A nationwide study of 893,212 U.S. births using Medicaid records has linked prenatal exposure to wildfire smoke during the middle and later stages of pregnancy with a higher risk of autism diagnoses in children, while finding no meaningful association with attention-deficit/hyperactivity disorder (ADHD). The research, led by Stefania Papatheodorou of the Rutgers School of Public Health, analysed national health data to explore how smoke from wildfires might influence early brain development.
Wildfire smoke is a complex mix of gases and fine particulate matter, known as PM2.5, with particles 2.5 micrometres or smaller in diameter. The study examined whether exposure to these particles during pregnancy relates to two common neurodevelopmental conditions in early childhood: autism spectrum disorder and ADHD.
In recent years, rising temperatures and droughts have intensified wildfires, producing heavier smoke plumes that can travel long distances. Researchers note that wildfire-derived fine particles tend to be more toxic due to their chemical makeup, smaller size, and frequent co-occurrence with high heat.
Scientists suggest several biological pathways by which prenatal exposure to fine particles could affect fetal development. A 2021 review argued that these particles can cross the placenta and trigger brain inflammation and oxidative stress, potentially disrupting neuron development and the maturation of neural circuits.
Brain imaging work has shown associations between prenatal fine particle exposure and changes in newborn brain development, including reduced myelin. Yet epidemiological evidence linking air pollution to specific childhood diagnoses has been mixed, with some studies finding no consistent ADHD risk from general particulate pollution.
“Wildfire smoke is an increasingly important public health concern, and its effects extend well beyond the communities where fires occur,” the study’s lead author Stefania Papatheodorou told PsyPost. “Smoke can travel long distances, exposing pregnant individuals hundreds or even thousands of miles away.”
To investigate on a national scale, the researchers drew on the U.S. Medicaid Analytic Extract database, covering more than 40 percent of births. The cohort comprised 893,212 mother-child pairs with pregnancies conceived on or after January 1, 2007, and delivered through 2014. The sample was restricted to women aged 12 to 55 who remained continuously enrolled in Medicaid from three months before pregnancy through one month after delivery and who had the same residential ZIP code at the start and end of pregnancy.
Participants averaged about 25 years old, with children nearly evenly split by sex. On the maternal side, 40.6 percent were White, 30.5 percent Black, 21.3 percent Hispanic or Latino, 4.1 percent Asian or Other Pacific Islander, and 3.5 percent of unknown race or ethnicity. Children were followed through the end of 2018, giving an average follow-up of 6.3 years for autism diagnoses and 7.1 years for ADHD diagnoses.
“An important strength is the inclusion of a large population of Medicaid-insured families, who are often underrepresented in research,” Papatheodorou said. “Understanding environmental risks in these populations is essential to developing public health responses that serve all families.”
Exposure levels were estimated with a machine-learning framework that mapped daily fine particulate concentrations across the contiguous United States, isolating the wildfire-derived fraction by subtracting background pollution and aggregating the data weekly for each mother’s ZIP code.
The team used survival models paired with distributed lag models to assess how exposure during each week of pregnancy related to later diagnoses. They tracked exposure across the first 33 weeks of pregnancy, a window selected to cover most pregnancies while including many births that occur prematurely.
Researchers controlled for numerous potential influences, including maternal age, race and ethnicity, parity, season of conception, prenatal folic acid prescriptions, smoking, alcohol or substance use, and nutrition. They also accounted for outdoor temperature, non-wildfire background pollution, and socio-economic factors such as ZIP-code income, education levels, population density, urban or rural status, obesity rates, and distance to the nearest hospital.
Across the full cohort, 8,357 children were diagnosed with autism and 22,302 with ADHD by the end of follow-up, corresponding to cumulative incidences of 1.9 percent and 10.4 percent respectively. Wildfire smoke accounted for between 4.4 percent and 7.5 percent of total fine particulate matter, with pregnancy-long averages of wildfire-specific PM2.5 around 0.55 micrograms per cubic metre versus about 9.4 micrograms per cubic metre from non-wildfire sources.
The study found that an interquartile range increase in cumulative wildfire PM2.5 across pregnancy—defined as 0.75 micrograms per cubic metre—was linked to a 30 percent relative increase in autism risk (hazard ratio 1.30, with a plausible range from a 7 percent decrease to a 59 percent increase). The association was not strictly linear, rising at low and moderate smoke levels before plateauing at the highest concentrations.
