Aeroallergens

Both outdoor and indoor aeroallergens sensitize and exacerbate allergic asthma; however, more than 90% of the average person's time in developed countries is spent indoors. Major indoor aeroallergens are derived from dust mites (see the image below), cockroaches, cat, dog, and other furry-animal debris, and fungi.

See All About Allergies: Be Ready for Spring, a Critical Images slideshow, to help identify a variety of allergens and symptoms.

Avoidance Measures and Interventions

The physician has the following 3 treatment options for individuals with allergic diseases, including allergic asthma:

• Aeroallergen avoidance

• Medications

• Allergen immunotherapy

Avoidance of the offending agent is the first line of treatment for allergic disorders and asthma. It is the most cost-effective treatment modality, and it sometimes results in complete resolution of an allergic problem. Modification of the home environment is the primary measure to reduce allergen exposure. A comprehensive strategy should include determining the allergens to which the individual is sensitive and implementing specific avoidance measures to reduce exposure to the offending allergens.

Dust mites

Dust mite bodies and fecal pellets are the major source of allergens in house dust. ^{[1, 2]} More than 80% of homes in the United States and 85% of daycare facilities have detectable levels of dust mite allergens ^[3] ; half of US homes have levels higher than the proposed threshold levels for sensitization, and about 25% have levels at or higher than those required to induce asthma. ^[3]

Avoidance strategies for dust mite allergens are as follows:

• Use impermeable (woven) covers (on the pillows, box spring, and mattress)

• Eliminate dust reservoirs when possible, including carpeting, upholstered furniture, stuffed animals, and drapery

• Vacuum weekly

• Wear a pollen mask when cleaning if you are allergic

• Wash bedding weekly

• Reduce indoor humidity

Furry animals

Furry pets include, for example, cats, dogs, ferrets, and rabbits.

Avoidance strategies for furry animal allergens are as follows:

• Remove the pet from the home or other dwelling

• Restrict the pet's access (eg, keep the pet out of the bedroom)

• Bathe the pet weekly (possibly helpful)

• Use impermeable (woven) covers on pillows, box springs, and mattresses

• Use high-efficiency particulate air (HEPA) central air conditioning filters (possibly helpful)

Rodents

Avoidance strategies for rodent allergens are as follows:

• Restrict access to the home, building, or other dwelling

• Eliminate food and water supply

• Use rodent predators

• Use rodent traps

Cockroaches

Avoidance strategies for cockroach allergens are as follows:

• Clean thoroughly

• Use pesticides – preferably gel or baits, selectively placed in the kitchen after thorough cleaning

• Eliminate food and water supply

• Extermination

Fungi

Most of the fungi recovered from an indoor environment emanate from outside. However, certain species, such as Penicillium and Aspergillus, can be found in greater quantities indoors.

Avoidance strategies for fungal allergens are as follows:

• Reduce spore infiltration – keep windows and doors closed as much as possible

• Control moisture – decrease the indoor humidity level to less than 50%, if possible

• Cleaning – a detergent solution with 5% bleach is useful for washable wallpaper and paneling

• Use high-efficiency particulate air (HEPA) central air conditioning filters

• Wear masks when cleaning if you are allergic

• Make sure there are no roof or water leaks

In 1921, R. A. Kern noted that a patient with asthma had a positive prick-puncture skin test to extracts obtained from her mattress. ^[4] The patient's asthma improved after she enclosed the mattress in heavy packing paper and thoroughly cleaned the room. In 1925, Storm van Leeuwen successfully treated individuals with asthma by moving them to high altitudes with lower allergen levels or enclosing them in an allergen-proof chamber. ^[5]

Despite the early recognition that environmental exposure to allergens causes allergic diseases, indoor allergen avoidance is still an underused treatment. Even when properly prescribed by physicians, the benefits of indoor allergen avoidance are often decreased by inadequate patient adherence. Atopic diseases, which affect up to 30% of the population in some countries, are an increasing problem, particularly in developed countries. Allergic diseases and asthma also cause substantial morbidity and mortality. Asthma affects almost 300 million people worldwide and accounts for 1 in 250 deaths. In the United States, approximately 3 million people are hospitalized for acute asthma each year. ^[6]  Half of all adults and 80% of children with asthma have an allergic component to their disease, and high levels of allergen exposure are linked to increased asthma symptoms in these individuals. ^[7]

