1. Department of Respiratory Medicine, First Affiliated Hospital, Xi'an Medical University, Xi'an 710077, Shaanxi, China
2. Department of Respiratory Medicine, Tangdu Hospital, Fourth Military Medical University, Xi'an, Shaanxi, China
3. Department of Respiratory Medicine, Gansu Provincial Hospital, Lanzhou, Gansu, China
4. Institute of Epidemiology and Health Statistics, School of Public Health, Lanzhou University, Lanzhou, Gansu, China