Associations between fine particulate matter, extreme heat events, and congenital heart defects

Background: Previous research reports associations between air pollution measured during pregnancy and the occurrence of congenital heart defects (CHDs) in offspring. The objective of this research was to assess if exposure to extreme heat events (EHEs) during pregnancy may modify this association. Methods: The study population consisted of 4,033 controls and 2,632 cases with dates of delivery between 1999 and 2007 who participated in the National Birth Defects Prevention Study, a multi-site case–control study in the United States. Daily data from the closest stationary fine particulate matter (PM 2.5 ) monitor within 50 km from the maternal residence were averaged across weeks 3–8 post-conception. EHEs were defined as maximum ambient temperature in the upper 95th percentile for at least 2 consecutive days or the upper 90th percentile for 3 consecutive days. Logistic regression models were adjusted for maternal age, ethnicity, education, and average humidity. Relative excess risks due to interaction (RERI) were calculated. Results: Compared with women with low PM 2.5 exposure and no exposure to an EHE, the odds of a ventricular septal defect in offspring associated with high PM 2.5 exposure was elevated only among women who experienced an EHE (odds ratio [OR] 2.14 95% confidence interval [CI] 1.19, 3.38 vs. OR 0.97 95% CI 0.49, 1.95; RERI 0.82 95% CI −0.39, 2.17). The majority of observed associations and interactions for other heart defects were null and/or inconclusive due to lack of precision. Conclusions: This study provides limited evidence that EHEs may modify the association between prenatal exposure to PM 2.5 and CHD occurrence.