"Indoor allergens" is a term coined by allergists to include allergens produced by dust mite, cockroach, mouse, rat, pet dander and mold (Diette et al. 2008). It is a useful term because exposure to most of these allergens occurs indoors. While mold fungi clearly can grow indoors, most exposure occurs outdoors, so they will not be included here.
There are two general approaches to the control of indoor allergens. One can attempt to control the sources (e.g., the dust mites, cats, cockroaches, etc.), or one can attempt to remove the allergens (e.g., the actual proteins that cause sensitization). Which of these two methods is preferred depends on the allergen source.
It is also important to note that the development of allergies is a complex process involving genetics and the environment. While environmental control can reduce symptoms in allergic individuals, there is conflicting evidence as to whether or not environmental control reduces the risk of developing allergies in susceptible individuals (Bush 2008).
Dust Mites
The study of indoor allergens began when the dust mite allergen was discovered. Before that time, patients with indoor symptoms were skin tested with extracts of house dust, usually from some standard source other than their own environment. Those that tested positive were considered "house dust" sensitive. Patients with house dust sensitivity were given a regimen to render their home "dust free." This included:
Remove all carpeting.
Use only washable drapes and comforters.
Minimize the use of upholstered furniture.
Remove all dust catchers (knick knacks, books, etc.).
Vacuum regularly with the sensitive person not doing the vacuuming.
The allergen control procedures described above are still advised today. However, a group of Dutch researchers, after long and persistent effort, identified the house dust mite as the source for house dust allergens (Dieges 2004). With that discovery, there was a specific target for remediation efforts and the study of the house dust mite intensified. Research revealed that the dust mite did not drink water, but absorbed it from the air (Arlian & Morgan 2003). The dust mite needed a place to live where both humidity and temperature remained high. Bedding turned out to be the ideal "nest" for these organisms.
Figure 1: Magnified photograph of a house dust mite.
Source: U.S. Food and Drug Administration
The requirement for high humidity indicated that lowering humidity could possibly control dust mite populations. Also, using barriers to prevent entry of dust mites into bedding could possibly prevent exposure. A number of studies confirmed both of these hypotheses and also the fact that washing bedding in hot water would kill dust mites and remove the allergen. Thus the industry of mattress and pillow encasings developed, and allergy sufferers returned to the hot water cycle of their washing machines. Lowering humidity was a more difficult problem. In occupied beds the humidity is inevitably high due to respiration (which yields CO2 and H20). Fortunately, the encasings and hot water washes were generally sufficient to control populations. The fact remains that the dust mites can also "nest" in carpeting and upholstered furniture providing the humidity is high. In the very humid parts of the world the only practical solution is to follow the early advice to remove carpets and upholstered furniture. If humidity is only high part of the year (as in the Northeastern U.S.), rugs can be used in the winter and stored in the summer. Our great grandparents did this and so can we. In very dry climates dust mites are much less of a problem and avoiding the addition of water to interior spaces will generally control their populations.
Chemical treatments have been studied for the reduction of dust mite allergens in carpeting. Benzyl Benzoate, 3% tannic acid spray and two carpet cleaners successfully reduced dust mite allergen concentrations in carpeting for approximately 2 weeks. Treatments were not effective for cat allergen. Other types of chemical treatment have not been sufficiently tested to warrant recommendation. Washing bedding in hot water continues to be recommended for control of dust mite allergen, although controlled trials have not been done to evaluate efficacy at preventing symptoms.
Cockroaches
Cockroaches were first linked to allergic disease in 1964 by Bernton and Brown. Of the 43% of the U.S. population that is allergic to at least one common indoor allergen, 26% are sensitive to German cockroaches (Blattella germanica). Allergens from the German cockroach and the American cockroach (Periplanata americana) have been isolated and are officially recognized.
The German cockroach colonizes the built environment, especially where food is prepared. German cockroach infestation is most common in the Northeastern U.S. The American cockroach generally lives outdoors or in sewers, but can also be found as the dominant indoor roach in tropical regions. Cockroaches have long been thought to carry infectious diseases, and some pathogens can be found on their bodies. However, there are no documented cases of any infection directly transferred from cockroaches to people.
Figure 2: Cockroach species that commonly inhabit human dwellings include the following: German cockroach (Blattella germanica); American cockroach (Periplaneta americana); Oriental cockroach (Blatta orientalis); brown-banded cockroach (Supella longipalpa); Australian cockroach (Periplaneta australasiae); smoky-brown cockroach (Periplaneta fuliginosa); and brown cockroach (Periplaneta brunnea).
