Prenatal Exposure to Wildfire PM 2.5 and Risk of ASD and ADHD in Children Enrolled in Medicaid

Importance Wildfires are an increasing source of fine particulate matter (PM 2.5 ), yet evidence on prenatal wildfire smoke exposure and long-term neurodevelopmental outcomes remains limited. Objective To examine whether prenatal exposure to wildfire-related PM 2.5 is associated with risk of autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) in children. Design, Setting, and Participants This was a population-based cohort study of the US nationwide Medicaid population using Medicaid Analytic Extract (MAX) data from 2007 to 2014 with follow-up through childhood. Included were mother-child pairs with continuous enrollment and stable residential zip code during pregnancy. Exposure Weekly wildfire smoke–attributable PM 2.5 at 1-km 2 resolution, aggregated to the zip code level across pregnancy. Main Outcomes and Measures Incident ASD and ADHD identified using validated claims-based algorithms. Associations were estimated using Cox proportional-hazards models with distributed lag nonlinear models. Results A total of 893 212 mother-child pairs (mother mean [SD] age, 24.7 [5.9] years) were included in this analysis. Children entered the cohort at birth (452 218 [50.6%] male). Mean (SD) observed follow-up was 6.3 (2.4) years for children diagnosed with ASD and 7.1 (1.9) years for children diagnosed with ADHD. Among 893 212 children, 8357 were diagnosed with ASD and 22 302 with ADHD. The cumulative incidence until age 8 years was 1.9% for ASD and 10.4% for ADHD. An IQR increase in cumulative wildfire PM 2.5 exposure (0.75 µg/m 3 ) during pregnancy was associated with increased risk of ASD (hazard ratio [HR], 1.30; 95% CI, 1.07-1.59) but not ADHD (HR, 1.03; 95% CI, 0.91-1.17). Associations for ASD were strongest during midpregnancy to late gestation compared with early gestation. Results were robust to sensitivity analyses. Conclusions and Relevance Results of this cohort study show that prenatal exposure to wildfire-related PM 2.5 may be associated with increased risk of ASD but not ADHD. These findings highlight the potential neurodevelopmental impacts of wildfire smoke and underscore the need for strategies to reduce exposure during pregnancy as wildfire activity intensifies.