SAN DIEGO-- Can lowering indoor humidity truly reduce patients' exposure to dust mites? Must patients with allergies always give up their pets? And what is the safest way to eliminate cockroaches? These were some of the provocative questions discussed during a symposium on allergen abatement at the 56th annual meeting of the American Academy of Allergy, Asthma, and Immunology.

A DUST MITE'S WORST ENEMY: DRY AIR

Dust mites, like humans, are largely composed of water and therefore require regular water intake to survive. But unlike humans, "dust mites derive water vapor from unsaturated air," said Larry Arlian, PhD, a professor of biological sciences at Wright State University in Dayton, Ohio. Because this can only occur when the relative humidity is above a critical level--55% to 75%, depending on the temperature--some researchers have suggested that keeping the indoor relative humidity below 50% would minimize house dust mites and the allergens they produce. Until recently, however, there were no convincing data to support this recommendation.

Dr. Arlian and colleagues studied the effects of varying levels of humidity on dust mites in a group of southwest Ohio homes. The homes were divided into three groups: those that used air conditioning only, those that used air conditioning plus a high-efficiency dehumidifier, and those that relied only on opening and closing windows for climate control. The dehumidifiers were placed in a central location and set to keep relative humidity below 50%. The homeowners in all three groups maintained their usual cleaning routines.

During the two summers included in the 18-month study, relative humidity averaged 46% and 45%, respectively, in the houses with dehumidifiers. Those levels were about 30% below the average summer outdoor relative humidity in southwest Ohio. "Neither of the other two groups of houses maintained a relative humidity below 50% during the summer," said Dr. Arlian. During the cooler months, however, relative humidity was between 40% and 50% in all three groups of houses because of dehumidification by forced-air furnaces.

At the beginning of the study, the houses with dehumidifiers had a live dust mite allergen level of about 400 µg per gram of dust on the bedroom floor, family room floor, and couch. By the end of the first winter, the allergen level had dropped to 8 µg/g; at the end of the study, it was 6 µg/g. Similarly, the mite allergen level in the same houses was high--18 µg/g--at the beginning of the study, but dropped to 5 µg/g by the end of the first winter and to 4 µg/g by the end of the study.

"Those levels are well below the threshold for the development of allergy symptoms," stressed Dr. Arlian in an interview with RESPIRATORY REVIEWS. In contrast, live mite counts and allergen levels in the other two groups fluctuated with changes in outdoor humidity, but they remained well above the threshold for symptom development throughout the study.

AVOIDING ANIMAL ALLERGENS

"The most effective solution for removing animal allergens from the home is simple: find a new home for the animal. Then clean aggressively afterward," suggested Robert A. Wood, MD, an associate professor of pediatrics at Johns Hopkins School of Medicine in Baltimore. This strategy reduces animal allergens to clinically insignificant levels in an average of four to six months, he said.

However, many patients with allergies refuse to give up their pets. In this case, what other steps can be taken to reduce the level of exposure to animal allergens? And how effective are these steps?

In general, washing pets is not very effective, unless it is done every one to two days. It may be more practical to keep pets from certain areas of the house, particularly the bedroom. Patients should also consider removing carpets, upholstered furniture, and other allergen reservoirs; using allergen-proof mattress and pillow covers; and increasing ventilation and/or installing high-efficiency particulate air (HEPA) filtration devices.

Unfortunately, even intensive measures like these may do little for highly allergic patients. For example, in one study, a reduction in bedroom allergen levels did not appear to improve symptom scores or other outcome measures in highly allergic patients. Furthermore, while adding a second HEPA unit in the living room did lead to reduced airway hyperresponsiveness in another study, it produced no improvement in any other outcome measures, according to Dr. Wood.

MANY WAYS TO KILL A COCKROACH

Removing cockroach allergens from the homes of children with asthma is particularly important because sensitivity to this allergen roughly triples the risk of hospitalization in these children, said Larry W. Williams, MD. Fortunately, many anticockroach agents are available, although some are more effective than others.

The insecticide fipronil is highly effective, comes in cockroach traps, and is available to consumers. "This agent enters the nervous system and kills by blocking GABAergic transmission," said Dr. Williams, an associate professor of allergy, immunology, and pediatrics at Duke University School of Medicine in Durham, NC. Roaches have not developed resistance to fipronil, and the agent is nontoxic to humans.

Hydramethylnon is another agent available to consumers that comes in a bait form and is extremely effective--provided it is placed near cockroach nests. Cockroaches die quickly after consuming the bait, and other cockroaches are poisoned when they eat their dead nest mates. Hydramethylnon is nontoxic to humans, has no odor, and resistance to it is minimal, noted Dr. Williams.

Other anticockroach agents include abamectin and anticholinesterases; however, these products can only be used by an exterminator. Studies have shown that abamectin dramatically decreases cockroach infestations, and despite nearly 10 years of use, resistance to it has not been reported.

