Prenatal exposure to wildfire-related PM2.5 and autism spectrum disorder in children born in California between 2001–2019

Wildfire smoke is an increasingly prevalent source of air pollution and contains a complex mixture of neurotoxic constituents that could affect neurodevelopment, yet its potential role in autism spectrum disorder (ASD) etiology remains unexplored. We examined associations between wildfire-specific PM 2.5 exposures and ASD risk in California births, considering exposure timing, intensity, and background air pollution. We analyzed 8.6 million births in California from 2001 to 2019, linking records to ASD diagnoses in California Department of Developmental Services records through 2022. Wildfire-related PM 2.5 exposures, including biomass burning and wildland-urban-interface (WUI) source tracers, were estimated for preconception, pregnancy, and post-pregnancy periods. Exposure metrics included continuous, percentile-based, and episode-based indicators. Logistic regression was used to estimate odds ratios, adjusting for sociodemographic covariates. Stratified analyses were conducted by background PM 2.5 , urbanicity, diagnostic era, and sex. Continuous wildfire-specific exposures during the pregnancy-period were weakly associated with ASD, but stronger effects emerged at high exposure percentiles and during intense wildfire episodes (≥35 µg/m 3 ). Associations were most pronounced among children in the lowest quartile of prenatal background PM 2.5 , with odds of ASD increasing by 50% for those in the > 90th percentile of wildfire PM 2.5 during pregnancy. WUI-related PM 2.5 was associated with ASD in metro areas, suggesting unique toxicity. Effects were stronger in non-metro regions, with the exception of WUI-related PM 2.5 , for births occurring after the implementation of DSM-5, and in White children. Sex differences were minimal, but showed stronger effect estimates for WUI-related PM 2.5 and high exposure episodes for males. Wildfire smoke exposure during pregnancy may increase ASD risk, particularly in low background air pollution and rural settings. High-intensity wildfire events were most strongly associated with ASD, underscoring the need for targeted policies, such as vegetation management, land use planning, and indoor filtration upgrades in the context of increasing wildfire frequency and severity.