Exposure to wildfire smoke during late pregnancy could increase the risk for autism in offspring, a new study showed.
Researchers analyzed more than 200,000 births in California and found that children who were prenatally exposed to wildfire smoke for more than 10 days in the third trimester had a 23% higher risk for autism diagnoses than unexposed children.
The findings come after California experienced one of the most destructive wildfire events in the state’s history last year.
“The fetal brain undergoes rapid growth and organization during late pregnancy, and our result suggests that the third trimester might be particularly a sensitive window when the developing brain is more vulnerable to environmental stressors such as wildfires,” principal investigator Mostafijur Rahman, PhD, MS, assistant professor of environmental health sciences at Tulane University in New Orleans, told Medscape Medical News.
The study was published online on January 20 in Environmental Science & Technology.
Rising Autism Rates
The rate of autism diagnoses among children has been steadily growing over the past decade, in part due to broader diagnostic criteria and increased screenings.
Autism has a strong genetic component, with recent research pinpointing four biologically distinct subtypes of autism.
However, “genetics alone do not explain why the prevalence of autism is high right now,” Rahman noted. It’s possible that environmental exposure during pregnancy might interact with genetic susceptibility and influence neurodevelopment, he added.
A 2021 study showed that exposure to particulate matter (PM) from air pollution during the third trimester was associated with a higher risk for autism in children. However, this new study is the first to specifically assess the link between exposure to wildfire smoke during pregnancy and the risk for autism.
The investigators included 204,374 mother-child pairs from 2006 to 2014 in their analysis.
Children were followed until age 5 and screened for autism at 18 and 24 months. Autism diagnoses were identified based on the International Classification of Diseases (ICD)-9 and ICD-10 codes.
Daily concentrations of PM with diameter less than 2.5 μm (PM2.5) from wildfires were assessed using a model that combined location, weather, and satellite data.
The investigators evaluated the number of days pregnant women were exposed to PM2.5 concentrations of about 3 μg/m3 and 5 μg/m3, and the number of smoke exposure waves, classed as consecutive days of smoke exposure.
Cox proportional hazards models were used to estimate hazard ratios (HRs) for autism diagnosis associated with wildfire smoke exposure. The models were adjusted for various child and mother demographic covariates, and birth year was included as a linear covariate to account for increasing rates of autism diagnoses.
They conducted a sensitivity analysis of families who didn’t move away from home during the wildfires, who were classed as “non-movers.” They hypothesized that associations would be stronger among non-movers due to less exposure misclassification.
A Key Window of Exposure
In the cohort, 3356 children were diagnosed with autism by age 5 (median age of diagnosis, 3 years), with four times as many boys diagnosed with autism as girls.
Investigators found significant associations between the number of days of wildfire smoke exposure in the third trimester of pregnancy and the risk for autism, particularly among non-movers.
Autism diagnoses were 10.8% higher in the group of pregnant individuals who were exposed to wildfire smoke for 1-5 days during their third trimester of pregnancy than in those who weren’t exposed (HR, 1.081; 95% CI, 1.010-1.215).
The likelihood of an autism diagnosis in offspring increased with the number of days of exposure: For 6-10 days of exposure, the associated risk was 11.8% (HR, 1.118; 95% CI, 0.957-1.307), and for more than 10 days, it was 22.5% (HR, 1.225; 95% CI, 1.043-1.440).
Consecutive exposure days were also associated with the risk for autism, with strong associations again in the third trimester.
However, among non-movers in the third trimester, exposure to 3 days of PM2.5 concentrations above 3 μg/m3 was associated with a statistically significant risk (HR, 1.137; 95% CI, 1.030-1.254), but not at levels above 5 μg/m3 (HR, 1.083; 95% CI, 0.946-1.239).
Mean wildfire PM2.5 concentration alone was not associated with the risk for autism.
Limitations of the study were that they didn’t measure other potential mediators such as stress and the possibility of misclassifying participants who may have evacuated during the wildfires.
Rahman said the next step in their work could be to study the effects of co-exposure to heat and wildfires on the risk for autism.
‘Striking’ Finding
A “striking” finding was the specificity of the increased risk in the third trimester, David Mandell, professor of psychiatry and director of the Penn Center for Mental Health at the University of Pennsylvania in Philadelphia, told Medscape Medical News.
Although there were clear increases in the risk for autism associated with the number of days of exposure, the same dose-response effect wasn’t observed among women who were exposed to higher concentrations of PM2.5 in the wildfire waves analysis, observed Mandell, who was not part of the study.
“That lack of a consistent dose-response relationship makes me approach the findings with a little more skepticism,” said Mandell, adding that he would like to see the data replicated to confirm the findings.
“This paper adds to the growing body of literature that shows that to the extent that environmental exposures are important, they occur in the prenatal period. It also suggests that if we are serious about preventing the sort of impairment and disability associated with autism, then we should be focused on cleaning up our environment,” Mandell said.
Rahman and Mandell reported having no relevant disclosures. This study was funded by the National Institute of Environmental Health Sciences and Kaiser Permanente Southern California Direct Community Benefit Funds.
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