Relative toxicities of major particulate matter constituents on birthweight in Massachusetts

Background: Maternal exposure to fine particulate air pollution (PM_2.5) during pregnancy has been linked to lower newborn birthweight, making it a toxic exposure because lower birthweight is a risk factor for chronic disease and mortality. However, the toxicity of major constituents of PM_2.5 and how they compare to each other remain uncertain. Methods: We assigned address-specific exposure to PM_2.5, elemental carbon (EC), organic carbon (OC), nitrate, and sulfate averaged over the entire period of pregnancy for each birth in Massachusetts from 2001 to 2012 using a high-resolution exposure model. Using multivariate regression adjusted for total PM_2.5, we estimated the relative toxicity of each constituent on continuous birthweight. Results: EC was more toxic per interquartile range increase compared with remaining PM_2.5 in single constituent models that estimated the effect of a constituent with adjustment for PM_2.5. OC, nitrate, and sulfate were each less toxic than their respective remaining PM_2.5 per interquartile range increase. When all constituents and total PM_2.5 were included in the same model, EC was most toxic, followed by nitrate, then OC and sulfate with similar toxicities. Sensitivity analyses using term low birth weight and small for gestational age also showed that EC was most detrimental as did averaging exposures over the third trimester of pregnancy. Scaling to unit mass increases also showed EC to be most toxic. Conclusion: Four major constituents of PM_2.5 had different relative toxicities on continuous birthweight. Our findings suggest that EC was most toxic, followed by nitrate, OC, and sulfate.