A new study of more than 200,000 children in southern California found a link between exposure to wildfire smoke in mothers and higher rates of autism in children. The causes of autism are not fully known and likely multifaceted, but the new research builds on existing evidence that air pollution may be tied to autism.
The study, published on Tuesday in Environmental Science & Technology, analyzed data for children born in the wildfire hot spot from 2006 to 2014. Pregnant women who were in their third trimester and exposed to as few as one to five smoke days were about 11 percent more likely to have a child who was diagnosed as autistic by age five than those who saw no smoke days. The more smoke days that mothers were exposed to, the higher the likelihood that their children would be diagnosed as autistic: women who were exposed to between six and 10 smoke days were 12 percent more likely to have a child who received such a diagnosis by age five, while this was 23 percent more likely among those who were exposed to more than 10 smoke days.
“This is one of the first large population-based studies to specifically examine prenatal wildfire smoke exposure and autism risk,” says Mostafijur Rahman, an assistant professor of environmental health sciences at Tulane University and an author of the new study. “Our findings suggest that wildfire smoke exposure during sensitive periods of pregnancy—particularly late pregnancy—may be associated with an increased risk of autism diagnosis in children.”
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Importantly, the study doesn’t identify a direct causal link between autism and wildfire smoke, Rahman says. Most experts believe autism is complex and likely arises from a combination of environmental and genetic factors.
Rather the study “highlights wildfire smoke as a potentially modifiable environmental risk factor that may contribute to risk in combination with other factors,” he says.
The study has several limitations. Some mothers included in the study may have had different levels of exposure to smoke than the researchers’ estimated. And wildfires can also be extremely stressful—an experience that may have also played a role.
David Mandell, a professor of psychiatry at the University of Pennsylvania who was not involved in the study, says that the paper’s association between wildfire smoke and autism is “certainly concerning” and “worthy of further attention.”
Mandell notes, however, that some children of mothers who experienced higher concentrations of wildfire smoke in her third trimester and didn’t move house during the study period didn’t show higher rates of autism—which is not the “dose-response gradient that one might expect,” he says.
Autism and its causes have been a central focus of the Trump administration and Secretary of Health and Human Services Robert F. Kennedy, Jr., who has, in the past, claimed that autism was most likely caused by a variety of environmental exposures, not all of which have been backed by solid science. According to the U.S. Centers for Disease Control and Prevention, around one in 31 children are diagnosed as autistic before age nine.
The new study jibes with past research showing children whose mothers were exposed to high rates of fine particulate pollution, as well as diesel exhaust and mercury, were more likely to be diagnosed as autistic than the children of those who breathed cleaner air.
Wildfires have become an ever-present risk in the U.S., especially for Californians. According to the U.S. Department of Agriculture, between 2003 and 2012, individual large wildfires in the western U.S. burned for an average of 52 days, up from just six days in the 1970s and 1980s.
“As wildfires become more frequent and intense due to climate change, understanding their potential long-term health impacts is increasingly important,” Rahman says.