There is increasing evidence that a significant proportion of illness symptoms and disease associated with building environments is due to particulatephase and, to a much lesser extent, gas-phase exposures to substances produced by a variety of organisms. These exposures include: airborne mold (fungi), bacteria and viruses, which may be viable (living) or nonviable; fragments of bacteria and mold which contain antigens, endotoxins, glucans, and/or mycotoxins; microbially produced volatile organic compounds; and immunologically active particles produced by insects, arachnids (mites, spiders), and pets (cats, dogs, etc.).