1. School of Public Health, Xi'an Jiaotong University Health Science Center, Key Laboratory of Trace Elements and Endemic Diseases of The National Health and Family Planning Commission, Xi'an, Shaanxi 710061, P.R. China
2. School of Public Health, Shaanxi University of Chinese Medicine, Xianyang, Shaanxi 712046, P.R. China
3. School of Public Health, Brown University, Providence, RI 02906, USA