Date: Jan. 19, 2000
Contacts: Neil Tickner, Media Relations Officer
Jennifer Cavendish, Media Relations Assistant
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Study Finds Strong Evidence That Exposure to Some Indoor Substances Can Lead to or Worsen Asthma

WASHINGTON -- As medical researchers struggle to understand the rising incidence of asthma – and its acute impact on poor, inner-city children – a new report from the Institute of Medicine (IOM) of the National Academies finds strong, causal evidence linking common indoor substances to the development or worsening of asthma symptoms in susceptible people. It also finds that a variety of strategies, such as removing a pet, intensive cleaning, prohibiting smoking, and controlling indoor humidity, may help alleviate asthma symptoms.

After reviewing the latest scientific studies, the committee that wrote the report concluded that exposure to allergens produced by house dust mites – found in nearly every indoor environment – can lead to asthma in children who are predisposed to developing the disease. In addition, exposure to these allergens, as well as those produced by cats and cockroaches, can aggravate symptoms in some asthma sufferers. The condition of asthmatic preschoolers also can be worsened by contact with secondhand tobacco smoke.

"The prevalence of asthma continues to rise dramatically in this country and the reason why is a mystery," said committee chair Richard B. Johnston Jr., professor, department of pediatrics, University of Colorado School of Medicine, and National Jewish Medical and Research Center, Denver. "People spend most of their time inside, and it's vital that we understand how the indoor environment may contribute to the disease. Fortunately there are actions people can take to limit their exposure and ease symptoms."

Scientists think that some people are genetically predisposed to develop the disease. Whether they do depends on a complex -- and at present poorly understood -- combination of factors that are partially inherited and partially environmental. For example, dust mites are present in most indoor environments, so their mere existence is not the sole factor that determines whether a person will develop asthma.

The committee evaluated a broad body of research on asthma and indoor air, including population studies, clinical research, and studies on air flow and humidity in buildings. It looked at a range of potential indoor animal, plant, and chemical allergens and irritants in two ways: the extent to which each could lead to the development of asthma -- the point at which a person first displays symptoms -- or could worsen the condition of people who already have the disease. The indoor agents included allergens from dust mites and cockroaches; fungi and mold; dander, hair, and saliva from domestic pets and other animals; viruses and bacteria; spores and materials from houseplants; and irritants from secondhand tobacco smoke, pesticides, cleaning and building materials, and other pollutants.

The committee found varying degrees of evidence for some of the allergens and irritants. For example, secondhand tobacco smoke was very strongly linked to worsening asthma symptoms in preschoolers, but the evidence was unclear in the case of older children.

Based on the strength of the evidence, findings were placed into the following five categories:

Sufficient evidence of a causal relationship. Evidence in this category is strong enough to conclude that an allergen or irritant causes symptoms to develop in predisposed individuals or to worsen in known asthmatics. The committee determined that exposure to material shed by house dust mites could lead to the development of the disease in susceptible people. Exposures that could cause symptoms to worsen in individuals sensitive to particular allergens are those associated with cats, cockroaches, and dust mites. In the case of preschool-aged children, exposure to secondhand smoke also could cause a worsening of asthma symptoms.

Sufficient evidence of an association. Evidence in this category is sufficient to conclude there is an association, but it stops short of a higher standard of proof needed for causality. The committee found sufficient evidence of an association between the worsening of asthmatic symptoms and exposure to dogs, fungi, molds, and rhinoviruses -- the group of viruses responsible for the common cold and other respiratory illnesses. In addition, using older or malfunctioning gas appliances in poorly ventilated kitchens can sometimes result in brief, high levels of nitrogen dioxide, which can lead to problems for asthmatics who also are exposed to other forms of indoor pollution. Sufficient evidence also exists that preschool children are at greater risk of developing asthma from exposure to secondhand tobacco smoke.

Limited or suggestive evidence of an association. A classification of limited or suggestive evidence means that while at least one major, high-quality population study has found a link between the disease and exposure, the evidence is not yet conclusive enough to say definitively that chance or study bias did not influence the results, or that the studies have isolated all of the variables that could have affected the outcome.

For biologic or chemical contaminants that could worsen asthma symptoms, limited or suggestive evidence exists regarding exposure to material shed by domestic birds; certain types of pneumonia and respiratory syncytial virus (RSV); secondhand tobacco smoke in older children and adults; formaldehyde fumes from furniture and building materials; and fragrances in personal care and household products. The committee found limited evidence of an association between the development of asthma in infants and their exposure to RSV and material shed by cockroaches.

Inadequate evidence to determine whether or not an association exists. The scientific evidence in this category was insufficient to determine whether an association exits between nonoccupational indoor exposure to a number of potential contaminants and the development or the worsening of asthma. These included pesticides, spores, and other exposures from houseplants, as well as domestic or wild rodents.

Limited or suggestive evidence of no association. For this category, several adequate studies are mutually consistent in showing no association between the action or agent and the outcome. The committee found that rhinoviruses did not appear to be associated with the development of asthma in adults.

Controlling Indoor Allergens

Researchers have wondered whether indoor air quality may play a role in the increasing rate of asthma nationwide. About 17.3 million Americans have this long-term respiratory disease. Since 1980, the prevalence of asthma and asthma-related hospitalizations and deaths has increased 75 percent. It is the most common chronic disease among children. Of particular concern are the high death rates among African Americans with asthma and in urban areas that have substantial poverty and minority populations. Moreover, the phenomenon is not limited to the United States. The prevalence of the respiratory disease in some countries -- including Australia, New Zealand, Ireland, and the United Kingdom -- exceeds that in the United States.

