Yangzong Yangzong*, Zumin Shi, Per Nafstad, Lise L Håheim, Ouzhu Luobu and Espen Bjertness
* Corresponding author: Yangzong Yangzong yang.zong@medisin.uio.no
BMC Public Health 2012, 12:860 doi:10.1186/1471-2458-12-860