Both outdoor and indoor aeroallergens sensitize and exacerbate allergic asthma and rhinoconjunctivitis. Major outdoor allergens include those derived from the pollens of trees, grasses, and weeds. Major indoor allergens are derived from dust mites, cockroaches, cat, dog, and other furry-animal debris, and fungi (see Table 1, below). ^[8] Allergens are located both in homes and in other indoor environments. For example, several studies show that most daycare facilities sampled have detectable levels of major indoor allergens; many contain levels sufficient to cause sensitization and exacerbation of allergic diseases. ^{[9, 10]}

Table 1. Major Indoor Aeroallergens (Open Table in a new window)

The physician has the following 3 treatment options for individuals with allergic diseases, including allergic asthma:

• Aeroallergen avoidance

• Medications

• Allergen immunotherapy

More than 90% of the average person's time in developed countries is spent indoors. This statistic underscores the importance of avoidance measures, especially in those who are allergic to indoor allergens.

Modern medicine has vastly increased the effective treatment of allergic disorders and asthma. However, the first line of treatment for these diseases is to avoid the offending agent. Therefore, the clinician should instruct those with allergies to indoor aeroallergens about practical matters to decrease exposure to these allergens. This is the most cost-effective treatment modality, and it sometimes results in complete resolution of the allergic problem.

Indoor allergens can be avoided using two primary techniques. First, the sensitized individual can be moved away from the allergen; second, the home environment can be modified to reduce allergen exposure. The latter strategy is usually more practical in clinical practice (see Table 2, below).

Controversy exists over the effectiveness of individual environmental control strategies, but a comprehensive approach to allergen avoidance is superior to a single intervention. ^[11] Conflicting results among studies that address primary prevention of atopic disorders have failed to lead to a consensus for recommendations for allergic subjects or parents who wish to decrease the risk of atopy in their children.

The recommendations for allergen avoidance cannot be uniform for everyone. A comprehensive strategy should include determining the allergens to which the individual is sensitive and implementing specific avoidance measures to reduce exposure to the offending allergens. Furthermore, no avoidance strategy is complete without a thorough discussion with the patient or the patient's family regarding the role of allergens in the disease.

The success of such a strategy depends on correct identification of relevant allergens, the physician's attitude regarding the effectiveness of avoidance measures, and patient motivation and means to apply this strategy.

Table 2. General Recommendations for Aeroallergen Avoidance (Open Table in a new window)

 

Dust mite debris is the major source of allergens in house dust. ^{[1, 2]} This microscopic arthropod primarily feeds on organic materials, including skin scales, fungi, and bacteria. Dust mites lack an organized respiratory system and their water supply is derived from the ambient air. Seasonal changes in relative humidity can affect the concentrations of dust mite allergen, and these fluctuations can contribute to allergic symptoms in the sensitized individual. ^[12]

In the home, mites typically infest objects that contain fabrics; for example, higher concentrations of mites are found in pillows, box springs, mattresses, bedding, carpets, throw rugs, drapery, stuffed animals, and upholstered furniture. Higher concentrations are usually found in older homes, in regions of high humidity, and in homes with heating units other than forced air. ^[3] Cold, dry air at high altitudes is not conducive to dust mite growth.

Both the mite bodies and fecal pellets are major sources of mite allergens, which become airborne when disturbed. ^[2] Dust mite allergens are predominately contained on aerodynamic particles 10 µm or larger in diameter and remain in the air for 30 minutes or less. In contrast, cat allergens are predominately carried on smaller particles, with an average size of 5 µm, and can remain in undisturbed air for days.

Allergens are named according to the first 3 letters of the genus and the first letter of the species (eg, the allergen for Dermatophagoides pteronyssinus is classified as Der p and Dermatophagoides farinae [see the image below] is classified as Der f). Allergens are grouped according to biologic function and homology. The major sources of dust mite allergens are the group 1 allergens Der p 1 and Der f 1.

