1. Silent Spring Institute, 29 Crafts Street, Newton, Massachusetts 02458, Department of Environmental Science, Policy and Management and School of Public Health, 137 Mulford Hall, University of California, Berkeley, California 94720, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78228, and Department of Environmental Health Sciences, Mailman School of Public Health, Columbia University, 60 Haven Ave B-109, New York, New York 10032