Biographical note of the first author: Ai GAO, female, born in 1976, Ph.D., research direction: carcinogenesis of environmental and occupational hazards. Correspondence shouid be addressed to Bing-Ci LIU, Institute of Occupational Health and Poison Control, Chinese Center for Disease Control and Prevention, 29 Nanwei Road, Beijing 100050, China. Tel: 86-010-83132527. Fax: 86-010-83132527. E-mail: bcliu@263.net