunlight for several hours was practiced in many cultures. It has been demonstrated that ultraviolet irradiation is lethal to many organisms including dust mites [242, 243].
Many harsh chemicals are known to kill dust mites or denature mite allergens in industrial and household settings. Agents like tannic acid, Benzyl benzoate, Disodium octaborate tetrahydrate, tri-n-butyl tin maleate, pirimiphos methyl and even "essential oils" like methyl eugenol have been described in the literature to effectively kill mites [244, 245, 246, 247, 248]. However, they are all dangerous at some concentration and cannot be recommended for use by patients or homeowners [199].
It has been suggested that freezing can be effective in killing dust mites and the recommendation to place small cloth items like stuffed animals in the freezer compartment of house hold refrigerators has been frequently given out by allergists. However, there is little evidence that this is effective. There may be some mite death due to desiccation in the dry environment of a household freezer. But, dust mite eggs have been shown to resist freezing at temperatures above −70° Celsius [222]. And, freezing is not effective in removing dust mite allergen from reservoirs because dust mite allergen is stable at low temperatures for extended periods of time [239].
Air conditioning would have a twofold impact on dust mite populations. The cool temperatures will slow mite metabolism and reproduction and reduce moisture need for mite survival. Microenvironments or increased humidity can be reduced using a dehumidifier and/or air conditioning. The absence of air conditioning has been shown to be a factor contributing to increased mite allergen levels in US homes [249]. Air conditioners must be operated for a long time to remove sufficient moisture from the air to effectively decrease room humidity. Mechanical ventilation heat pump recovery units in the UK failed to achieve the desired mite reduction results [250].
Evidence on clinical benefits of dehumidification using mechanical ventilation with dehumidifiers remains scanty [225]. Although dehumidification and air conditioning doubtlessly reduce overall dust mite exposure [251], the difficulty in using dehumidification alone in damp environments to decrease dust mite antigen exposure has been described in a recent Cochrane review [225].
Summary of current recommendations
Most publications on allergy and dust mite control would agree that a comprehensive program of personal hygiene, bed hygiene, properly fitted allergen-impermeable covers, cleaning, dehumidification or air conditioning and appropriate food storage in very damp climates can reduce exposure to house dust mite allergens. It is a stretch further to conclude that the above steps can improve symptoms in those already allergic to dust mites. However, depending on the sensitivity and life style of the allergic person, prudent efforts over an extended period of time are likely to result in gradual improvement in health. The fact that current studies do not provide sufficient evidence for critical reviews to conclude there is unequivocal benefit is no reason to abandon logical and prudent efforts to reduce mite exposure.
