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Objective: This study of a birth cohort in the city of Tongliang in Chongqing, China, evaluated the relationship between two prenatal exposures (polycyclic aromatic hydrocarbons(PAH) and environmental tobacco smoke(ETS)) and child intelligence quotient (IQ) as measured by the Wechsler Preschool and Primary Scale of Intelligence at age 5 years. A coal-fired power plant was the major source of ambient PAH in this city. We tested the hypothesis that, after adjusting for potential confounders, prenatal exposure to these pollutants would be associated with lower IQ scores at 5 years of age. Methods: Nonsmoking mothers and children were enrolled before delivery. PAH exposure was measured by DNA adducts in umbilical cord white blood cells using High-Performance Liquid Chromatography-Fluorescence. Estimated exposure to environmental tobacco smoke was based on personal interview. At age 5 years, scores for verbal, performance, and full scale IQ were obtained. Multiple regression was used to test the main effects of adducts and environmental tobacco smoke on IQ and to explore the interactions between these exposures on IQ. Results: after adjusting for potential confounders, neither DNA adducts nor exposure to environmental tobacco smoke had significant main effects on IQ. However, significant interactions between adducts and environmental tobacco smoke were observed on full scale (p=0.025) and verbal (p=0.029) IQ scores, indicating that the adverse effects of prenatal PAH exposure became greater as exposure to environmental tobacco smoke increased. The interaction on performance IQ score was not significant (p=0.135). Conclusion: These results suggest that exposure of pregnant women to emissions of PAHs from the coal-burning plant, in combination with prenatal exposure to environmental tobacco smoke, may have adversely affected cognitive function of children at age 5. The polluting coal-fired plant has since been closed by the government, with likely important benefits to child health and development. [Copyright &y& Elsevier]