1. The Department of Occupational and Environmental Health; School of Public Health; Ningxia Medical University; 750004, 1160 Shengli Street, Xingqing District Yinchuan 750004 People's Republic of China
2. School of Public Health; Key Lab of Public Health Safety; Ministry of Education; WHO Collaborating Center for Occupational Health, Fudan University; Shanghai 200032 China