Rutgers study links prenatal wildfire smoke exposure to autism risk

A Rutgers-led analysis of health insurance records found that exposure to fine particles from wildfire smoke during pregnancy was associated with a higher risk of autism diagnosis in children, with the strongest associations during the second and third trimesters.

The study in JAMA Pediatrics included 893,212 mother-child pairs enrolled in Medicaid, with children born between 2007 and 2014. Each 0.75-microgram-per-cubic-meter increase in the average concentration of wildfire-related fine particles during pregnancy was associated with a 30% higher risk of a subsequent autism diagnosis.

Wildfire smoke can travel far beyond the communities where fires are burning, and exposure may occur even when smoke is not visible. Our findings highlight the importance of awareness of wildfire events and their potential effects on air quality during pregnancy."

Stefania Papatheodorou, epidemiologist, Rutgers School of Public Health and lead author of the study

The association was strongest for exposures after the 15th week of pregnancy. Papatheodorou emphasized, however, that brain development continues throughout pregnancy. "Pregnancy is a period of rapid brain development, when environmental exposures may interfere with important developmental processes," Papatheodorou said. "Our findings suggest that the middle and later stages of pregnancy may be particularly relevant for understanding the potential effects of wildfire smoke on the developing brain and risk of autism."

The researchers focused on wildfire fine particulate matter, known as PM_2.5, which consists of inhalable particles generally 2.5 micrometers or smaller in diameter. These particles can penetrate deep into the lungs and contribute to inflammation throughout the body, providing one possible pathway through which exposure could affect pregnancy and fetal development. The study did not directly examine these biological mechanisms.

Previous research suggests that wildfire-related particles may be more harmful than particles from some other sources. In the Rutgers-led study, fine-particle pollution from sources other than wildfires was associated with a smaller increase in autism diagnosis risk. Two earlier studies, both limited to California, reached different conclusions about wildfire smoke and autism. This new study expanded the geographic scope to the contiguous United States, using modeled estimates of weekly wildfire smoke concentrations for each mother's residential ZIP code.

That broader perspective is increasingly relevant as major wildfires expose distant communities to smoke. In 2023, for example, smoke from Canadian wildfires affected air quality across New Jersey, New York and other parts of the United States. Because the study examined births before those events, the authors called for research in more recent birth cohorts.

The study was observational and cannot establish that wildfire smoke causes autism. Its focus on Medicaid enrollees may also limit how broadly the findings apply. Although results were similar across alternative analytic approaches, unmeasured factors and uncertainty in exposure estimates remain possible sources of bias.

"These findings add to the evidence on prenatal air pollution exposure and child neurodevelopment," Papatheodorou said. "Additional studies are needed to confirm the association and better understand the potential effects of wildfire smoke."

For pregnant women, Papatheodorou recommended awareness about wildfire events, checking local air-quality reports and following guidance for sensitive groups when pollution levels are elevated. Measures to reduce exposure include limiting time outdoors during smoke episodes, keeping smoke out of the home and using air filtration when available. A well-fitting particulate respirator can also help reduce exposure when going outdoors is necessary.

She encouraged obstetricians and other clinicians to incorporate discussions of environmental hazards, including wildfires and extreme heat, into routine prenatal care.

"Environmental counseling during pregnancy from clinicians is inadequate," Papatheodorou said. "Sometimes clinicians are not even aware of these risks." 

"Understanding how prenatal environmental exposures may contribute to autism, alongside genetic and other factors, remains an important research priority," Papatheodorou said.