Researchers say a new study suggests a link between air pollution and autism, and that women exposed to elevated levels of fine particulate matter during pregnancy may face a significantly higher risk of delivering a child with autism.

Their risk is twice as high as seen in pregnant women who live in regions with low levels of particulate matter pollution, they say.

The risk is highest during a woman's third trimester of pregnancy, the researchers at the Harvard School of Public Health report in the journal Environmental Health Perspectives.

In a study of children of more than 116,000 women involved in the Nurses' Health Study II, which started in 1989, researchers gathered data on where their mothers lived during the pregnancies and the level of particulate pollution in those locations.

While an analysis of that data showed no association between autism and pollution before pregnancy or in its early stages, elevated levels of exposure experienced during the third trimester significantly increased the risk of autism, the researchers said.

The evidence of a link between exposure to pollution during pregnancy and an increased risk of autism "is becoming quite strong," says study leader Marc Weisskopf, a Harvard epidemiologist.

"This not only gives us important insight as we continue to pursue the origins of autism spectrum disorders," he says, "but as a modifiable exposure, opens the door to thinking about possible preventative measures."

While it isn't clear how particulate matter might be linked to autism, researchers point out that the fine particles carry many contaminants and can penetrate cells, possible disrupting brain development.

Fine particulate matter, up to 2.5 micrometers in diameter, are often found in smoke, haze or city smog, and can be emitted by fires, industrial smokestacks or vehicles.

The Harvard study is just the latest linking various forms of air pollution to autism risks. Earlier this year a study by the University of California, Davis, suggested women who live in areas of heavy pesticide use during pregnancy are two-thirds more likely to have children with autism or other developmental delays than women whose homes were distant from such sites.

Diagnoses of autism in the United States rose from one in 150 children in 2000 to one in 68 in 2010, although scientists are divided on whether its truly an increase in incidence or if the numbers reflect a greater awareness that leads to a greater diagnosis rate.

Increasing evidence of links between fine particulate pollution and lung cancer, asthma and cardiovascular disease led the Environmental Protection Agency to announce stricter air quality standards last year that U.S. states must meet by 2020.

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