1. Department of Pulmonary and Critical Care Medicine, Center of Respiratory Medicine, China-Japan Friendship Hospital, Beijing, P. R. China
2. Graduate School, Peking Union Medical College, Chinese Academy of Medical Sciences, Beijing, P. R. China
3. Department of Pulmonary and Critical Care Medicine, Center of Respiratory Medicine, National Center for Respiratory Medicine, Beijing, P. R. China
4. Department of Pulmonary and Critical Care Medicine, Center of Respiratory Medicine, Institute of Respiratory Medicine, Chinese Academy of Medical Sciences, Beijing , P. R. China
5. Department of Pulmonary and Critical Care Medicine, Center of Respiratory Medicine, National Clinical Research Center for Respiratory Diseases, Beijing, P. R. China
6. Department of Pulmonary and Critical Care Medicine, Center of Respiratory Medicine, WHO Collaborating Centre for Tobacco Cessation and Respiratory Diseases Prevention, Beijing, P. R. China