The risk seemed to be linked to what is called particulate matter pollution and nitrogen dioxide -- commonly known as smog -- the researchers noted.
Autism is a neurodevelopmental condition that appears to be caused by both genetic and environmental factors. Autism spectrum disorder is an umbrella term that includes a range of symptom severity. Common factors include problems with communication, social interaction and repetitive behaviors. An estimated one in 88 U.S. children has some form of autism.
Geraldine Dawson, chief science officer of Autism Speaks and a professor of psychiatry at the University of North Carolina at Chapel Hill, wrote an accompanying journal editorial.
"Over the past decade, there has been an explosion of research on autism, which has paralleled the dramatic increase in autism prevalence," Dawson said. "However, we still lack effective treatments and more research funding is needed to address the public health crisis that autism has become."
New research methods are helping to explain the biology of autism and its causes, according to Dawson, who said the latest study "confirms earlier studies that exposure to environmental toxins, especially during the prenatal period, can increase the risk of autism."
Environmental toxins may be one factor contributing to abnormalities in immune functioning that have been associated with autism, she added.
Dawson noted that another study in the same journal issue found more evidence of abnormal immune function in people with autism.
"We don't know whether these immune abnormalities are a cause or a response to having autism. Either way, these findings are an important clue that the immune system plays a role in autism," she said. "There is an urgent need for more research on environmental factors that can influence prenatal brain development and potentially increase risk fo
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