Key Points
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The major indoor allergens include dust mite, animal danders, cockroach, rodents and mold.
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Indoor allergen exposure and sensitivity should be considered for all patients with chronic allergic symptoms, including asthma, allergic rhinitis and atopic dermatitis.
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Allergen avoidance should be considered the first line of therapy for patients with indoor allergen sensitivities.
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Allergen avoidance should be approached with a comprehensive specific environmental control strategy based on the patient’s sensitivities and exposures.
There is no doubt that aeroallergens play a major role in the pathogenesis of allergic disease, including asthma, allergic rhinitis and atopic dermatitis. Among these, the indoor allergens are of particular importance. These principally include the allergens of house dust mites, domestic pets, molds and pests such as cockroaches and rodents. The relative importance of these different allergens varies in different environments depending on a variety of geographic, climatic and socioeconomic factors. All studies agree, however, that children with asthma have a high likelihood of becoming sensitized to whichever of these allergens are prominent in their local environments. This chapter focusses on the possible role that allergen avoidance may play in the management of allergic disease.
As a general concept, it is important to recognize that allergen avoidance should be based on knowledge of the patient’s specific allergic sensitivities as well as their environmental exposures. Based on this information, it is important to recommend a comprehensive strategy to reduce exposure to as many relevant allergens as possible. In fact, most of the studies in which environmental control has been proven effective are those that utilize a multifaceted approach tailored to the patient’s sensitivities and environment, which requires both thoughtful consideration and detailed patient education.
Dust Mites
Dust mites are arachnids that live in the dust that accumulates in most homes, particularly the dust contained within fabrics. Favorite habitats include carpets, upholstered furniture, mattresses, pillows and bedding materials. Their major food source is shed human skin scales, which are present in high numbers in most of these items. The major dust mite species known to be associated with allergic disease are Dermatophagoides pteronyssinus and Dermatophagoides farinae. Other mites, including Euroglyphus maynei and Blomia tropicalis, are also important in some areas, although their distribution is considerably more limited. Dust mites grow optimally in areas that are both warm and humid, and they grow very poorly when the relative humidity remains below 40%. Dust mites grow from eggs to adults over the course of about 4 weeks and adult dust mites live for about 6 weeks, during which time females produce 40 to 80 eggs.
Assessment of dust mite exposure has been accomplished largely through the analysis of settled dust samples. Although some studies have not been able to show a relationship between dust mite levels and allergic sensitization or disease activity, there is now general agreement that dust mite levels of greater than 2 µg of group 1 allergen per gram of dust should be considered a risk factor for sensitization and that levels greater than 10 µg/g of dust are a risk factor for increased asthma morbidity. Airborne sampling for dust mite allergen has not proven useful in assessing exposure.
The prevalence of dust mite sensitivity in patients with asthma or allergic rhinitis varies considerably from one geographic area to another. For example, studies have demonstrated sensitization rates ranging from 5% in asthmatic children in Los Alamos, New Mexico, to 66% in Atlanta, Georgia, to 91% in Papua, New Guinea. These differences are roughly proportional to differences in mite exposure in these areas of the world.
In addition to the relationship between mite exposure and mite sensitization, there is also evidence that mite exposure contributes to the development of asthma. In a prospective trial, Sporik and colleagues demonstrated a significant increase in asthma, as well as mite sensitivity, in 11-year-old children who had experienced high mite exposure during infancy. Other studies have demonstrated a striking association between asthma development and mite sensitivity, although these studies lacked a prospective evaluation of mite exposure. While these studies suggest that mite avoidance early in life could potentially prevent the development of asthma, studies of prevention have yielded inconsistent and mostly disappointing results.
Extensive evidence also exists to support a relationship between ongoing mite exposure and disease activity. With regard to chronic symptoms, Vervloet and colleagues demonstrated a significant correlation between medication requirements and current mite exposure in a group of mite-sensitive adult asthma patients. Custovic and colleagues also demonstrated a relationship between mite exposure and asthma severity as evidenced by bronchial hyperreactivity (BHR), peak expiratory flow rate variability and forced expiratory volume in 1 second (FEV 1 ). Several studies have also demonstrated mite exposure to be a risk factor for acute asthma and emergency room visits. The most compelling evidence for the role of dust mites in asthma comes from studies of allergen avoidance, either through environmental control in the home or the removal of mite-allergic patients from their homes. Two classic studies from the early 1980s provided dramatic evidence for the potential benefits of dust mite avoidance. Platts-Mills and colleagues investigated the effects of mite avoidance by placing nine young adults with mite-induced asthma in a hospital setting for a minimum of 2 months. All patients experienced reduced symptoms, seven had reduced medication requirements, and five showed at least an 8-fold reduction in bronchial reactivity. In the second study Murray and Ferguson studied 20 mite-allergic asthmatic children in a controlled trial of mite avoidance in the patients’ homes. They found significant reductions in asthma symptoms, days on which wheezing was observed, days with low peak flow rates, and BHR in the group using active mite control measures.