The concentration of dust mite allergens (an indirect assessment of exposure) is measured in micrograms per gram (μg/g) of settled dust from samples obtained by vacuuming a defined area of a carpet or mattress. The proposed threshold concentration of allergen exposure required to sensitize to dust mites is 2 μg of group 1 allergen (Der p 1 and Der f 1) per gram of dust. Evidence that sensitization may occur at even lower levels also exists; hence, an individual genetically predisposed to develop allergic diseases may not have a safe level of exposure. Levels of 10 μg/g of dust induce allergic symptoms or asthma in sensitized persons, but lower levels may also cause symptoms. ^[13]

More than 80% of homes in the United States and 85% of daycare facilities have detectable levels of dust mite allergens. ^[3] Furthermore, half of US homes have levels higher than the proposed threshold levels for sensitization; about 25% have levels at or higher than those required to induce asthma. ^[3]

Avoidance measures

Impermeable covers

Several studies show that using impermeable mattress covers is not an effective strategy by itself to reduce the symptoms of allergic rhinitis or asthma in dust mite-sensitized subjects. ^{[11, 14]} However, a study by Morgan et al demonstrates significant benefit of impermeable covers when used as part of a comprehensive and individualized allergen avoidance program. ^[15]

Impermeable covers are composed of either vinyl or tightly woven fabrics and serve as effective dust mite barriers. Because vinyl is impermeable to air, it tends to be hotter and less comfortable than a woven cover, which is more comfortable but, usually, more expensive. In addition, nonwoven mattress encasements accumulate mite allergen, in contrast to woven encasements. ^[16] In either case, the covers used should encase the pillows, box spring, and mattress. These coverings may be purchased from retail stores, Web sites (eg, www.natlallergy.com), mail-order catalogs, and allergy specialty stores.

Elimination of dust reservoirs

The removal of dust reservoirs remains an important part of the avoidance program, although it can be difficult and expensive. Removal of carpets, draperies, and upholstered furniture decreases mite exposure. Eliminating carpets from the bedroom or home can increase injuries from falls. Removal of stuffed toys, books, or other items that collect dust, particularly in the bedroom can be helpful.

Vacuuming

Weekly vacuuming is recommended, and vacuum bags should have 2 layers or a HEPA filter to decrease aerosolized allergen exposure because of the air disturbance. Individuals allergic to dust mites should try to avoid vacuuming or being present when the bag is changed. ^[17] The use of a pollen mask (eg, Nexcare All-Purpose Mask [3M, St Paul, Minn]) can also be helpful for sensitized individuals who are unable to avoid vacuuming.

Dehumidifiers

Because dust mites depend on a humid environment to live, reducing the relative humidity level to less than 50% can be helpful. ^[13] However, a level of relative humidity low enough to effectively restrict mite population growth is difficult to maintain in most locations. Central dehumidifiers are expensive and are not very effective, particularly in areas that have a high humidity all year. Bedroom dehumidifiers are not effective and are not recommended for routine use.

Washing

Dust mites have 90% to 98% mortality in cold water washing, indicating that most mites die by drowning. ^[18] Residential hot water should be set below 49º C (120º F) given the effectiveness of kiling dust mites at lower temperatures to prevent scalding of occupants. ^[13]

Drying

All dust mites were killed when a blanket was dried for 10 minutes, and 99% of dust mites were killed when a duvet (comforter) was dried for 1 hour in a clothes dryer. ^{[19, 20]}

Air purifiers

Air purifiers alone are not useful for dust mite allergen control because dust mites do not remain airborne for long periods. Thus, the routine use of air purifiers is not recommended because clinical benefits are not documented.

Freezing

Dust mites die when frozen; therefore, stuffed toys and other such items can be placed in a domestic freezer at -15º C (5º F) for at least 16 hours to decrease dust reservoirs. ^[21] In climates where the outside temperature drops below freezing, rugs, blankets, mattresses, or other items can be placed outside for 2-3 days to kill dust mites.

