Relative toxicities of major particulate matter constituents on birthweight in Massachusetts

Abstract

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.

Figure 1.

Estimated effects on birthweight per IQR increase in a PM_2.5 constituent adjusted for PM_2.5 in Massachusetts from 2001 to 2012 (n = 725,919). PM_2.5 constituents include EC, OC, nitrate, and sulfate. Point estimate and 95% CI per individual constituent are in purple whereas those for PM_2.5 are in teal. Four separate linear models for each of the four constituents were run and included adjustment for PM_2.5 and the following covariates: maternal age, race, marital status, smoking, education, parity, chronic diabetes, gestational diabetes, chronic high blood pressure, high blood pressure during pregnancy, Kessner index of adequacy of prenatal care, mode of delivery, clinical gestational age, year of birth, newborn sex, and Medicaid-supported prenatal care. IQRs (µg/m³): PM_2.5 = 2.0, EC = 0.2, OC = 1.4, nitrate = 0.5, and sulfate = 1.5.

Figure 2.

Estimated effects on birthweight per IQR increase in PM_2.5, EC, OC, nitrate, and sulfate. PM_2.5 constituents include EC, OC, nitrate, and sulfate in Massachusetts from 2001 to 2012 (n = 725,919). Point estimate and 95% CI per individual constituent are in purple while that per PM_2.5 is in teal. A single multi-constituent linear regression with the four constituents and PM_2.5 was run and included the following covariates: maternal age, race, marital status, smoking, education, parity, chronic diabetes, gestational diabetes, chronic high blood pressure, high blood pressure during pregnancy, Kessner index of adequacy of prenatal care, mode of delivery, clinical gestational age, year of birth, newborn sex, and Medicaid-supported prenatal care. IQRs (µg/m³): PM_2.5 = 2.0, EC = 0.2, OC = 1.4, nitrate = 0.5, and sulfate = 1.5.