Maternal exposure to outdoor air pollution and congenital limb deficiencies in the National Birth Defects Prevention Study

Abstract

Background: Congenital limb deficiencies (CLDs) are a relatively common group of birth defects whose etiology is mostly unknown. Recent studies suggest maternal air pollution exposure as a potential risk factor.

Aim: To investigate the relationship between ambient air pollution exposure during early pregnancy and offspring CLDs.

Methods: The study population was identified from the National Birth Defects Prevention Study, a population-based multi-center case-control study, and consisted of 615 CLD cases and 5,701 controls with due dates during 1997 through 2006. Daily averages and/or maxima of six criteria air pollutants (particulate matter <2.5 μm [PM_2.5], particulate matter <10 μm [PM₁₀], nitrogen dioxide [NO₂], sulfur dioxide [SO₂], carbon monoxide [CO], and ozone [O₃]) were averaged over gestational weeks 2-8, as well as for individual weeks during this period, using data from EPA air monitors nearest to the maternal address. Logistic regression was used to estimate odds ratios (aORs) and 95% confidence intervals (CIs) adjusted for maternal age, race/ethnicity, education, and study center. We estimated aORs for any CLD and CLD subtypes (i.e., transverse, longitudinal, and preaxial). Potential confounding by co-pollutant was assessed by adjusting for one additional air pollutant. Using the single pollutant model, we further investigated effect measure modification by body mass index, cigarette smoking, and folic acid use. Sensitivity analyses were conducted restricting to those with a residence closer to an air monitor.

Results: We observed near-null aORs for CLDs per interquartile range (IQR) increase in PM₁₀, PM_2.5, and O₃. However, weekly averages of the daily average NO₂ and SO₂, and daily max NO₂, SO₂, and CO concentrations were associated with increased odds of CLDs. The crude ORs ranged from 1.03 to 1.12 per IQR increase in these air pollution concentrations, and consistently elevated aORs were observed for CO. Stronger associations were observed for SO₂ and O₃ in subtype analysis (preaxial). In co-pollutant adjusted models, associations with CO remained elevated (aORs: 1.02-1.30); but aORs for SO₂ and NO₂ became near-null. The aORs for CO remained elevated among mothers who lived within 20 km of an air monitor. The aORs varied by maternal BMI, smoking status, and folic acid use.

Conclusion: We observed modest associations between CLDs and air pollution exposures during pregnancy, including CO, SO₂, and NO₂, though replication through further epidemiologic research is warranted.

Keywords: Air pollution; Carbon monoxide; Congenital limb deficiencies; Nitrogen dioxide; Sulfur dioxide.

Fig. 1.

Gestational-period-specific CO concentrations and odds of CLDs in the restricted (residential address within 20 km of air monitoring station) and original analyses, adjusting for maternal education, race/ethnicity, age, and study center.