Most subsequent trials of mite avoidance have yielded similar results. Ehnert and colleagues studied 24 children with asthma and mite sensitivity in a 1-year trial of mite avoidance. The patients were divided into three groups. The first had their mattresses, pillows and comforters covered with impermeable encasements; the second had their mattresses and carpets treated with an acaricide (benzyl benzoate); and the third had their mattresses and carpets treated with placebo. Significant reductions in dust mite allergen levels were found only in the group with mattress and pillow encasements. Similarly, a highly significant reduction in BHR was noted in that group compared with the other two. In another study, Peroni and colleagues studied mite avoidance by moving asthmatic children to a high-altitude environment and demonstrated significant reductions in total immunoglobulin E (IgE) levels, dust mite-specific IgE levels, methacholine reactivity and response to dust mite bronchoprovocation.
However, it is also important to recognize that there have been several important negative studies of mite avoidance, especially when using a single intervention such as bedding encasements, and that meta-analyses on the efficacy of mite avoidance have yielded conflicting results. For example, in one study Woodcock and colleagues studied the efficacy of impermeable bed covers in over 1,100 adults with asthma and dust mite sensitivity. While the impermeable covers resulted in significant decreases in mite allergen in mattress dust, asthma symptoms were not significantly reduced. These studies again point to the need for a comprehensive allergen reduction plan rather than relying on single interventions.
Dust Mite Control Measures
Although a variety of approaches to dust mite control have been studied, there is still some controversy as to the specific measures that are necessary to reduce mite exposure sufficiently to control disease. This controversy arises for three major reasons. First, some environmental control measures have not been adequately studied to make any firm conclusions. Second, for some measures, studies of their efficacy have yielded conflicting results. Third, in many studies a combination of environmental control measures was used, making it difficult to determine which measures actually led to the benefit that was observed. Specific environmental control measures will therefore be reviewed individually. They are summarized in Box 22-1 .
First Line (Necessary and Cost Effective)
Encase mattresses and pillows
Wash bed linens every 1 to 2 weeks, preferably in hot water
Remove stuffed toys
Regularly vacuum carpeted surfaces
Regularly dust hard surfaces
Reduce indoor relative humidity (dehumidify and do not add humidity)
Second Line (Helpful but More Expensive)
Remove carpets, especially in the bedroom
Remove upholstered furniture
Avoid living in basements
Third Line (Limited or Unproven Benefit)
Acaricides
Tannic acid
Air cleaners
It is very clear that allergen-proof encasements for mattresses and pillows significantly reduce dust mite exposure. In the study by Ehnert and colleagues, polyurethane mattress encasements produced a 91% decrease in mite allergen by day 14 of treatment, which rose to 98% by month 12 of the study. Encasements of the mattress, pillows and box springs should therefore be recommended for all patients with mite sensitivity. In addition, although encasements had been constructed of impermeable plastic or vinyl materials that were very uncomfortable, they are now also available in tightly woven fabrics that are considerably more comfortable.
The effects of vacuum cleaning on mite exposure have been studied. Live mites are difficult to remove from carpeting, and it is clear that vacuum cleaning in the absence of other measures will provide only limited benefit. However, regular vacuum cleaning does remove significant amounts of dust from carpets, which will at least help to reduce the allergen reservoir. Patients should also be warned that vacuuming creates considerable disturbance, with transient increases in airborne mite levels. Vacuum cleaners equipped with special bags or HEPA filters help prevent this problem and may be of some added benefit. There is some evidence that wet vacuum cleaning or steam cleaning may provide additional benefit, although one study showed that wet vacuum cleaning led to a subsequent increase in mite numbers.
A variety of carpet treatments, including acaricides such as benzyl benzoate and denaturing agents such as tannic acid, have also been developed in an effort to control dust mite allergen exposure. At best, both approaches provide only modest, short-lived effects and should not be recommended for routine use.
