Methods Thirty-two Wister rats were distributed randomly into 4 groups:A,B,C and D. Group A was used as sham control. Group B,C and D were exposed to passive cigarette smoking daily for 2,3 and 4 months,respectively.

• 方法将32只Wister大白鼠随机分成4组,分别予以不吸烟、被动吸烟2个月、3个月和4个月;