1 Beijing Chao-Yang Hospital, Capital Medical University, Beijing 100020, China; Beijing Institute of Respiratory Medicine, WHO Collaborating Center for Tobacco or Health, Beijing, China; Department of Respiratory Medicine, Capital Medical University, Beijing, China. Electronic address: danxiao@263.net.
2 Clinical Trials Service Unit, Oxford University, Oxford, UK.
3 Beijing Institute of Respiratory Medicine, WHO Collaborating Center for Tobacco or Health, Beijing, China; Department of Respiratory Medicine, Capital Medical University, Beijing, China; Beijing Hospital, National Clinical Research Centre for Respiratory Diseases, Medical Research and Education Department, National Health and Family Planning Commission of the People's Republic of China, Beijing, China.