In contrast, anticholinesterases, although widely used, have limited effectiveness and an unpleasant odor. "Resistance is a real problem with these agents, and there's been increasing concern about their human toxicity," Dr. Williams added. Hydramethylnon and abamectin are equally effective and probably the best anticockroach agents available when used properly, he told RESPIRATORY REVIEWS. However, simply eliminating cockroaches isn't enough. "After killing the roaches, the allergen remains in household dust for at least six months," Dr. Williams said.

Thorough cleaning is therefore also necessary. This usually reduces cockroach allergen dramatically in kitchens but not as much in living rooms and bedrooms, which typically have carpets and other allergen reservoirs. These problems can be minimized with allergen-proof mattress and pillow covers and by routinely washing all bed linens.

FUNGAL ALLERGENS ARE ELUSIVE

Determining indoor fungal allergen exposure is difficult because the available detection methods are limited. The most common ones are culture analysis of fungi from dust or air samples and microscopy of airborne spores. But culture analysis misses fungi that are not viable in a culture medium, creating false-negative results, Robert Bush, MD, told RESPIRATORY REVIEWS. Spore microscopy is frequently inconclusive because many fungi have similar-looking spores.

Nonetheless, these tests can be helpful, especially when combined with simultaneous testing of outdoor air in the immediate area. Results may suggest whether indoor fungal contamination originated outdoors. "If you have high levels of Penicillium or Aspergillus in your house but don't find them in outdoor air samples, they probably originated indoors," said Dr. Bush, chief of the allergy section at the William S. Middleton VA Hospital in Madison, Wis. In contrast, indoor Alternaria contamination typically originates outdoors, he noted.

Because fungi require moisture, reducing indoor relative humidity to less than 50% is vital to reducing fungal allergen levels. Besides installing dehumidifiers, homeowners should heat closets that are on outside walls (to prevent condensation during cold weather), repair water leaks, make sure clothes are dry when hung in closets, and limit the number of houseplants. In addition, washable contaminated surfaces should be cleaned with a diluted bleach and detergent solution. "Many fungi are hydrophobic, and you need a detergent to kill them," Dr. Bush explained.

Humidifier tanks, refrigerator drip pans, and other appliances that may harbor fungi must also be cleaned. It may be necessary to call the appropriate professional when fungal contamination of the ventilation system is suspected. "Anything you cannot clean or decontaminate appropriately may need to be discarded," concluded Dr. Bush. Perhaps the most common example is water-damaged carpeting.

--Timothy Begany

Suggested Reading
Wood RA, Chapman MD, Adkinson NF Jr, Eggleston PA. The effect of cat removal on allergen content in household-dust samples. J Allergy Clin Immunol. 1989;83:730-734.
Williams LW, Reinfried P, Brenner RJ. Cockroach extermination does not rapidly reduce allergen in settled dust. J Allergy Clin Immunol. 1999;104(pt 1):702-703.
Arlian LG, Neal JS, Vyszenski-Moher DL. Reducing relative humidity to control the house dust mite Dermatophagoides farinae. J Allergy Clin Immunol. 1999;104(pt 1):852-856.

 

How Common is Dust Mite Exposure?

Mounting evidence suggests that exposure to indoor allergens from dust mites and cockroaches is a risk factor for allergic diseases and asthma. But how widespread is such exposure? To find out, researchers at the National Institute of Environmental Health Sciences (NIEHS) collected indoor dust samples from five or six different sites in 831 homes from 75 different areas across the United States.[1] Demographic and health information about the home occupants was also collected. Based on the study results, the researchers estimate that over 45% of the US housing stock, or about 44 million homes, have bedding with dust mite allergen concentrations that exceed 2 µg per gram of dust--a level that has been associated with the development of allergies. Furthermore, about 23% of American homes (22 million dwellings) are estimated to have bedding with dust mite allergen concentrations that exceed 10 µg per gram of dust, a level associated with the trigger of asthma symptoms in asthmatic patients who react to these allergens. "This study suggests that a large number of US homes contain dust mite allergen levels that pose a significant risk for the development of allergies and asthma," said Patrick Vojta, PhD, of the NIEHS, one of the authors of the study. "There are housekeeping practices as well as allergen-proof bedding covers that can be used to reduce exposures to high levels of allergens. For people who are not allergic to these allergens, steps to reduce exposure may reduce the chance of developing allergies and asthma. For those who are already allergic and/or asthmatic, steps to reduce exposure may decrease the frequency and severity of the symptoms of these diseases," he added. The researchers also found that 17% of household occupants reported having problems with cockroach infestations in the year before the study. And the study results suggest that cockroach allergen may be present in bedding in over 6% of all US homes, representing almost 6 million households. However, the researchers believe that the number of homes with detectable cockroach allergen is much higher than either of these percentages suggest, because they have not yet analyzed dust samples from the kitchen, which is typically the most common site of cockroach activity. --Deborah L. O'Connor

Reference 1. Vojta PJ, Friedman W, Clickner R, et al. Prevalence of dust mite and cockroach allergens in U.S. bed dust: Results from the first national allergen survey. Am J Respir Crit Care Med. 2000;161:A794.