Wildland fire smoke, severe maternal morbidity, and health equity: A population-based cohort study in California

Despite the recent increase in the intensity and frequency of wildland fires in the United States, there is limited investigation into the impact of wildland fires on pregnancy-related mortality and morbidity. This study examined associations between maternal exposure to wildland fire smoke and severe maternal morbidity (SMM), or potentially life-threatening complications related to pregnancy or delivery, in a population-based cohort of California birthing people between 2007 and 2018. We measured pregnancy smoke-day burden using the Hazard Mapping System smoke product and wildland fire-specific fine particulate matter (PM_2.5, particles ≤2.5 μm) using bias-corrected Community Multiscale Air Quality Model results, linked to birthing people's residential locations. Logistic regression models estimated severe maternal morbidity risk associated with pregnancy smoke-day burden and average wildland fire-specific PM_2.5, during the whole pregnancy and within each trimester, adjusting for individual-level sociodemographic and health covariates. To investigate whether the impact of wildland fire smoke was more pronounced among racially marginalized populations, we conducted stratified analysis by race and ethnicity. In a sample of 4,584,110 individuals with singleton births, exposure to high pregnancy smoke-day burden and average wildland fire-specific PM_2.5, compared to low exposure, was associated with elevated risk of SMM (OR = 1.16, 95% CI: 1.13-1.19; OR = 1.12, 95% CI 1.09-1.14, respectively). Exposure during the second and third trimesters was more strongly associated with SMM, compared to first trimester exposure. Exposure to pregnancy smoke-day burden was most strongly associated with SMM among births to Asian and Pacific Islander individuals, and exposure to wildland fire-specific PM_2.5 was more strongly associated with SMM among births to Asian and Pacific Islander and Black individuals. Results demonstrate the urgency to understand how wildland fire smoke affects maternal health, as well as its role in shaping racial and ethnic inequities in SMM.