Lawrence R.Curtis is professor and head of the Department of Environmental and Molecular Toxicology at Oregon State University. His research interests are focused on understanding the cellular level processes that determine bioaccumulation of persistent chlorinated hydrocarbons and polycyclic aromatic hydrocarbons and the trophic transfer and ecotoxicology of persistent organic contaminants. Dr. Curtis is on the editorial board of the Journal of Toxicology and Environmental Health: Critical Reviews and has served as chair of the Membership Committee of the Society of Toxicology. He received his M.Sc. from the University of South Alabama and his Ph.D. in pharmacology and toxicology from the University of Mississippi Medical Center.

Charles N.Haas is the L.D.Betz Chair Professor of Environmental Engineering at Drexel University. He is widely recognized for his research in the areas of microbial and chemical risk assessment, hazardous waste processing, industrial wastewater treatment, waste recovery, and water and wastewater disinfection processes. Dr. Haas is a fellow of the American Academy of Microbiology and is the founding editor in chief of Quantitative Microbiology. He is also a member of the Council of the Society for Risk Analysis. He received his M.S. in environmental engineering from the Illinois Institute of Technology and his Ph.D. from the University of Illinois.

William E.Halperin is professor and chairman of the Department of Preventive Medicine and Community Health at the New Jersey Medical School. Before joining the faculty of the medical school, Dr. Halperin was a senior scientist with the National Institute for Occupational Safety and Health and also held the position of deputy director. His research interests are in occupational medicine, occupational epidemiology, and public-health surveillance. Dr. Halperin was a member of the NRC Committee on Risk Assessment Methodology and currently serves on the NRC Committee on Toxicology and its Subcommittee on Spacecraft Water Exposure Guidelines. He received his M.D., M.P.H., and Dr. P.H. from Harvard University and is certified by the American Board of Preventive Medicine and the American Board of Occupational Medicine.

Ellen Z.Harrison is director of the Cornell Waste Management Institute, a program of the Cornell Center for the Environment that develops solutions for waste-management problems and addresses broader issues of waste generation and composition, waste reduction, risk management, environmental quality, and public decision-making. Ms. Harrison has been involved for many years in the assessment of health and environmental risks from land application of sewage sludges. She has served as cochair of the Northeast Regional