Acaricides

Studies investigating the use of acaricides on carpeting to kill dust mites have yielded mixed results. Benzyl benzoate kills 90% of mites in culture within 12 hours, however, these effects are short-lived and the acaricide on the carpet should be reapplied every 2-3 months. ^[22] Furthermore, a decrease in dust mite allergens in carpeting may not be clinically beneficial. ^[23]

Duct cleaning

Because dust mites do not typically reside in air ducts, cleaning air ducts does not decrease exposure.

Relocation

Although not very practical, moving to locations that are less favorable for dust mite growth can be beneficial. For example, high altitudes or arid locales are associated with less mite growth.

Furry pets include, for example, cats, dogs, rodents, ferrets and rabbits. In the United States, 68% of households have one or more domestic pet. ^[24] The two most common animals are cat and dog; thus, these animals are major contributors to indoor allergen exposure. Cases of sensitization and allergy to rabbit, gerbil, hamster, ferret, and other animals are also described in the literature. For this reason, it is important to elicit an adequate pet history. ^{[25, 26]}

Of the households with a cat, 17% of the individuals who live with them are sensitized based on a positive skin prick test to cat extract. However, only 5% of dog owners have a positive skin prick test to dog extract. ^[27]  In a study by Arbes et al, all sampled homes in the United States, even those that have never had an animal in the house, contain dog allergen and most contain cat allergen. ^[28] Furthermore, most homes contain levels that exceed the proposed levels for both sensitization and exacerbation of asthma symptoms.

Felis domesticus (Fel d 1), the major cat allergen, is produced primarily in the sebaceous glands and is secreted onto the skin and fur. ^[29] This allergen is also present in the salivary glands; thus, cats add additional allergen to their fur when they clean themselves. The allergen is carried on small particles and remains airborne for long periods. Thus, decreased ventilation in the home leads to increased levels of cat allergen. Unlike dust mite allergen, cat allergen can be found even at high levels on the walls and other surfaces within the home. ^[30] The major dog allergens, Canis familiaris 1 (Can f 1) and Canis familiaris 2 (Can f 2), have physical properties similar to those of cat allergens. Hypoallergenic cats and dogs do not exist, as published in the lay literature. 

An article by Vredegoor et al demonstrates that Can f 1 levels are significantly higher in hair and coat samples in dog breeds referred to as hypoallergenic; that is, they are no less allergenic than any other dogs. Although some variations exist in other characteristics, such as whether a dog was bathed, no measured parameters significantly influenced the conclusions of the study. ^{[31, 32]} These data are confirmed by another study by Nicholas CE et al, which indicates that there is no evidence for differential shedding of allergens by dogs grouped as hypoallergenic. ^[33]

The amount of exposure to an animal allergen necessary to cause sensitization is controversial. The proposed threshold level for sensitization to cat allergen is 1 µg of Fel d 1 per gram of dust, and the level to cause symptoms in susceptible individuals with asthma is 8 µg of Fel d 1 per gram of dust. For dog allergen, the proposed levels for sensitization and exacerbation are greater than 2 and 10 µg/g of Can f 1 per gram, respectively. However, lower levels are reported to be associated with sensitization. Observational studies suggest that high exposure to cat and dog allergen early in life is associated with a decreased risk of pet allergy. ^{[34, 35]} However, an earlier study suggests that the opposite is true. ^[36]

Avoidance measures

Removal of the pet

One of the most important recommendations for the family—removal of the pet—may also be the most difficult to accept, since some pets are considered to be a part of the family. However, everyone must understand that continued exposure to a pet and its allergens occurs if the pet is kept in the home. Those who are allergic to a pet need to implement effective lifestyle modifications to reduce their exposure to animal dander. ^[27] Once the pet is removed, the time required for allergen levels to decrease to levels at which allergic problems no longer occur can be as long as 6 months. ^[37]

Restrict pet access

If the family decides to keep the offending pet, the pet should be kept outdoors. A less desirable option is to keep the pet in one area of the home and out of the bedroom.

