Ranking Asthma's Indoor Culprits: Dust Mites Top List

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Jan. 19, 2000 (Washington) -- The household is a hostile environment for asthmatics, containing a variety of agents that might cause or worsen an attack, but a new report rates these threats in a way that is ultimately aimed at helping people breathe more easily. The microscopic but ubiquitous dust mite apparently is the only definitive indoor cause among individuals without previous symptoms.

"[The] dust mite is clearly, any way you cut it, important, and so is environmental tobacco smoke, and then cats and cockroaches in terms of flaring up asthma ... those are bad actors," Richard Johnston Jr., MD, tells WebMD. Johnston headed the committee that produced the analysis called "Clearing the Air: Asthma and Indoor Air Exposures" for the Institute of Medicine (IOM). The IOM is an independent advisory group chartered by the National Academy of Sciences.

The IOM document hopes, in part, to address the increasing problem of asthma in the U.S. -- particularly among inner-city blacks.

While asthma appears to be caused by a mix of genetic and environmental factors, the condition, which afflicts some 17 million Americans, is still poorly understood, according to Johnston. "The document serves as a guide, basically. It's a strategic plan and statement of where the needs are," says Johnston, a professor of pediatrics at the University of Colorado School of Medicine.


In its review, the IOM committee surveyed a wide variety of studies linking asthma and indoor air. Among the issues studied were animal, plant, and chemical allergens and irritants, as well as dust mites, cockroaches, fungi and mold, dander, hair and saliva from domestic pets, and a number of other factors.

The results were then divided into agents that could cause or lead to the development of asthma and things that could make the condition worse. Only the dust mite gathered enough evidence to rank as an asthma cause. However, environmental tobacco smoke in young children was strongly associated with the breath-stealing disease. There was limited data on the relationship between cockroaches and asthma in infants, says the IOM committee, and there wasn't enough information to show if cats, dogs, or fungi and molds were asthma causes.


However, when it comes to making asthma worse, cats, cockroaches, and dust mites led the list. There was also an association between asthma worsening and dogs, fungi and molds, as well as cold viruses. There wasn't enough information to link rodents, houseplants, or pesticides with worsening symptoms.


Johnston says it was surprising that cats and roaches rank so low as possible asthma causes. "The only agent we can really feel that you could invoke causality on, with any degree of certainty, in terms of developing asthma in an individual who has not had asthma symptoms previously, is a dust mite," he says.

The Environmental Protection Agency commissioned the IOM report to make sure that its public health policies were consistent with the best available science. But Johnston says much more research needs to be done before taking action to clear indoor air. For instance, while covering up a mattress to reduce a person's exposure to dust mites might work well for an individual, it's not clear that's good public health strategy.

"Too much ignorance remains regarding the biologic changes that permit the disease to emerge and recur ... and the means of effective exposure mitigation and prevention," writes Johnston in the preface to the report. The report suggests studying why it is that some become sensitive to asthma-inducing substances but others don't. Another area of importance is prenatal tobacco smoke exposure, which, says Johnston, increases the risk the infant will develop asthma later in life.


The IOM committee says doctors and public planners should work more closely to build structures where inside air is cleaner and healthier. While it wasn't mentioned directly in the report, Johnston says there is a strong association between poverty and asthma.

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