1. BIC-ESAT and SKL-ESPC, College of Environmental Sciences and Engineering, Center for Environment and Health, Peking University, Beijing 100871, China
2. Department of Epidemiology and Biostatistics, MRC Centre for Environment and Health, Imperial College London, London W2 1PG, U.K.
3. Peking University Hospital, Peking University, Beijing 100871, China