Correspondence to: Dr. Aaron Blair, National Cancer Institute, 9609 Medical Center Drive, Rockville, MD 20892. E-mail: blaira@mail.nih.gov;2. National Institute for Occupational Safety and Health, Cincinnati, Ohio;3. National Exposure Research Laboratory, U.S. Environmental Protection Agency, North Carolina;4. Division of Epidemiology and Genetics, National Cancer Institute, Rockville, Maryland;5. Department of Biological Sciences, North Carolina State University, Raleigh, North Carolina;6. Epidemiology Branch, National Institute for Environmental Health Sciences, North Carolina;7. College of Public Health, University of Iowa, Iowa City, Iowa