The research, led by Stefania Papatheodorou, an OB-GYN and epidemiologist at the Rutgers School of Public Health, analyzed Medicaid health insurance records for nearly 900,000 mother-child pairs across the United States born between 2007 and 2014. Scientists from Rutgers, the Harvard T.H. Chan School of Public Health, Emory University, Massachusetts General Hospital and other institutions used modeled estimates of weekly wildfire smoke concentrations in the areas where each pregnant person lived.

The study found that for every incremental rise in air pollution from wildfire smoke, the risk of autism rose by 30%. The association was strongest during the second and third trimesters, though brain development continues throughout pregnancy.

The culprit is fine particulate matter known as PM 2.5 — particles roughly 30 times smaller than the width of a human hair that can be inhaled deep into the lungs and enter the bloodstream. Such pollution is already known to cause respiratory and cardiovascular problems, particularly in vulnerable populations such as children, seniors and people with conditions like asthma.

"We already know that exposure to wildfire smoke is harmful," Papatheodorou said. "There is very little evidence on how exposure to wildfire smoke during pregnancy could affect neurodevelopment during childhood. That's why we decided to combine these questions together and examine this association in a large database."

Because the study was observational, it cannot establish that wildfire smoke causes autism, and Papatheodorou said additional research is needed to confirm the association.

Still, she urged obstetricians to factor environmental hazards such as wildfires and extreme heat into routine prenatal care. She recommended that pregnant people limit time outdoors when air quality is poor and use home air purification systems.

"It doesn't have to be visible for the outside [particulate matter] levels to be harmful," Papatheodorou said. "So, instead of waiting until they see a big cloud coming, they can always check the air quality on their phones, which will alert them that, for example, the levels are not appropriate for groups that are sensitive."

The findings carry particular weight for the Philadelphia region, where wildfire smoke from Canada blanketed the area for five days in 2023, prompting schools to switch to virtual learning and causing delays at Philadelphia International Airport. A report by the New Jersey Department of Health found that the 2023 wildfires drove a spike in emergency room visits for new asthma diagnoses in the state.

Climate scientists expect wildfires to become more frequent as regions farther north, including Canada, grow hotter. High temperatures dry out trees and plants, increasing fire risk, while trapped stagnant air can worsen pollution. New Jersey firefighters have recently battled an unseasonably high number of wildfires triggered by severe drought conditions.

"The population disease burden is going to increase with the increased exposure to fires. So, it is something that we need to pay a lot of attention to," Papatheodorou said. "If people are aware of the health risks, they can take measures to protect themselves."