1Department of Humanities and Social Medicine, Ajou University School of Medicine, Suwon, Korea
2Department of Occupational and Environmental Medicine, Pusan National University, Yangsan Hospital, Yangsan, Korea
3Department of Preventive Medicine, College of Medicine, Seoul National University, Seoul, Korea
4Department of Preventive Medicine, Yonsei University College of Medicine, Seoul, Korea
5Department of Occupational and Environmental Medicine, Wonju Severance Christian Hospital, Wonju College of Medicine, Yonsei University, Wonju, Korea
6Department of Preventive Medicine and Institute of Occupational and Environmental Medicine, Wonju College of Medicine, Yonsei University, Wonju, Korea
© The Author(s). 2016
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.