Wildfire Smoke Exposure During Pregnancy and Preterm Birth in California

Background/Aim Owing to climate change and fire suppression policy, wildfires in the Western United States are a significant source of air pollution that threatens to reverse decades of progress in air pollution reduction; effects of wildfires on preterm birth (PTB) are not well understood. We assessed associations between prenatal exposure to wildland fire (wildfire and prescribed fire) smoke and risk of PTB in California. Methods The population included all live singleton births in California, 2007-2015. PTB was defined as gestational age <37 weeks. We assigned wildland fire smoke exposure to geocoded maternal residence at birth with weekly average concentrations of particulate matter ≤2.5 microns attributable to fires from USEPA’s Community Multiscale Air Quality Model (fire PM2.5) and smoke frequencies (smoke days) from National Oceanic and Atmospheric Administration’s Hazard Mapping System. We used logistic regression to estimate odds ratios (OR) for PTB associated with exposures in the final 6 weeks of pregnancy and Cox proportional hazard models to estimate hazard ratios (HR) for PTB in relation to weekly exposures. Models adjusted for maternal age, education, race/ethnicity, season, medical insurance, and smoking. Results Our study included 3,342,267 births. A one-smoke day increase in any of the final 6 weeks of the pregnancy was associated with increased odds of PTB, ranging from OR=1.01 (95%CI:1.01-1.02) in the final week of pregnancy to OR=1.03 (95%CI:1.02-1.03) 5 weeks before birth. A 10µg/m3 increase in fire PM2.5 in the final 6 weeks was associated with ORs ~1.04 (95%CIs covered 1.02-1.06). In survival analyses, HR=1.01 (95%CI:1.01-1.02) for a one-day increase in smoke frequency and HR=1.03 (95%CI:1.02-1.04) for a 10µg/m3 increase in fire PM2.5. Conclusion Wildland fire smoke is modestly associated with PTB. Results can inform efforts to slow climate change, manage forests, and reduce wildfire smoke exposure to pregnant people. Keywords wildfire; pregnancy; preterm birth; climate; air pollution