Because of the limitations of both vacuuming and chemically treating carpets, carpet removal is always best when feasible, especially from the bedroom of the allergic person. Bed linens, stuffed animals and other soft furnishings also provide excellent environments for dust mite growth. Objects such as stuffed animals should be removed whenever possible. The mite content of bedding materials and other objects that cannot be removed can be reduced by washing. Washing in hot water (greater than 55°C) is ideal in that it both removes allergen and kills dust mites. These water temperatures, however, may not be available in many homes due to safety concerns. It is important to note, therefore, that washing in cooler water does not kill mites but does remove most live mites as well as mite allergens very effectively. Weekly washing of all bed linens in a hot cycle is therefore recommended for all mite-allergic patients. Dry cleaning also kills dust mites, as does tumble drying at temperatures greater than 55°C for at least 20 minutes.
Dust mites are susceptible to the effects of low as well as high temperatures. Freezing in a typical household freezer for 24 hours will kill most dust mites, although the mite allergen in the object will not necessarily be reduced. Exposing carpets to direct sunlight for several hours will also kill dust mites because of the high temperature, the low humidity or both. It has also been shown that electric blankets will reduce mite growth. None of these methods have been established in clinical trials.
Because of the reliance of dust mites on humidity for growth, it has been suggested that methods capable of reducing relative humidity would be useful in the control of mite exposure. Korsgaard and Iversen demonstrated that dust mite growth could be significantly reduced by keeping indoor humidity below 7 g/kg by ventilation, whereas Arlian demonstrated that mite growth could be prevented by maintaining relative humidity below 35% for at least 22 hours a day. Air conditioning and dehumidification may also help to deter mite growth and should be used whenever possible; humidifiers should be avoided. It is clear, however, that achieving low humidity will be difficult or impossible in very humid environments. A prime example of this fact is the difficulty in eliminating dust mites from carpets over cement slab floors in basements.
Finally, air filtration devices are frequently purchased by patients for the control of their dust mite allergies; however, there is little evidence to support their use. One would logically not anticipate much effect because of the fact that dust mite allergens do not remain airborne for extended periods and would therefore not be available for filtration in most instances.
In summary, effective dust mite control can be accomplished in most homes with a combination of mattress and pillow covers, hot washing of bed linens, removal of stuffed animals and other soft furnishings, and carpet removal. Because of the convincing benefits provided through dust mite avoidance in mite-sensitive asthmatic patients, these measures should be routinely recommended, and compliance with these recommendations should be reassessed at each subsequent visit.
Animal Allergens
Animal allergens are also potent causes of both acute and chronic asthma and allergy symptoms. 4, Cat and dog allergens are the most important, although significant exposure to a wide variety of other furred animals is not uncommon. Sensitivity to cat and dog allergens is very common in asthmatic children, and in some settings these are clearly the dominant indoor allergens. This fact was best demonstrated in a study conducted in Los Alamos, New Mexico. In this environment, where cat and dog allergens are common but exposure to dust mite and cockroach allergens is rare, IgE antibody to cat and dog allergens was detected in 62% and 67% of asthmatic children, respectively. The presence of this IgE antibody was highly associated with asthma, whereas sensitivity to mite or cockroach allergen was not associated with asthma in this setting.
A number of studies have investigated the distribution of cat and dog allergens in the home and other environments. Using settled dust analysis, it has been shown that levels of cat and dog allergens are clearly highest in homes housing these animals. However, it is also clear from a number of studies that the vast majority of homes contain cat and dog allergen even if a pet has never lived there. This widespread distribution of cat and dog allergens has also been documented in a variety of other settings, including office buildings and schools. Whereas most of the environments with no animals have relatively low allergen levels compared to those with a cat or dog, it is not uncommon to find rather high levels in some of these homes. This widespread distribution is presumed to occur primarily through passive transfer of allergen from one environment to another. The particles carrying animal allergens appear to be very sticky and, unlike dust mite allergens, can be found in high levels on walls and other surfaces within homes.
The characteristics of airborne cat allergen have also been extensively studied. Cat allergen has been shown to be carried on particles that range from less than 1 µm to greater than 20 µm in mean aerodynamic diameter. Although estimates have varied, studies agree that at least 15% of airborne cat allergen is carried on particles less than 5 µm. Dog allergen is distributed very much like cat allergen, with about 20% of airborne dog allergen being carried on particles less than 5 µm in diameter.
Cat allergen can also be detected in air samples from all homes with cats and from many homes without cats. In an attempt to determine the clinical significance of this unsuspected cat exposure, patients were challenged in an experimental cat exposure facility to varying levels of cat allergen. It was found that allergen levels of less than 100 ng/m 3 were capable of inducing upper and lower respiratory symptoms as well as significant pulmonary function changes. These levels are similar to those found in homes with cats as well as a subset of homes without cats, suggesting that even patients without known cat exposure may be exposed to clinically significant concentrations of airborne cat allergen on a regular basis.
