A register-based study of the association between air pollutants and hypertensive disorders in pregnancy among the Japanese population

Background: Ambient air pollution is hypothesized to be a risk factor for hypertensive disorders in pregnancy, one of the major pregnancy complications. Past studies have reported the supporting evidence, however this mainly referred to the Western population, and results from trimester-specific analysis have been varied. In this study, we focused on exposure during the first trimester of pregnancy (placental development stage), and tested the hypothesis among the Japanese population. Methods: We drew on data from the Japan Perinatal Registry Network database, and studied 36,620 singleton pregnant women without medical complications, in western Japan (Kyushu and Okinawa districts) between 2005 and 2010. In addition, data on ozone, suspended particulate matter (SPM), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂) concentrations were obtained. The nearest monitoring station to the respective birthing hospital was used as a reference point for assigning average concentrations of each pollutant during the first trimester of pregnancy for each woman. The logistic regression model was applied to assess the association between quintiles of each pollutant and hypertensive disorders in pregnancy. Results: Mean concentrations during the first trimester were 41.3ppb for ozone, 27.4μg/m³ for SPM, 11.8ppb for NO₂, and 3.2ppb for SO₂. High exposure to ozone was associated with an increased risk of hypertensive disorders in pregnancy (for highest quintile vs. lowest: odds ratio=1.20, 95% confidence interval=1.01-1.42). With regard to SPM, NO₂ and SO₂, we did not obtain the results with constant directionality. Conclusions: Ozone exposure during early pregnancy may be a risk factor for hypertensive disorders in pregnancy.