1. Department of Occupational and Environmental Medicine, Ajou University School of Medicine, Suwon, Korea.
2. Department of Occupational and Environmental Medicine, The Catholic University of Korea College of Medicine, Seoul, Korea.
3. Department of Preventive Medicine, Institute for Occupational Health, and Graduate School of Public Health, Yonsei University College of Medicine, Seoul, Korea.
4. Incheon Workers' Health Center, Incheon, Korea.