A study of more than 200,000 mother-child pairs finds that repeated days of wildfire smoke during late pregnancy may modestly raise autism risk, highlighting a critical window for protecting expectant mothers.
Study: Prenatal Exposure to Wildfire and Autism in Children. Image credit: Lumppini/Shutterstock.com
Prenatal exposure to wildfire, emitted particulate matter (PM) during the third trimester, may increase autism risk in children, according to a study published in Environmental Science & Technology.
Wildfire smoke is becoming a widespread prenatal exposure
Environmental and health risks from wildfires are increasing across the United States, as more regions are experiencing the rapid spread of fires in forests and grasslands, and the smoke generated by these fires is spreading across a broad area of the country.
Wildfires are a major source of fine particulate matter (PM2.5), an inhalable airborne pollutant with diameters of 2.5 micrometers or less. Exposure to these pollutants is associated with severe health risks, including respiratory disease, cardiovascular disease, and premature death.
Prenatal, during pregnancy, exposure to PM2.5 has been linked to increased risk of neurodevelopmental disorders in children. PM2.5 generated from different sources, including tailpipe exhaust and residential heating, has been found to increase autism risk in children.
Wildfire PM2.5 contains high levels of carbonaceous compounds, such as levoglucosan and PAHs, and metals, as compared to PM2.5 from other sources. Exposure to these toxic components of wildfire PM has been linked to decreased gestational age and birth weight and to an increased risk of cognitive deficit in adults.
Given the significant health impact of wildfire smoke, the current study was designed to investigate whether prenatal exposure to wildfire smoke-derived PM2.5 increases the risk of developing autism in children.
Measuring smoke days, intensity, and exposure waves
The study included 204,374 mother-child pairs from a large pregnancy cohort of the Kaiser Permanente Southern California healthcare system. Children were followed through the first five years of life for the screening of autism.
The average wildfire PM2.5 exposure concentration, number of days of smoke exposure, and number of smoke exposure waves were calculated across pregnancy and within individual trimesters. Wildfire “waves” were defined as multi-day smoke events consisting of two or more consecutive days, or three or more consecutive days, in which wildfire-specific PM2.5 concentrations exceeded predefined thresholds of 0, 3, or 5 μg/m³, combining both duration and intensity of exposure.
Third-trimester smoke exposure days linked to autism risk
The study found that about 60% of mother-child pairs were exposed to wildfire PM for more than five days during pregnancy. The median number of exposure days was eight, with a mean of approximately 9.6, indicating infrequent exposure.
A separate analysis of mother-child pairs who did not change residential addresses during pregnancy (nonmovers) revealed more exposure days and higher exposure concentrations.
The study identified 3,356 autism cases. The risk analysis revealed a significant association between increased numbers of exposure days during the third trimester and a higher risk of developing autism in children by the age of five years. This risk was strongest among nonmovers. The estimated increases in risk were modest in magnitude.
Specifically, the study found that prenatal exposure to wildfire PM for one to five days during the third trimester was associated with a significantly higher risk of autism compared to complete non-exposure.
Regarding wildfire exposure waves, which combined duration and intensity, the study found that exposure to 2-day and 3-day wildfire waves during the third trimester was significantly associated with increased autism risk in the entire study population and among nonmovers. Some wave-based associations were also observed when exposure was assessed throughout pregnancy, depending on the PM2.5 threshold used.
Findings support targeted protection for pregnant women
The study reveals that prenatal exposure to wildfire smoke-derived PM2.5 during the third trimester is associated with an increased risk of autism in children. Notably, the study highlights that the frequency and duration of exposure, measured as days or waves, rather than average PM2.5 concentration alone, were associated with autism risk.
The study considered two specific PM2.5 concentration thresholds, 3 and 5 μg/m³, for the risk analysis. However, a consistent dose-dependent increase in autism risk was not observed. As explained by researchers, this lack of dose-response effect may be due to a smaller sample size and a lower number of participants in higher exposure thresholds.
Furthermore, there remains the possibility of evacuation, temporary relocation, or the use of indoor particle filters on high-intensity exposure days. These factors may lead to exposure misclassification and may partly explain why associations were stronger among nonmovers than movers.
The study identifies the third trimester of pregnancy as a window of higher susceptibility to wildfire exposure-related autism risk. The third trimester of pregnancy is associated with rapid fetal brain development characterized by increasing neuronal connectivity and organization, and rapid gray matter growth. An acute exposure to wildfire PM may affect these processes, leading to a higher risk of neurodevelopmental disorders in children, including autism.
Wildfire PM2.5 contains high levels of carbonaceous components, which are biologically plausible contributors to autism risk based on prior toxicological and epidemiological evidence. Furthermore, wildfire PM is known to increase preterm birth risk, which subsequently may increase the risk of developing autism in children.
Wildfire-induced anxiety and depression experienced by mothers during pregnancy can also increase the risk of autism in children. Because maternal psychological stress was not directly measured, it may represent a potential mediating pathway or unmeasured factor influencing the observed associations, rather than a fully controlled confounder.
Future studies should consider maternal stress level and household activities, climatic temperature, and other components of air pollution, such as heavy metals, as potential confounders to better interpret the effect of wildfire PM on autism risk. As an observational cohort study, the findings demonstrate associations rather than definitive causation, and the authors note that results were not adjusted for multiple statistical testing.
Given these findings, researchers advise public health authorities to prioritize protecting pregnant women from wildfire smoke, particularly in late pregnancy.
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Prenatal wildfire smoke exposure linked to higher autism risk in children