Biographical note of the first author: Li-Wen HU, female, born in 1981, doctoral student at the School of Public Health, China Medical University, majoring in occupational and environmental health. Correspondence should be addressed to Professor Yang LIU, Department of Environmental Health, School of Public Health, China Medical University, No. 92 Second Road Heping District Shenyang 110001, P. R. China. Tel: 86-24-23256666-5407. Fax: 86-24-23264417. E-mail: cmuliuyang@yahoo.com