Bathing the pet

The recommendation for a dog owner who is allergic to his or her pet is to bathe the pet at least weekly. ^[38]  However, the beneficial effects of reducing allergen levels by regular bathing are more likely to be successful for dogs because of the rapid buildup of the allergen burden in cats. Can f 1 levels may be reduced below baseline values after bathing, superior to vacuuming the fur of a dog, which results in minimal allergen decline. ^[39] Studies evaluating pet bathings have yielded conflicting results; however, in all cases, the effects are transient.

Vacuuming

A study of the effects of vacuuming the carpet on cat allergen levels in the home fail to show any beneficial effects even when modern HEPA filters are used. In fact, the amount of cat allergen found increased, possibly because of the sweeping motion of the brushes on the carpeting and the air disturbance from the exhaust flow. ^[38]

Other measures

Impermeable coverings are recommended for the pillow, box spring, and mattress. Air purifiers may be helpful because animal allergens remain airborne much longer than dust mite allergens; however, data to support the efficacy of air purifiers are lacking. Elimination of the reservoirs for allergens is an important component of the avoidance strategy.

Chemical treatment with tannic acid or hypochlorite bleach (0.05% solution) modifies the allergens so that they are no longer allergenic. Such treatment represents a temporary measure, at best, given that allergens reaccumulate. There is no evidence that tannic acid treatment improves respiratory health, while hypochlorite bleach can increase respiratory symptoms. ^[38]

Of the 4500 known species of cockroaches in the world, four are pests, including the oriental cockroach (Blatella orientalis), German cockroach (Blatella germanica), American cockroach (Periplaneta americana), and the brown-banded cockroach (Supella loongipalpa).The 2 most commonly encountered species are the German (Blatella germanica) and the American (Periplaneta americana) cockroaches (see the image below). Cockroach allergens are derived from fecal material, secretions, and dead bodies. The airborne allergen particles from cockroaches are similar to dust mite aeroallergens in that they are relatively large (>10 µm in diameter). As such, they tend to become airborne after disturbance and then quickly fall. The proposed threshold levels for sensitization and asthma symptoms are 0.04 and 0.08 µg of Bla g 2 per gram of dust, respectively. ^[40]

An association between cockroach sensitization and more frequent episodes of asthma in individuals in the emergency department is described. Cockroach exposure in subjects with asthma who reside in inner-city areas could account for the disproportionately high morbidity in this population, and the association of low socioeconomic status and cockroach allergy appears to be independent of age, sex, and race. Several studies demonstrate that cockroach allergy is found not only in the inner city but also in any substandard housing conditions or where apartments are infested with cockroaches. ^{[41, 42]}

Avoidance measures

Pesticides

An important aspect of cockroach eradication is the proper use of effective pesticides. Because the kitchen is the main source of both food and water for these insects, this room contains the most roach allergen. Pesticide spray is effective, but selective placement of gels or baits is preferred. Potential targets should include any cracks or other points of entry throughout the house.

Cleaning

Before pesticide application, a thorough cleaning is recommended so that the cockroaches are more likely to feed on the gel or bait. All surfaces with potential contact with the insect should be cleaned to reduce allergen levels. Particular attention should be paid to kitchen cabinets and the space around household appliances. The addition of liquid bleach facilitates removal of the allergens. Daily vacuuming of carpets reduces cockroach allergen exposure. ^[40]

Elimination of food and water supply

Access to food and water should be eliminated to effectively eliminate cockroaches from the home. Food should be stored in sealed containers, and any sources of standing water should be removed. Routine chores such as washing the dishes and removing the trash from the home should be done daily. Food left out overnight promotes cockroach infestation.

Fungi are widely distributed throughout the world. They grow mycelium and reproduce asexually by budding and sexually through spores. These spores become airborne and have the potential to sensitize and cause symptoms in susceptible individuals. ^[43] The optimal conditions for fungal growth vary greatly among different species, but all require oxygen, a carbohydrate source, and water. Optimal temperatures vary but are usually between 18°C (64.4°F) and 32°C (89.6°F).

