Air pollution and birth weight among term infants in California.

OBJECTIVE To examine associations between birth weight and air pollution among full-term infants in California. METHODS We matched exposure data collected from air pollution monitors for small particles (PM(2.5)) and carbon monoxide (CO) to California birth records for singleton births delivered at 40 weeks' gestation in 2000 using the locations of the monitors and mother's residence. Pollution measurements collected within 5 miles of the mother's residence, averaged for the time period corresponding to the duration of pregnancy and each trimester, were used as exposure variables. Logistic and linear regression models were used to estimate the associations between the pollution measures and 2 pregnancy outcomes: small for gestational age (SGA) and birth weight. Variations of the models were used to examine the robustness of the findings. RESULTS The adjusted odds ratio for SGA for exposure in the highest compared with lowest quartile of PM(2.5) was 1.26 (95% confidence interval [CI]: 1.03-1.50). We found no association between CO and birth weight or SGA after controlling for maternal factors and PM(2.5) (mean birth weight difference: 2.6 g; 95% CI: -20.6 to 25.8). The difference in mean birth weight for infants with a 9-month exposure in the highest quartile of PM(2.5) compared with that of infants who were exposed in the lowest quartile was -36.1 g (95% CI: -16.5 g to -55.8 g); this difference was similar after controlling for CO. We did not find PM(2.5) exposure during a particular trimester most important for assessing birth weight; trimester-level associations were similar to those found using the 9-month exposure variable. CONCLUSIONS We found an increased odds of SGA and a small difference in mean birth weight between infants with the highest and lowest exposures to PM(2.5) but not CO. These findings have important implications for infant health because of the ubiquitous exposure to fine particulate air pollution across the United States.