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Large US Study Links Air Pollution to Autism

FILE - A mouse brain that was exposed to polluted air shows an enlarged lateral ventricle (right) compared with a mouse whose air was clean and filtered. Enlarged lateral ventricles are associated in humans with autism and schizophrenia. (University of Rochester Medical Center)

Women exposed to the highest levels of fine-particulate air pollution late in their pregnancies are two times more likely to give birth to a child with autism. That’s according to the largest study to date looking at the link between autism and pollution. The neurodevelopmental disorder is marked by repetitive behaviors and difficulties with social interaction and communication.

Researchers analyzed data from 116,000 mothers and their children in all 50 states.

Investigators, led by Marc Weisskopf of Harvard University, found women exposed to high levels of fine-particulate matter air pollution, during their third trimester of pregnancy, were at highest risk of having an autistic child.

Their increased risk was double that of women who lived in areas with low levels of particulate pollution.

The Harvard study found that timing of exposure was critical. The researchers detected no increased risk of autism spectrum disorder in children whose mothers were exposed to high levels of pollution before becoming pregnant. And the study found airborne pollution does not appear to increase the risk of children developing autism after they are born.

Weisskopf, an environmental and occupational epidemiologist, says the finding is compelling even though it is not evidence of a direct link between pollution and autism.

“Finding an association like this that’s very specific in time rules out a lot of other possible explanations for that,” said Weisskopf. "So it really ratchets up the strength our confidence that we got something really related to the air pollution here.”

To the extent possible, Weisskopf says expectant mothers should avoid exposure to air pollution.

“You can avoid being in extremely polluted cities during pregnancy if possible,” said Weisskopf. "You can also choose to go running in a park rather than next to a street. But that said, I think also it’s very important to recognize that autism spectrum disorders is a multi-factorial disorder. And there are lots of reasons why risk could be increased.”

Autism, which runs the spectrum from mild to severe impairment, is believed to be caused by a combination of environmental and genetic factors that predispose individuals to the disorder.

Paul Wang, head of medical research at Autism Speaks, an organization that advocates on behalf of families with autistic children, says the findings, published online in Environmental Health Perspectives, are very compelling. Wang notes the last three months of pregnancy are a time of accelerated neurological development in the fetus.

For now, he says the study does not change the autism group’s basic recommendation.

“For parents of children who have already been diagnosed, the focus should continue to be on treatment for them, on the behavioral, educational therapies that are available that we know can help children with autism,” said Wang.

Meanwhile, Weisskopf says researchers are trying to identify the specific substances in fine-particulate air pollution that increase the risk of autism, as well as women who are at greatest risk for giving birth to a child with the disorder.

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