Control of Animal Allergens
At the present time much less is known about the control of animal allergens than about the control of dust mite allergens. In particular, there are still very few studies on the clinical benefit of environmental control measures for animal allergens. Although it is assumed that removing an animal from the home will lead to clinical improvement in patients who have disease related to their pets, even this simple concept has undergone little investigation. One prospective study did evaluate patients with asthma who were sensitized to furry animals, with some choosing to find their pet a new home and others electing to keep it. After 1 year, there was a significant improvement in airway hyperresponsiveness and a reduction in inhaled corticosteroid use in the pet removal group compared with the pet keeping group. Even fewer data are available regarding the potential benefits of methods that might be used in lieu of animal removal. The overall approach to the control of animal allergens is summarized in Box 22-2 .
Remove source (e.g. find a new home for the pet):
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Allergen levels fall slowly – benefit would not be expected for weeks to months.
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Follow by aggressive cleaning to remove reservoirs of allergen.
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Possible role for tannic acid to augment allergen removal.
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If the pet is not removed:
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Install air cleaners, especially in the bedroom.
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Remove carpeting, especially in the bedroom.
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Encase mattresses and pillows.
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Wash animals (not likely to help unless done at least twice a week).
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These measures may not reduce allergen levels enough to help highly allergic patients.
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To begin, it should be stated that in any asthmatic patient who is known to be cat or dog sensitive and whose asthma is believed to be related to a significant degree to the pet, the most appropriate recommendation is to remove the pet from the home. This is clearly the correct advice from a medical standpoint, and it should be recommended strenuously. A number of potential alternative measures will also be discussed, however, because of the high proportion of patients who are either reluctant or completely unwilling to remove a household pet.
Once a cat has been removed from the home, it is important to recognize that the clinical benefit may not be seen for a period of at least several months because allergen levels fall quite slowly after cat removal. In most homes, levels of settled dust will have fallen to those seen in homes without cats within 4 to 6 months of cat removal. Levels may fall much more quickly if extensive environmental control measures are undertaken, such as removal of carpets, upholstered furniture and other reservoirs from the home, whereas in other homes the process may be considerably slower. This information points to the fact that thorough and repeated cleaning will be required once the animal has been removed. It has also been shown that cat allergen may persist in mattresses for years after a cat has been removed from a home, so new bedding or impermeable encasements must therefore also be recommended.
A number of studies have investigated other measures that might help to reduce cat allergen exposure without removing the animal from the home. De Blay and colleagues demonstrated significant reductions in airborne Fel d 1 with a combination of air filtration, cat washing, vacuum cleaning and removal of furnishings, although these results were based on a small sample size and did not include any measure of clinical effect. When cat washing was evaluated separately in that study, dramatic reductions in airborne Fel d 1 were seen afterward. Subsequent studies, however, have presented conflicting results. Klucka and colleagues studied both cat washing and Allerpet/c (Allerpet, Inc., New York, New York) and found no benefit from either treatment. In addition, Avner and colleagues studied three different methods of cat washing and found transient reductions in airborne cat allergens after each. There was no sustained benefit, however, with levels returning to baseline within 1 week of washing. Results regarding dog allergen are very similar to those with cat, suggesting the need to wash dogs at least twice a week to achieve any meaningful reduction in allergen exposure.
Information is very limited as to the clinical benefits of these environmental control measures if one or more pets is allowed to remain in the home. Studies have evaluated different combinations of control measures, and although all have shown reductions in allergen levels, clinical effect was less consistent. Two studies showed a clear benefit, one showed benefit only in the group in which environmental control was performed along with intranasal steroid treatment, and the fourth showed no clinical benefit whatsoever. It therefore still remains to be seen whether allergen exposure can be sufficiently reduced to produce a clinical effect in the absence of animal removal.
While the notion of hypoallergenic cats and dogs is increasingly popular, there are no studies confirming that any specific breeds are predictably less allergenic. Further, there is no evidence that the size, hair length, hair type or degree of shedding have any effects on indoor allergen levels or allergenicity.
In families who insist on keeping their pets, the following should be recommended pending more definitive studies. The animals should be restricted to one area of the home and certainly kept out of the patient’s bedroom. HEPA or electrostatic air cleaners should be used, especially in the patient’s bedroom. Carpets and other reservoirs for allergen collection should be removed whenever possible, again focussing on the patient’s bedroom. Finally, mattress and pillow covers should be routinely used. Although tannic acid has been shown to reduce cat allergen levels, the effects are modest and short-lived when a cat is present, so this treatment should not be routinely recommended. Similarly, cat and dog washing appears to be of such transient benefit that it is only likely to add significantly to the other avoidance measures if it is done at least twice a week.
