Background: Maternal exposure to particulate matter under 2.5 u.m in aerodynamic diameter (PM2.5) during pregnancy is a well-established risk factor for decreased birthweight. Recently, there has been increased interest in how neighborhood racial and economic segregation modify health impacts of PM2.5. We therefore investigated the association between PM2.5 and birthweight at different levels of racial and economic segregation. Methods: For each birth in Massachusetts from 2001 to 2013, we calculated the average PM2.5 maternal exposure during pregnancy using the geocoded maternal residence at birth, infant birthdate, and clinical gestational age. Racial residential segregation (RRS) and economic residential segregation (ERS) indices were calculated at Census block group level and matched to maternal residence. RRS and ERS range from -1 to 1, with -1 indicating geographic concentration of the least privileged and 1 the most privileged. We quantified the association between PM2.5 and continuous birthweight overall and in each quartile of RRS and ERS with regression analysis, controlling for individual risk factors. Results: In 783,966 singleton full-term live births, an interquartile range (IQR) increase in PM2.5 (2.3 ug/m3) was associated with a 20g (95% CI: 18, 23) decrease in birthweight. RRS and ERS individually modified this association. The estimated birthweight decrease per IQR increase in PM2.5 was 6 g (95% CI: 0, 12) in the lowest quartile of RRS (1Q); 22 g (95% CI: 17, 28) in 2Q; 17 g (95% CI: 12, 22) in 3Q; and 5 g (95% CI: 0, 9) in 4Q. For ascending quartiles of ERS, the estimated decreases per IQR increase in PM2.5 were 12 g (95% CI: 7, 18), 24 g (95% CI: 19, 29), 14 g (95% CI: 9, 19), and 15 g (95% CI: 10, 20) respectively. Conclusions: Neighborhood racial and economic segregation modified the association between PM2.5 and decreased birthweight. The impacts of PM2.5 were most severe among those who lived in neighborhoods in the middle quartiles of RRS and ERS.
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