1. a Institute of Occupational Medicine Ltd., 8 Roxburgh Place, Edinburgh EH8 9SU, Scotland, U.K.
2. b Division of Environmental and Occupational Health, School of Public Health, Box 197 Mayo, 420 Delaware Street SE, Minneapolis, MN 55455
3. c Pollution Abatement Division, Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Herts SG 1 2BX, England, U.K.