Source: Centers for Disease Control and Prevention
Exposure to cockroach allergens clearly leads to both sensitization and symptoms of asthma (Chew et al. 2008). Guidelines for exposure leading to sensitization, (2-4 Units Bla g 1 per gram of vacuumed, sieved house dust), and symptoms, (8 Units Bla g 1 per gram), have been suggested.
As for other indoor allergens, two approaches to exposure reduction may be employed. Cockroach populations can be suppressed, and the allergens may be removed by cleaning. Studies show that without cockroach suppression, cleaning is not an effective means for significantly reducing allergen loads. However, pest control alone can be effective. Studies evaluating the efficacy of commercial pest management were disappointing (Sever et al. 2007). However, research entomologists were able to achieve significant population reductions. The steps they used are as follows:
18 cockroach traps placed throughout the home for 1 week.
Bait applications applied with amounts depending on number of cockroaches at each trap site.
Bait used: 2.15% Hydramethylnon (Maxforce Roach Killer Gel Bait; Bayer Environmental Science, Research Triangle Park, NC).
Baits placed: months 0, 1, 3, 6, and 9 in areas where traps had cockroaches.
New traps placed at each visit.
So, to put this into practical terms: Each home must be entered first to place traps, then a week later to place bait. Traps are placed where cockroaches are most likely to be. Bait is applied wherever there are cockroaches in the traps. Each time new bait is applied, new traps are laid. This process is continued approximately monthly until no more cockroaches appear in the traps.
Rodent Pests
Rodent infestation is common. More than 50% of randomly selected U.S. homes had detectable mouse allergen (Cohn et al. 2004) and prevalence was even higher in the inner city, especially in detached homes (Berg et al. 2007). Asthma is an epidemic in the inner city, and sensitization to rodent allergens is a risk factor for having asthma (Donahue et al. 2008). Rodent allergens are also common in classrooms.
Integrated pest management is required to reduce infestations with rodents. This includes application of pesticides, plugging access to the indoors, and preventing access to any food. The latter includes storing dry foodstuffs in sealed metal containers. Thorough, vigorous, and frequent cleaning to remove residual allergens must follow pest management. Even with intensive management and cleaning, rodent allergen was shown to only fall by about 42% in one study (Eggleston 2005).
Figure 3: Mouse and rodent allergens are common in houses and classrooms.
Source: U.S. Department of Housing and Urban Development
Cats
Cat allergen is a conundrum. Individuals do become sensitive to cat allergen, and develop asthma as a result. However, in epidemiological studies, concentrations of cat allergen are correlated with a decreased risk of asthma (Takkouche et al. 2008).
Cat allergen cannot be completely removed. A cleaning protocol that included vacuuming, dry mopping, then wet mopping and shaking pillows and small rugs outdoors reduced dust levels and, to a small extent, cat allergen concentrations (Adgate et al. 2008). The type of carpet apparently does not affect the ease of removing cat allergen, but allergen can be removed more easily from new rather than from old carpets (Causer et al. 2008). High efficiency filtration in a central forced air system was found to remove up to 55% of airborne cat allergen.
Removal of the pet cat will eventually lead to allergen reduction after 4-6 months. Thorough cleaning and dust removal will speed this along. Room air cleaning is not an efficient method for removing exposure (Diette et al. 2008), nor is washing the pet or isolating the pet from the bedroom (Eggleston 2005).
Dogs
Sensitization to dog allergen is strongly associated with asthma (Korppi et al. 2008). As with cat allergies, early exposure to dogs may lessen the risk of sensitization and subsequent asthma development. However, this effect, while seen in univariate analyses, disappears when multivariate techniques are used.
Removal of dog allergen is similar to that described for cat allergen. Dogs can survive as primarily outdoor animals, whereas the average life span for an outdoor cat is only about 2 years.
Other Indoor Allergen Sources
Almost any animal, fungus or plant can release allergens into the indoor environment. In order to decide on a remediation protocol, the nature of the source and the dynamics of allergen dispersal need to be considered. For mammals, allergen is generally contained in skin secretions, saliva, and/or urine. For arthropods, feces and eggs may contain the allergen, or secretions may be at fault. For many plants, the majority of allergens are contained in pollen. In each of these cases, exposure prevention involves removing the source, or somehow blocking the pathway of the allergen. In domestic environments, removal of sources is by far the best approach to reducing allergen exposure.
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