1. Ph.D. Student, Dept. of Geography and Environmental Engineering, Johns Hopkins Univ., 3400 N. Charles St., Baltimore, MD 21218 (corresponding author).
2. Associate Professor, Dept. of Environmental Health Sciences and Dept. of Biostatistics, Johns Hopkins Bloomberg School of Public Health, 615 N. Wolfe St., Baltimore, MD 21205.
3. Professor, Dept. of Geography and Environmental Engineering, Johns Hopkins Univ., 3400 N. Charles St., Baltimore, MD 21218.