John D. Spengler, Ph. D., is Director of Environmental Science and Engineering at Harvard University. A pioneer in the early efforts to establish indoor air quality as an important area of health concerns in the United States, he has expertise in instrumentation, risk assessment, hazard communication, and exposure assessment.John F. McCarthy, Sc.D., is president of Environmental Health & Engineering, Inc., of Newton, Massachusetts, a company that has investigated air quality in more than 1600 buildings since 1988. Dr. McCarthy is an expert in the assessment of complex pollutant exposures, use of biological markers in assessment, aerosol technology, control of toxic air pollutants, the development of health and safety programs and building commissioning.Jonathan M. Samet, M.D., M.S., is chairman of the Department of Epidemiology at Johns Hopkins University in Baltimore, Maryland. A pulmonary physician and epidemiologist, he is an expert on the health effects of pollutants in indoor and outdoor environments and on environmental risk assessment.