Indoor Exposures Found to Promote Asthma.

The Evidence

Researchers have wondered whether indoor air quality has played a role in the increasing rate of asthma nationwide. About 17.3 million Americans have asthma. Since 1980, the prevalence of asthma and asthma-related hospitalizations and deaths has increased 75 percent. Asthma 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. In some countries, including Australia, New Zealand, Ireland, and the United Kingdom, the prevalence of this disease exceeds that in the United States.

The U.S. Environmental Protection Agency (U.S. EPA) has asked the Institute of Medicine (IOM) of the National Academies to assess the scientific literature regarding asthma and indoor air quality. The goal is to ensure that U.S. EPA's public health strategies and outreach are based on sound science.

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. Dust mites, for example, are present in most indoor environments, so their mere existence cannot be the sole factor that determines whether a person will develop asthma.

The IOM 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 assessed a range of potential allergens and irritants for two kinds of impact: the extent to which a substance could lead to the development of asthma and the extent to which it could worsen the condition of people who already have the disease. The substances 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 that these substances were contributing to asthma. The substances were divided into five categories according to the strength of the evidence.

1. Sufficient Evidence of a Causal Relationship

The evidence was strong enough for the committee to conclude that an allergen or irritant causes symptoms to develop in predisposed individuals or to worsen in known asthmatics:

* Material shed by house dust mites can lead to the development of the disease in susceptible people.

* Allergens associated with cats, cockroaches, and dust mites can cause symptoms to worsen in people sensitive to those allergens.

* Exposure to secondhand smoke causes asthma symptoms to worsen in preschool-aged children.

2. Sufficient Evidence of an Association

The evidence was sufficient for the committee to conclude that there is an association, but it did not meet the higher standard of proof needed to establish causality:

* Exposures to dogs, fungi, molds, and rhinoviruses--the group of viruses responsible for the common cold and other respiratory illnesses--are associated with the worsening of asthmatic symptoms.

* Older or malfunctioning gas appliances used in poorly ventilated kitchens can sometimes result in brief, high levels of nitrogen dioxide, which can lead to problems for asthmatics also exposed to other forms of indoor pollution.

* Secondhand smoke is associated with an increased risk of developing asthma among preschool-aged children.

3. Limited or Suggestive Evidence of an Association

While at least one major, high-quality population study has found a link between the disease and a given type of exposure, the evidence was not yet conclusive enough for the committee 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:

* Material shed by domestic birds, certain types of pneumonia and respiratory syncytial virus (RSV), formaldehyde fumes from furniture and building materials, and fragrances in personal care and household products all may worsen asthma symptoms.

* Secondhand tobacco smoke may worsen symptoms in older children and adults.

* Exposure to RSV and material shed by cockroaches also may contribute to the development of asthma in infants.

4. Evidence Inadequate to Determine an Association

The scientific evidence was insufficient for the committee to determine whether an association exists between nonoccupational indoor exposure to a number of potential contaminants and the development or worsening of asthma. The following factors fall into this category:

* pesticides,

* spores and other exposures from houseplants, and

* domestic or wild rodents.

5. Limited or Suggestive Evidence of No Association

Several adequate studies are mutually consistent in showing no association between the action or agent and the outcome. For instance, the committee found that rhinoviruses did not appear to be associated with the development of asthma in adults.

Controlling Indoor Allergens

The committee found that the effectiveness of steps to control indoor allergens or pollutants varies widely They considered the strategies discussed below.

Removing Pets and Pests

Although this strategy may be unpopular, it can be extremely effective, especially when followed by a thorough cleaning of the home, including carpets, bedding, and upholstered furniture.

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. When possible, removal of 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. Removing standing water also can help in eliminating cockroaches. In more humid climates, effective techniques include the use of air conditioning to lower indoor humidity. In drier climates the same effect may be achieved simply by opening windows for an hour each day.

Setting Priorities

While certain interventions work at the individual level, it is not known whether they all will be effective at a community level. 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 the question of 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. Such information would aid in the design of more effective interventions.

The IOM committee called for more interaction among 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 example.