Background:Prenatal exposure to fine particulate matter air pollution with aerodynamic diameter ≤2.5μm (PM2.5) has been associated with preterm delivery and low birth weight (LBW), but few studies have examined possible effect modification by PM2.5 oxidative potential.Objectives:The aim of this study was to evaluate if regional differences in the oxidative potential of PM2.5 modify the relationship between PM2.5 and adverse birth outcomes.Methods:A retrospective cohort study was conducted using 196,171 singleton births that occurred in 31 cities in the province of Ontario, Canada, from 2006 to 2012. Daily air pollution data were collected from ground monitors, and city-level PM2.5 oxidative potential was measured. We used random-effects meta-analysis to combine the estimates of effect from regression models across cities on preterm birth, term LBW, and term birth weight and used meta-regression to evaluate the modifying effect of PM2.5 oxidative potential.Results:An interquartile increase (2.6 μg/m3) in first-trimester PM2.5 was positively associated with term LBW among women in the highest quartile of glutathione (GSH)-related oxidative potential [odds ratio (OR)=1.28; 95% confidence interval (CI): 1.10, 1.48], but not the lowest quartile (OR=0.99; 95% CI: 0.87, 1.14; p-interaction=0.03). PM2.5 on the day of delivery also was associated with preterm birth among women in the highest quartile of GSH-related oxidative potential [hazard ratio (HR)=1.02; 95% CI: 1.01, 1.04], but not the lowest quartile [HR=0.97; 95% CI: 0.95, 1.00; p-interaction=0.04]. Between-city differences in ascorbate (AA)-related oxidative potential did not significantly modify associations with PM2.5.Conclusions:Between-city differences in GSH-related oxidative potential may modify the impact of PM2.5 on the risk of term LBW and preterm birth. https://doi.org/10.1289/EHP2535

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