Department of Occupational and Environmental Health, and Key Laboratory of Environment and Health, Ministry of Education & Ministry of Environmental Protection, and State Key Laboratory of Environmental Health (incubating), School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei, PR China; Department of Epidemiology, Brown University School of Public Health, Providence, RI, USA. Electronic address: zengqiang506@hust.edu.cn.