From the Department of Epidemiology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD.
NOTES
Reprint requests to Dr. Jonathan M. Samet, Johns Hopkins Bloomberg School of Public Health, 615 N. Wolfe Street, Suite W6041, Baltimore, MD 21205 (e-mail: jsamet{at}jhsph.edu).
Received for publication August 14, 2001. Accepted for publication August 30, 2001.