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Children born to mothers exposed to high levels of fine particulate pollution during pregnancy have a higher chance of being diagnosed with autism, according to a new study led by Harvard University.

The study found autism risks are especially high when women are exposed during their third trimester of pregnancy. Published online today in the journal Environmental Health Perspectives, the paper adds to a growing body of recent research linking autism to air pollution.

“Our data add additional support to the hypothesis that maternal exposure to air pollution contributes to the risk of autism spectrum disorders,” said Marc Weisskopf, a study author and associate professor of environmental and occupational epidemiology at Harvard School of Public Health, in a statement.

The researchers studied a group of more than 116,000 female nurses from all 50 states. They identified 245 children born to the nurses between 1990 and 2002 who were diagnosed with autism spectrum disorder, as well as a control group of 1,522 children born without autism during that same period.

The authors gathered data from U.S. EPA on the levels of fine particulate matter — particles about a 13th the width of a human air — that the mothers were exposed to before, during and after pregnancy. They also collected data on where the participants lived during those times.

According to Weisskopf, the results showed that a woman who lives in an area that is in the highest 25 percent of fine particulate matter levels during pregnancy is more than twice as likely to have a child diagnosed with ASD than a woman who lives in an area in the lowest 25 percent.

The study linked exposure during the third trimester of pregnancy specifically with an autism diagnosis.

Weisskopf said that there are many factors contributing to autism risk, including genetics, and that air pollution itself may not be responsible for doubling the risk of autism.

But, he noted, “the specificity of our findings for the pregnancy period, and third trimester in particular, rules out many other possible explanations for these findings.”

Study authors did not find any association with autism for fine particle exposure occurring before and after pregnancy. They also did not find any increased risk of autism for mothers exposed to coarse particle pollution.

The study was funded by Israel’s Environment and Health Fund, the National Institutes of Health, the U.S. Department of Defense and the Autism Speaks Foundation. Along with the Harvard School of Public Health, the University of California, Davis, participated in the study.

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