1. J. M. Samet is in the Department of Epidemiology, Johns Hopkins University, Bloomberg School of Public Health, Baltimore, MD 21205, USA.
2. D. M. DeMarini is at the U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA.
3. H. V. Malling is at the National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA.