need for increased awareness and knowledge of indoor allergens among primary care physicians, allergists, patients, and others.

Because of the large number of people affected by allergic disease, the committee anticipates that this report will be of interest to a broad array of readers (e.g., medical practitioners, public policymakers, members of the public whose health is adversely affected by the indoor environment). Consequently, several sections of background material are presented in this chapter; they should be useful to readers in understanding the rest of the report. The sections below present a brief historical perspective on allergies, the indoor environment, and the twentieth-century innovations that have contributed to current concerns; several concepts and definitions that the committee used as a framework for its discussions are also discussed. The chapter concludes with a brief statement regarding the report's scope and organization.

ALLERGENS AND THE INDOOR ENVIRONMENT

For more than 5,000 years, observers have recorded episodes of disease and demise that today are recognized as allergic reactions. For example, the death of King Menes of Memphis in about 3000 B.C. was the result either of anaphylaxis from the sting of a hornet or of being trampled by a hippopotamus. (A question arises because hornet and hippo share the same ancient Egyptian word.) During the sixteenth, seventeenth, and eighteenth centuries, reactions were noted (mostly to foods) that were surely allergic in nature, and some surprisingly intuitive observations were made regarding cause and effect. During this period, cats, dogs, horses, feathers, and many foods were suspected of causing asthma. In the early nineteenth century, it was recognized that pollen caused "hay fever" and that dust from beaten carpets produced similar symptoms. The first experimental challenges with pollen apparently were done by Kirkman in 1835, followed by Blackley's extensive investigations into both allergy and aerobiology in the 1870s (Blackley, 1873).

More recently, much progress has been made in recognizing specific causes of asthma and hay fever and in defining the mechanisms by which symptoms are elicited. Yet in spite of increasing knowledge of the mechanisms of allergic disease and of the agents that cause sensitization and symptoms, we have modified indoor environments in ways that may contribute significantly to exposure to these agents and to the development of allergic disease. For example, we have adopted central heating on a broad scale, which means that we heat all parts of our houses, even when they are not occupied. By the late 1960s and 1970s it became obvious that much energy was wasted in houses and other buildings owing to excessive heat transfer through surfaces such as walls, ceilings, and windows or through



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