Higher outdoor levels of fungi are associated with concomitant high levels in the indoor environment. Indeed, most of the fungi recovered from an indoor environment emanate from outside. However, certain species, such as Penicillium and Aspergillus, can be found in greater quantities inside buildings and homes. Another source of indoor fungi are the transport of outdoor spores on clothing and animal fur into the indoor environment. ^[44]

In children with allergies, 63% of their homes have increased fungal levels. ^[45] Sixty-six percent of asthma sufferers have sensitization to one or more fungi on skin-prick testing or serum IgE measurement. ^[46]

The threshold levels of allergens that are required to sensitize and exacerbate allergic symptoms have yet to be established. Culture methods are usually used to determine exposure, using spore counts or quantitation of colony-forming units. An indoor source of fungal growth is suggested if the indoor concentration of individual types of fungi are greater indoors than outdoors. ^[47]

Avoidance measures

Reduce spore infiltration

Because most of the fungal spores detected in a home are derived from outdoors, windows and doors should be kept closed as much as possible.

Moisture control

Decrease the indoor humidity level to less than 50%, if possible. Ensure there are no roof or water leaks in the home. Use air conditioning at times of high humidity, ventilate bathrooms and kitchens by using exhaust fans, and heat all rooms in the winter. ^[46]

Cleaning

A detergent solution with 6% bleach is useful for washable wallpaper and paneling. The disinfectant is prepared by adding 0.25-1.5 cups of bleach per gallon of water. ^[48] Removal of the contaminated source may be necessary to reduce allergen exposure. Vacuuming frequently may also reduce fungal spore levels. ^[46]

Air filters

HEPA filters are more effective than electrostatic air filters in removing fungal allergens from the air ^[49]  However, efficacy for such filters is questionable. 

Masks 

Activities such as cleaning and vacuuming can expose the individual to various fungal allergens; therefore, a pollen mask is useful, especially for the allergic individual, in such situations. ^[46]

Rodents are the largest population of mammals in the world. The order Rodentia includes rat, mouse, guinea pig, hamster and gerbil. Rodents with less known clinical significance include beaver, muskrat, porcupine, woodchuck, chipmunk, squirrel, prairie dog, chinchilla, and others. Eighty-two percent of homes had detectable mouse Mus m1 allergen in a national US housing survey. Levels of rodent allergen are higher in older, mobile, and low-income homes and also in high-rise apartments. Studies also show increased levels of rodent allergens in schools and daycare centers. Exposure to the major mouse allergen, Mus m 1, above 1.6 µg per gram of dust is associated with sensitization and may be a significant cause of asthma. ^[50]

Laboratory animal facility workers typically develop sensitization within the first three years of employment. Approximately 1/3 of these workers develop allergy to the animal and 1/3 of those who develop allergies will develop asthma. An atopic history and the intensity of exposure are risk factors for developing sensitization and symptoms. ^[50] Please refer to the “Environmental Assessment and Exposure Reduction of Rodents: a Practice Parameter” for recommendations to reduce rodent exposure in the animal facility setting. ^[50]

Avoidance measures

Restricting access into building

A rat may gain entry into the home through a hole as small as 3/4 and a mouse as small as 1/4 of an inch. All openings, including pipe entries should be sealed or covered with metal mesh to prevent entry and openings under doors should be minimized. Debris and clutter located near the building should be removed to avoid masking evidence of rodent entry and as a potential rodent shelter. A non-vegetative border is recommended around the building perimeter to prevent the rodent from going undetected during movement from the outside to the inside of the building. ^[50]

Elimination of food and water supply

Food sources, such as cereal, grain, and pet food should be stored in sealed containers. Garbage should be removed from the building on a regular basis.

Use of rodent predators

Cats, owls, and snakes are rodent predators. A cat in the home may deter a mouse from entering the dwelling and reduce the mouse population. However, having a cat fails to completely eliminate a mouse population and is not a significant rat deterrent. One study indicates that 84% of subjects who were sensitized to mouse are also sensitized to cat so this intervention could cause more harm from the cat versus rodent sensitization. ^[50]

Rodent traps

Three main types of rodent traps exist: snap traps, live traps, and glue boards. The rodent triggers the snap traps. A pea-size amount of bait is placed on the trap and the trap placed along the perimeter of the room. Live traps use a baited cage with a one-way door to capture the rodent. Glue boards can be used to capture rodents and also droppings and hair. ^[50]