The U.S. Environmental Protection Agency asked the IOM to assess the scientific literature regarding asthma and indoor air quality in an effort to ensure that its public health strategies and outreach are based on sound science.

The committee found that the effectiveness of steps to control indoor allergens or pollutants that can cause or aggravate asthma varies widely. Some strategies include:

Removing pets and pests. Although the strategy may be unpopular, the removal of a pet known to be the source of an allergen may ease symptoms in sensitized individuals. A thorough cleaning of the home -- especially carpets, bedding, and upholstered furniture -- is a sound approach to controlling exposures to pet dander, mite or cockroach material, mold, or other problematic biologic agents. In addition, the combination of cleaning and pest extermination applied consistently and conscientiously over time is a more effective long-term measure than either cleaning or extermination by itself.

Eliminating chemical pollutants. A complete cessation of smoking in the home and other indoor environments appears to be the only reliable means of protecting young children from exposure to secondhand smoke. Where possible, removing the source also works well for a range of other problematic chemical exposures.

Controlling indoor humidity. The degree of humidity in the home is the key to controlling dust mite and fungal growth, and removing standing water can help in eliminating cockroaches. In more humid climates, effective techniques include using air conditioning to lower indoor humidity. Drier climates may simply require opening windows for even an hour each day to lower the relative humidity in the house.

Setting Priorities

Because asthma is a complex illness, more needs to be learned about the many variables that determine its development and severity, the committee said. For example, more research is needed to better judge whether various environmental interventions are effective in improving the health of asthmatics. While some interventions work at an individual level, it is not known whether they will be effective at a community level. These programs may have to be adapted to the special circumstances of target populations, such as poor and inner-city residents. Individuals living in public or rental housing, for instance, may not have the resources or authority to make changes to their environments, such as replacing carpeting, removing excess moisture, or exterminating pests.

Of particular interest is determining how some people become sensitive to certain allergens and develop asthma, the report says. There also is a great need for studies that examine the role of prenatal exposures and whether the age of first exposure influences the development of sensitization. This would aid in the design of more effective interventions.

The committee also called for more interactions between researchers, clinicians, public health professionals, and those who are responsible for the design and function of indoor environments, such as engineers, architects, and materials manufacturers. Better research is needed on optimal levels of humidity and ventilation for healthier indoor air, for example.

In general, it is not possible to quantify the risk of developing asthma from exposure to contaminants in indoor air, the committee said. The relative risk of exposure depends on widely ranging variables, such as the type of indoor environments and characteristics of people exposed.

A committee roster follows. The study was funded by the U.S. Environmental Protection Agency. The Institute of Medicine is a private, nonprofit organization that provides health policy advice under a congressional charter granted to the National Academy of Sciences.

Read the full text of Clearing the Air: Asthma and Indoor Air Exposures for free on the Web, as well as more than 1,800 other publications from the National Academies. Printed copies are available for purchase from the National Academy Press Web site or at the mailing address in the letterhead; tel. (202) 334-3313 or 1-800-624-6242. Reporters may obtain a pre-publication copy from the Office of News and Public Information at the letterhead address (contacts listed above).

Division of Health Promotion and Disease Prevention

Committee on Asthma and Indoor Air

Richard B. Johnston Jr., M.D. * (chair)
Department of Pediatrics
National Jewish Medical and Research Center
University of Colorado School of Medicine

Harriet A. Burge, Ph.D.
Associate Professor
Department of Environmental Health
Harvard School of Public Health

William J. Fisk, M.S., P.E.
Staff Scientist and Group Leader
Indoor Environment Department
Environmental Energy Technologies Division
Lawrence Berkeley National Laboratory
Berkeley, Calif.

Diane R. Gold, M.D., M.P.H.
Assistant Professor of Medicine
Harvard Medical School, and
Assistant Professor
Department of Environmental Health
Harvard School of Public Health

Leon Gordis, M.D., Dr.P.H.*
Department of Epidemiology
Johns Hopkins School of Hygiene and Public Health, and
Department of Pediatrics
Johns Hopkins School of Medicine

Michael M. Grunstein, M.D., Ph.D.
Department of Pediatrics
Children's Hospital of Philadelphia

Patrick L. Kinney, Sc.D.
Assistant Professor
Division of Environmental Health Sciences
Columbia School of Public Health
New York City

Herman E. Mitchell, Ph.D.
Senior Research Scientist
Rho Federal Systems Division, and
Adjunct Professor of Biostatistics
University of North Carolina School of Public Health
Chapel Hill

Dennis R. Ownby, M.D.
Professor of Pediatrics
Medical College of Georgia

Thomas A.E. Platts-Mills, M.D., Ph.D.
Department of Medicine and Microbiology, and
Division of Allergy, Asthma, and Clinical Immunology
University of Virginia Health Sciences Center

Sampson B. Sarpong, M.B., Ch.B.
Assistant Professor
Department of Pediatrics
University of Chicago Children's Hospital

Sandra Wilson, Ph.D.
Senior Staff Scientist and Chair
Department of Health Services Research
Palo Alto Medical Foundation Research Institute
Palo Alto, Calif.


David A. Butler, Ph.D.
Study Director

* Member, Institute of Medicine