Geographic Variation in the Association between Ambient Fine Particulate Matter (PM(2.5)) and Term Low Birth Weight in the United States

BACKGROUND: Studies on the association between prenatal exposure to fine particulate matter ≤ 2.5 μm in aerodynamic diameter (PM(2.5)) and term low birth weight (LBW) have resulted in inconsistent findings. Most studies were conducted in snapshots of small geographic areas and no national study exists. OBJECTIVES: We investigated geographic variation in the associations between ambient PM(2.5) during pregnancy and term LBW in the contiguous United States. METHODS: A total of 3,389,450 term singleton births in 2002 (37–44 weeks gestational age and birth weight of 1,000–5,500 g) were linked to daily PM(2.5) via imputed birth days. We generated average daily PM(2.5) during the entire pregnancy and each trimester. Multi-level logistic regression models with county-level random effects were used to evaluate the associations between term LBW and PM(2.5) during pregnancy. RESULTS: Without adjusting for covariates, the odds of term LBW increased 2% [odds ratio (OR) = 1.02; 95% CI: 1.00, 1.03] for every 5-μg/m(3) increase in PM(2.5) exposure during the second trimester only, which remained unchanged after adjusting for county-level poverty (OR = 1.02; 95% CI: 1.01, 1.04). The odds did change to null after adjusting for individual-level predictors (OR = 1.00; 95% CI: 0.99, 1.02). Multi-level analyses, stratified by census division, revealed significant positive associations of term LBW and PM(2.5) exposure (during the entire pregnancy or a specific trimester) in three census divisions of the United States: Middle Atlantic, East North Central, and West North Central, and significant negative association in the Mountain division. CONCLUSIONS: Our study provided additional evidence on the associations between PM(2.5) exposure during pregnancy and term LBW from a national perspective. The magnitude and direction of the estimated associations between PM(2.5) exposure and term LBW varied by geographic locations in the United States. CITATION: Hao Y, Strosnider H, Balluz L, Qualters JR. 2016. Geographic variation in the association between ambient fine particulate matter (PM(2.5)) and term low birth weight in the United States. Environ Health Perspect 124:250–255; http://dx.doi.org/10.1289/ehp.1408798