Outdoor air pollution and term low birth weight in Japan

Introduction: Evidence has accumulated on the association between ambient air pollution and adverse birth outcomes. However, most of the previous studies were conducted in geographically distinct areas and suffer from lack of important potential covariates. We examined the effect of ambient air pollution on term low birth weight (LBW) using data from a nationwide population-based longitudinal survey in Japan that began in 2001. Methods: We restricted participants to term singletons (n = 44,109). Air pollution concentrations during the 9 months before birth were obtained at the municipality level and were assigned to the participants who were born in the corresponding municipality. We conducted multilevel logistic regression analyses adjusting for individual and municipality-level variables. Results: We found that air pollution exposure during pregnancy was positively associated with the risk of term LBW. In the fully adjusted models, odds ratios following one interquartile range increase in each pollutant were 1.09 (95% confidence interval: 1.00, 1.19) for suspended particulate matter (SPM), 1.11 (0.99, 1.26) for nitrogen dioxide (NO₂), and 1.71 (1.18, 2.46) for sulfur dioxide (SO₂). Specifically, effect estimates for SPM and NO₂ exposure at the first trimester were higher than those at other trimesters, while SO₂ was associated with the risk at all trimesters. Nonsmoking mothers were more susceptible to SPM and NO₂ exposure compared with smoking mothers. Conclusions: Ambient air pollution increases the risk of term LBW in a nationally representative sample in Japan.