“This describes a relative difference in the rate of diagnosis during follow-up,” Papatheodorou explained. “Even a moderate association can matter at the population level when an exposure affects many pregnancies, but translating this result into an absolute increase in risk requires additional information.”
She added: “One notable finding was that the association with autism was apparent in a national cohort rather than a population selected for proximity to major wildfires. This reinforces the importance of considering transported smoke and cumulative exposure during pregnancy, rather than focusing only on people living near a fire.”
By contrast, the same 0.75 microgram per cubic metre increase in prenatal wildfire smoke showed no meaningful link with ADHD, yielding a hazard ratio of 1.03 with a plausible range from a 9 percent decrease to a 17 percent increase. Non-wildfire background pollution was associated with a smaller, 10 percent rise in autism risk and was not linked to ADHD. In weekly analyses, risk rose gradually from early to late pregnancy for both outcomes, strongest for autism during mid-to-late gestation, particularly after around week 15.
“We also did not observe a clear association with ADHD,” the author noted, adding that this discrepancy invites further investigation into possible biological pathways, timing of exposure, or diagnostic practices.
Examining the data by child sex, autism risk appeared about 31 percent higher across both boys and girls, though the estimate for girls was less precise. Week-by-week analyses suggested higher risk for boys across most of gestation, with increases for girls mainly in mid-to-late pregnancy.
Autism risks were elevated across all racial and ethnic groups, and regional analysis suggested higher but imprecise estimates in the South for both conditions. The authors cautioned that regional differences could reflect variations in fire behaviour and smoke composition, climate, background pollution, socio-economic factors, healthcare access, and local diagnostic practices.
The findings held up under sensitivity tests that extended the pregnancy window to 37 weeks or restricted analyses to children diagnosed after age five. The researchers also tested the robustness of their results against unmeasured confounding, estimating that a hidden factor would need to be linked to both higher wildfire smoke exposure and autism by a risk ratio of at least 1.92 to fully explain away the observed association.
In discussing the broader literature, the authors noted partial alignment with studies linking airborne pollutants to neurological and psychiatric outcomes, including research associating certain pollution components with higher risks of autism and attention disorders. They also observed that wildfire smoke may be tied to short-term mental health issues among youth, reflecting findings from other settings.
“Our findings suggest that wildfire smoke exposure during pregnancy may have implications for children’s longer-term neurodevelopment, in addition to its better-known effects on respiratory and cardiovascular health,” Papatheodorou told PsyPost. “We observed an association with autism diagnoses, but our study does not establish that wildfire smoke causes autism. The broader message is that protecting people from wildfire smoke matters even far from an active fire.”
The authors acknowledge several limitations, including exposure assignment based on residential ZIP codes confirmed only at the start and end of pregnancy, which cannot capture moves or outdoor time or indoor filtration on smoky days. They also note that autism diagnoses came from health care records, meaning access to evaluation could influence recorded diagnoses. The study population—low-income Medicaid beneficiaries—may not automatically generalise to higher-income groups or reflect the heavier wildfire conditions seen in more recent years.
Lead author Papatheodorou emphasised that the study is observational and should not be read as proving a causal link between wildfire smoke and autism. “Our exposure estimates represent outdoor pollution where participants lived, rather than the amount of smoke each individual actually inhaled,” she cautioned. “Autism has a complex, multifactorial basis. These findings should not be interpreted as identifying a single cause.”
Looking ahead, the researchers plan to extend their work to examine whether associations differ across autism subtypes and to explore other neurodevelopmental outcomes, while considering how environmental and social conditions may shape susceptibility. Their aim, they say, is to translate the findings into prevention by identifying when reducing exposure matters most and which populations require additional support.
In summary, the study argues that wildfire smoke is a widespread environmental exposure with potential neurodevelopmental implications, underscoring the need for cleaner indoor air, better information, and support for affected families alongside traditional public health concerns.
The study, “Prenatal Exposure to Wildfire PM2.5 and Risk of ASD and ADHD in Children Enrolled in Medicaid,” lists Stefania Papatheodorou as lead author, with a collaborative group of researchers including Hayon Michelle Choi, Krista F. Huybrechts, Sonia Hernandez-Diaz, Yang Liu, Danlu Zhang, Matthew Shupler, Michael Leung, Wanyu Huang, Xinye Qiu, Yaguang Wei, Christopher J. McDougle, Evgenia Papatheodorou, Joel D. Schwartz, Brent A. Coull, Marc G. Weisskopf and Antonella Zanobetti.
