A U.S. study of Medicaid records found an association between prenatal wildfire-particle exposure estimates and later autism diagnoses, but its design cannot establish that smoke caused the outcome.
Published online Oct. 26, 2026, in JAMA Pediatrics, the study analyzed records for 893,212 mother-child pairs. The children were born from 2007 to 2014, and diagnoses were tracked through 2018. Stefania Papatheodorou, an epidemiologist at Rutgers School of Public Health, led the research.
For a 0.75-microgram-per-cubic-meter difference in cumulative prenatal wildfire PM2.5 exposure, the estimated hazard of an autism spectrum disorder (ASD) diagnosis was 30% higher (hazard ratio, 1.30; 95% confidence interval, 1.07 to 1.59). The association was strongest during midpregnancy to late pregnancy, particularly after about 15 weeks. The same exposure difference showed no meaningful association with attention-deficit/hyperactivity disorder diagnoses (hazard ratio, 1.03; 95% confidence interval, 0.91 to 1.17).
Researchers built weekly pregnancy exposure estimates from modeled daily concentrations on a 1-square-kilometer grid, then summarized them by residential ZIP code. PM2.5 refers to fine particles 2.5 micrometers or smaller. ZIP-code estimates cannot capture each person’s exposure, including time indoors, and the authors said residual confounding may remain.
Mothers had to be continuously enrolled in Medicaid from three months before the last menstrual period through one month after delivery. Their recorded residential ZIP code also had to match at the last menstrual period and delivery; the study did not establish that they stayed in the same ZIP code at every point between those dates.
Earlier California studies reported mixed findings. One found an association among people who did not move and experienced more than 10 days of wildfire smoke during the third trimester, but not with average wildfire PM2.5 alone. Another, based on 8.6 million California births from 2001 to 2019, found mostly null results, with higher ASD odds in a subgroup exposed to very high wildfire pollution and low background PM2.5. The JAMA Pediatrics authors said differences in exposure measurement, study populations and analysis may help explain the variation.
“Additional studies are needed to confirm the association and better understand the potential effects of wildfire smoke,” Papatheodorou said.
