- Correspondence to Dr Jian Li, Institute of Occupational and Social Medicine, Center for Health and Society, Faculty of Medicine, University of Düsseldorf, Universitätsstrasse 1, Düsseldorf 40225, Germany;
- Received February 12, 2013
- Revised February 12, 2013
- Accepted March 17, 2013
- First published July 23, 2013.
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