Associations of PM2.5 and PAHs from wildland fires with pregnancy outcomes: evidence based on high-resolution exposure assessment.

Wildfire threats are increasing dramatically and compromising decades of progress in air quality improvements. However, research on wildfire PM2.5 and preterm birth (PTB) remains limited. No studies have examined wildfire-emitted polycyclic aromatic hydrocarbons (PAHs), a potential driver of wildfire PM2.5 toxicity, or specifically investigated preterm premature rupture of membranes (PPROM), a leading cause of PTB. We used statewide birth certificate data in California and estimated hourly concentrations of PM2.5 and particulate-phase PAHs from wildland fires at a 1 km resolution in 2017 and 2018, based on a two-stage machine learning model integrating the Community Multiscale Air Quality model outputs. We applied the generalized linear mixed model to examine the associations of wildfire PM2.5 and PAH exposures during pregnancy with PTB and PPROM. We examined the effect modification by maternal characteristics and other environmental exposures and calculated the population-attributable fractions of PTB. We observed positive associations of maternal exposures to wildfire PM2.5 and PAHs with PTB and PPROM among 627,494 births, particularly during the second trimester. Every interquartile range increase in wildfire PM2.5 was associated with a 13% (95% CI: 12%, 14%) higher risk of PTB. Race/ethnicity, pre-pregnancy body mass index, season of conception, non-wildfire PM2.5 exposure, and temperature exposure were important effect modifiers. Approximately 6.96% of PTB cases can be attributed to higher exposure to wildfire PM2.5 during pregnancy. These findings highlight the need for targeted public health interventions to reduce wildfire smoke exposure during pregnancy, especially among vulnerable populations.