WASHINGTON, DC-- Exposure to dust mites appears to be the best-documented trigger of asthma in persons susceptible to the disease. A new analysis on the pathogenetic role of various indoor substances has identified the lowly dust mite as a household allergen clearly capable of causing a first episode of asthma in those who are genetically at risk.[1]

A combination of complex factors, at least partially environmental, are believed to contribute to the epidemic-like rise in asthma. Some 17.3 million Americans have the disease, and the number of cases has risen about 75% since 1980, especially among blacks living in inner cities, where exposure to indoor allergens is believed to be most severe. Despite a growing literature showing that indoor substances, including proteins shed by dust mites, cats, and cockroaches, can cause or aggravate asthma in those who are prone to the disease, confusion still exists over which of these are the prime suspects in asthma's pathogenesis.

The latest findings, from a committee set up by the Institute of Medicine, offer a new perspective concerning the extent to which these indoor irritants contribute to asthma or exacerbate its symptoms. The key findings from the report:

• Sufficient evidence exists to prove a causal relationship between exposure to house dust mites and the development of asthma in susceptible children (Table 1).
• Enough evidence has been gathered to support an association between environmental tobacco smoke and the development of asthma in younger children. Although both exposures are detrimental, maternal smoking during pregnancy appears to be a stronger contributor to the development of asthma than exposure to smoking after birth. Environmental tobacco smoke has been conclusively shown to exacerbate asthma symptoms.
• Only limited or suggestive evidence exists for associations between cockroach exposure and the development of asthma. Nevertheless, cockroach exposure clearly aggravates asthma symptoms.

DUST MITES ARE CLEARLY A CULPRIT

Commenting on the significance of his committee's findings, Richard B. Johnston, Jr, MD, told RESPIRATORY REVIEWS: "This is an appraisal by an impartial group of scientists about how well these relationships have been proven. What comes out strongly is that the dust mite is a bad actor. There needs to be a full-court press on reducing exposure to dust mites. It's a causal relationship." This conclusion was based on several lines of evidence, including the results of several clinical studies and population-based, case-control, and prospective epidemiologic investigations. However, according to Dr. Johnston, "Unquestionably, tobacco smoke is also an important factor in development of the disease."

Dr. Johnston, a professor of pediatrics at the University of Colorado School of Medicine and the National Jewish Medical and Research Center in Denver, was chair of the Committee on the Assessment of Asthma and Indoor Air, whose work was supported by the Environmental Protection Agency. The committee was organized under the auspices of the Institute of Medicine and included 11 other scientists who have been leading investigators in the importance of indoor irritants in the pathogenesis of the disease.

STRATEGIES FOR SYMPTOM CONTROL

The committee found that strategies to limit dust mite growth have proved effective in reducing symptoms in controlled trials, and it urged that these steps be part of routine management of asthma in mite-allergic individuals.

Given the role that cockroach exposure plays in asthma exacerbations, the committee found that the "combined use of cockroach extermination and control of potential reservoirs of allergens in beds, carpets, furnishings, and clothing through cleaning can achieve a short-term decrease in cockroach allergen levels in indoor environments." However, extermination alone appears to be ineffective because significant allergen levels remain in settled dust. Similarly, cleaning alone in the absence of complete extermination does not eliminate the source of the allergen.

In evaluating steps to limit exposure to other allergens, the committee pointed out that there is "limited or suggestive evidence" of an association between removal of a cat from the home and improvement of symptoms or lung function in cat-allergic patients with asthma. Since removal of a cat from the home may prove unpopular, the committee recommended that thorough cleaning--especially of carpets, bedding, and upholstered furniture--is a sound strategy for controlling exposure not only to pet dander, but also to mites, cockroach material, mold, and other biologic agents that may aggravate asthma.

The committee offered these observations on other ways to control indoor irritants:

• Complete cessation of smoking in the home and other indoor locations appears to be the only reliable means of protecting young children from secondhand smoke.
• Controlling humidity in the home is important because it is the key to controlling dust mite and fungal growth; removing standing water can help control cockroaches.
• Research is still inadequate to determine whether pesticides, houseplant spores, or material from domestic or wild rodents are major causes or aggravators of asthma.
• There is suggestive evidence, but not proof, that mold, dog dander, and pollution from poorly burning gas stoves also play a role in worsening asthma.

FUNGAL ALLERGENS

Damp conditions were implicated in the report as a problem for asthma patients. Although the factors related to dampness that may actually lead to asthma exacerbations are not yet confirmed, it is suspected that they probably relate to dust mite and fungal allergens. In view of this connection, the report suggests that the entry of fungal spores from outdoors can be substantially reduced in mechanically ventilated buildings by pressurizing and filtering incoming air. But the report stops short of recommending fungi removal as a key strategy in the abatement of asthma symptoms, noting that there is only limited or suggestive evidence that such steps may reduce the levels of fungi in the indoor environment; fungi, the report says, are difficult to kill--and dead fungal material probably contains allergens that can become airborne.

The intent of the report is primarily to raise awareness of the importance of environmental "inciting agents," according to Dr. Johnston. "It will serve as a guide to mitigation efforts," he said, alerting health care professionals to the importance of indoor irritants and what may be done to limit their effect. Measures such as encasing mattresses and pillows in plastic covers, opting for leather sofas instead of upholstery, and removing carpets can help deprive dust mites of their favored environment.

--Stu Chapman

Reference
1. Committee on the Assessment of Asthma and Indoor Air, Institute of Medicine. Clearing the Air: Asthma and Indoor Air Exposures. Washington, DC: National Academy Press; 2000.