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Two new studies suggest that rising autism rates might be connected at least in part to air pollution from traffic. They are not the first to show a link between exposure to pollutants during pregnancy or early life and the diagnosis of autism spectrum disorders. But both studies look at large populations and find a link with relatively low levels of pollutants.

In a study of 132,256 births in Vancouver, Canada, published Monday in JAMA Pediatrics, researchers detected an association between exposure to roadway pollution in utero and later diagnosis with autism. The study’s strengths were its large size and its method of diagnosing autism, which can be inconsistent.


The second study, published earlier this fall in Environmental Epidemiology (but not yet available online), found a link between pollution exposures during the first months of life and later diagnosis with autism. That study looked at more than 15,000 infants born between 1989 and 2013 in Denmark.

Most of the 10 previous studies examining air pollution and autism, including one published earlier this year using data from Israel, found a similar connection.

Although no one study is definitive, the accumulation of research suggests that the link is real and that more needs to be done to reduce emissions, said Amy Kalkbrenner, an associate professor and environmental epidemiologist at the University of Wisconsin, Milwaukee, who was not involved in either study.


Kalkbrenner said she does not want to alarm young families, but does want to get the word out that pollution from cars and trucks may pose a risk. “If we care about autism, we should be concerned,” she said.

Small exposures may be one of the cascading triggers for autism — along with genetics and other causes, such as infections during pregnancy, Kalkbrenner said.

It’s possible, she and others said, that exposure to high levels of pollution cause early fetal death — the pregnancies simply don’t survive. Low-level exposures, meanwhile, could have more subtle effects on brain development, she said.

This finding is consistent with other research into air pollution that shows different health effects from different levels of exposure, she and others said.

Paradoxically, autism diagnosis rates have skyrocketed as pollution rates have plummeted. Both Vancouver and Denmark are considered to have very clean air.

There’s no good explanation for that yet, said Dr. Bruce Lanphear, a co-author of the Vancouver study, and a professor at Simon Fraser University in British Columbia, Canada. “Part of the way it might make sense,” he said, “is that the composition of air pollution may have changed.”

Traffic is not the only pollutant linked to autism. In the study in Denmark, researchers found a connection between autism and higher levels of sulfur dioxide, potentially connected to the shipping industry, according to first author Beate Ritz. Other research in rural areas has connected autism to a mother’s exposure to certain pesticides. None of these epidemiological studies can prove that pollution causes autism, but any research intentionally exposing children to pollutants in order to definitively show a link would be unethical.

In the Danish study, which was conducted before a change in the definition of autism, researchers found a connection between air pollution and full-blown autism as well as Asperger’s, but not with something called pervasive developmental disorder. Autism is defined by social and communication difficulties, and repetitive behaviors. Asperger’s includes the same symptoms, but usually with less severe communication challenges. PDD is a group of disorders characterized by delays in socialization and communication skills. All three are now folded into one condition called autism spectrum disorders. The difference researchers found suggests that pollution may have a more specific effect on autism than previously realized, said Ritz, a professor of epidemiology in the Center for Occupational and Environmental Health at UCLA.

She and Lanphear both said they became interested in the topic from personal experience. Ritz’s first child was born below ideal weight, and Lanphear’s second child suffered respiratory problems in infancy — both while living near a highway. Lower birth weight and respiratory issues are also linked to pollution.

Ritz said she moved out from under a freeway before her second pregnancy. Although noting that one person’s experience doesn’t prove anything, Ritz said her second child was born at a healthier weight. “I was the lucky one who could actually act,” she said, “and feel better about having acted.”

Governments should also take notice of this data, Lanphear added, saying that schools and new residential developments should be located away from busy roadways to avoid a variety of potential problems from pollution.

“There is an urgency over the next decade or two to find ways to dramatically reduce our exposures to air pollution, and in this case particularly in pregnant woman,” he said. “I would reiterate that we don’t have definitive evidence, but collectively, we have enough.”

This story has been updated to say that Beate Ritz’ research found an association between air pollutant exposure during early infancy and later autism diagnosis.

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