California petroleum refinery retirements as a natural experiment to assess effects on adverse birth outcomes

Background: Numerous studies have linked residential proximity to refineries with higher pollutant exposures and adverse health outcomes, but few have examined impacts on birth outcomes. Four petroleum refineries in California have ceased operations since 2010, providing a natural experiment to evaluate their perinatal health impacts. Methods: We conducted a retrospective cohort study of singleton births between 2008 and 2022. The study population included infants conceived within 3 years prior or after one of four retired California refineries in four regions based on air quality management district boundaries. We conducted a difference-in-differences analysis using logistic regression to evaluate whether refinery closures led to reduced odds of adverse birth outcomes in exposed versus unexposed communities. Results: Retirements of most refineries resulted in a reduced odds of term low birth weight (tLBW) and small-for-gestational age (SGA) births among exposed populations during the postclosure period. The strongest associations were observed for the Alon Bakersfield refinery, with a 32% decrease in the odds of tLBW (95% confidence interval [CI] = 0.55, 0.83) and 19% decrease in the odds of SGA (95% CI = 0.74, 0.89). Conclusion: Our results indicate that petroleum refinery retirements in California were associated with a reduced likelihood of tLBW and SGA in nearby populations, suggesting that living near active refineries may lead to reduced fetal growth. Our findings suggest oil refinery retirements may improve birth outcomes in surrounding communities.