Cockroach Allergen
The importance of cockroach allergen in asthma and allergy has been recognized only over the past 30 years. It is now clear that cockroach allergens play a major role in asthma, particularly in urban areas. Significant cockroach exposure has been demonstrated in a number of cities, and the prevalence of cockroach sensitivity in urban patients with asthma has been shown to range from 23% to 60%. In addition, cockroach exposure has been associated with higher rates of sensitization. The combination of cockroach exposure and cockroach sensitization has been shown to be a risk factor for increased asthma morbidity and acute asthma exacerbations.
In the first comprehensive study on the problem of asthma in inner city children, 1,528 children with asthma from eight major inner city areas were extensively investigated with regard to the factors, both allergic and otherwise, that contributed to their disease. Although sensitivity to cockroaches, dust mites and cats were all common (36.8%, 36.9% and 22.7%, respectively), exposure to cockroach allergen was much more common than exposure to either dust mite or cat (50.2%, 9.7% and 12.8%, respectively). The combination of cockroach sensitivity and high cockroach exposure was associated with significantly more hospitalizations, unscheduled medical visits for asthma, days of wheezing, missed days from school, and nights with sleep loss because of asthma. Such a correlation was not seen for dust mite or cat allergens. These data argue persuasively that cockroach allergen is a major factor, if not the major factor, in the high degree of morbidity seen in this patient population.
Although there are at least 50 cockroach species in the USA, only four or five are domiciliary. Two species, the German cockroach ( Blattella germanica ) and the American cockroach ( Periplaneta americana ), are the most common causes of both household infestation and allergic sensitization. Several allergens from each species have been identified and characterized. The most important among these are Bla g 1, Bla g 2 and Per a 1. There is significant cross-reactivity between B. germanica and P. americana, although most patients in the USA are primarily sensitized to B. germanica. The source of the major cockroach allergens is still not completely clear, although they do appear to be secreted or excreted, suggesting that they may also be digestive proteins.
The distribution of cockroach allergens has been studied in a number of settings. The highest levels tend to be found in kitchens, although the allergen is widely distributed through the home, including the bedroom. In fact, in the inner city asthma study noted above, the 50.2% exposure rate was found in bedroom dust samples. It has been suggested that cockroach allergen levels of greater than 2 units per gram are associated with sensitization and levels greater than 8 units per gram are associated with disease activity. Cockroach allergen has also been detected at significant concentrations in schools in urban Baltimore. Finally, studies have shown that cockroach allergen is very much like dust mite allergen, with little or no measurable airborne allergen in the absence of significant disturbance.
Cockroach Allergen Control
Extensive study has been performed on the chemical control of cockroach infestation, and a variety of pesticides and traps are readily available. These include chlorpyrifos, diazinon, boric acid powder and bait stations that contain hydramethylnon. All of these agents, with the exception of boric acid, can reduce cockroach numbers by 90% or more, whereas boric acid reduces numbers by 40% to 50%. Several studies have shown that cockroach extermination is possible in most homes and that a combination of extermination and thorough cleaning can reduce cockroach allergen levels by 80% to 90%, although studies to date have not convincingly demonstrated that cockroach eradication alone is capable of significantly reducing disease activity.
In addition to these measures, integrated pest management also includes other strategies that help to reduce cockroach infestation including eliminating food sources and hiding and entry points ( Box 22-3 ). All foods should be stored in sealed containers and the kitchen should be cleaned regularly. Finally, extensive cleaning should be performed after extermination to remove the cockroach debris as completely as possible. Even with the most aggressive measures, however, it may be difficult to reduce cockroach exposure adequately in some environments. This is particularly true of the older, multiple dwelling units that house a preponderance of inner city residents. A more encouraging study related to inner city asthma did demonstrate a convincing benefit from a multifaceted, allergen-specific environmental control program in asthmatic children living in urban areas. In that study, cockroach extermination was part of a global environmental treatment that also included education, a HEPA filtered vacuum cleaner, allergen-proof bedding encasings and a HEPA filter in the child’s bedroom. Bla g 1 in floor dust was reduced by 53% compared to 19% in the control group but, more importantly, symptoms were also significantly reduced in the treated group. This trial supports the concept that integrated environmental avoidance strategies have the highest likelihood of producing beneficial clinical effects.
Regular and thorough extermination
Thorough cleaning after extermination
Extermination of neighboring dwellings
Roach traps
Repair leaky faucets and pipes
Repair holes in walls and other entry points
Behavioral changes to reduce food sources
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Clean immediately after cooking
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Clean dirty dishes immediately
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Avoid open food containers
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Avoid uncovered trash cans
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