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Cigarette smoke is one of the most important environmental risk factors for the development of cervical intraepithelial neoplasms (CINs). Given that the prevalence of cigarette smoking in Taiwanese women is very low, compared with Caucasian women (3-4% vs. ∼28%, respectively), direct smoking is likely a minimal risk factor in the Taiwanese group. However, the evaluation of the association between indirect tobacco exposure, or exposure to environmental tobacco smoke (ETS), and CIN risk in Taiwanese women may be enlightening. The authors designed a community-based nested case-control study to examine the association between ETS exposure and CIN risk among Taiwanese women who were nonsmokers. The study was conducted in Chia-Yi, a city in southwestern Taiwan. The test population comprised adult females (n = 32,466) who had undergone Pap smear screening during the 14-mo period that preceded this study. Potential cases were chosen from individuals who demonstrated positive Pap smear results, which were indicative of at least a level II cervical intraepithelial neoplasm (i.e., ≥ CIN2), as confirmed by cervical biopsy (n = 116). The authors chose 2 matched controls for each case; criteria used were (a) the 2 controls for each case had to be approximately the same age (± 2 yr) as the case; (b) each case and respective control lived in the same residential area; and (c) each control had a negative Pap smear, determined during the same time frame (± 6 mo) that the matched study subject was tested. A total of 100 study subjects and 197 control females (3 cases had only 1 control) underwent interviews by public health nurses, who obtained information about the participants' active and passive smoking histories and other potential risk factors. The incidence of cigarette smoking was low (5.0% [n = 5]) in study subjects and lower in controls (3.1% [n = 6]). The authors' final analysis was restricted to 89 nonsmoking pairs (89 subjects and 175 controls). Of the affected subjects, 85.4% reported exposure to passive smoke at home during their adulthood, vs. 61.1% of controls. The authors found that, after adjusting for other covariates, there was a 2.73-fold increased risk of contracting CIN (95% confidence interval [CI] = 1.31, 5.67) among ETS-exposed individuals. The authors assessed cumulative dose in terms of pack-years (i.e., average daily exposure [number of packs smoked] multiplied by the number of years the same exposure continued). Compared with nonsmokers, women exposed to 1-20 pack-yr and more than 20 pack-yr had 1.90-fold (95% CI = 0.72, 5.03) and 2.99-fold (95% CI = 1.10, 8.09) increased risks, respectively, of developing CIN. The authors concluded that lifetime ETS exposure is a major determinant for contracting cervical neoplasms among nonsmoking women in Taiwan.