1. Department of Internal Medicine and Environmental Health Center, Kangwon National University Hospital, Chuncheon, Korea.
2. Department of Respiratory Center, Kangwon National University Hospital, Kangwon National University School of Medicine, Chuncheon, Korea.
3. Department of Internal Medicine, Seoul St. Mary's Hospital, The Catholic University of Korea College of Medicine, Seoul, Korea.
4. Department of Internal Medicine, Konkuk University School of Medicine, Seoul, Korea.
5. Department of Internal Medicine, CHA Bundang Medical Center, CHA University, Seongnam, Korea.
6. Department of Internal Medicine, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Korea.
7. Division of Pulmonology, Department of Internal Medicine, Hanyang University Guri Hospital, Hanyang University College of Medicine, Guri, Korea.
8. Department of Internal Medicine, Ewha Womans University Mokdong Hospital, Ewha Womans University College of Medicine, Seoul, Korea.
9. Division of Pulmonary and Critical Care Medicine, Department of Medicine, Kangbuk Samsung Hospital, Sungkyunkwan University School of Medicine, Seoul, Korea.
10. Department of Pulmonary and Critical Care Medicine and Clinical Research Center for Chronic Obstructive Airway Diseases, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.