1. Department of Environmental & Occupational Medicine, The Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne, NE2 4HH, England, Walther-Straub-Institut für Pharmakologie und Toxikologie, Ludwig-Maximilians-Universität München, D-80336 München, Germany, and Institute of Environmental and Occupational Toxicology, Casella Postale 108, CH-6780 Airolo, Switzerland