People have always considered the indoor environment a refuge from the dangers of the outdoors, offering protection from weather, wild animals, and, in the late nineteenth and twentieth centuries, air pollution—long viewed as primarily an outdoor phenomenon. In recent years, however, questions have been raised about potential health problems arising from indoor air pollution. The resultant concerns are related to modern lifestyles and the fact that most Americans spend more than 90 percent of their time inside a building of one kind or another. Indeed, if hours in transit between locations are considered, most Americans are indoors nearly 24 hours a day (Figure 1). In addition, there is a trend in newer construction toward energy-efficient, relatively airtight, structures. These factors taken together have raised increasing concern about the potential adverse health effects of indoor air quality.
Motivated by this concern, several agencies of the federal government asked the Institute of Medicine (IOM) to undertake an assessment of the public health significance of indoor allergens. The IOM responded by assembling a committee of experts in such fields as allergy and immunology, epidemiology, mycology, engineering, industrial health, pulmonology, education, and public policy. The study undertaken by the committee had three primary objectives: (1) to identify airborne biological and chemical agents found indoors that can be directly linked to allergic diseases; (2) to assess