Exposure to high levels of air pollution during pregnancy may double the chances that a child will have autism, a team of Harvard researchers said Tuesday.

In the first national study looking at a possible link between autism and air pollution levels, researchers found that women living in places with the highest levels of pollution while they were pregnant were up to twice as likely to give birth to a child with autism as compared to those living in locations with the lowest levels.

For the study, researchers at the Harvard School of Public Health looked at data from a long-term study of more than 116,000 nurses dating back to 1989 and focused on 325 women who had a child with autism and 22,000 who had a child without the developmental disorder. They used air pollution data from the U.S. Environmental Protection Agency to estimate the women’s exposure depending on when they were pregnant and where they lived at the time.

Ultimately, the researchers found that the children of moms living in the 20 percent of locations with the highest levels of diesel particulates or mercury in the air were twice as likely to have autism as those living in the 20 percent of places where exposure was the lowest.

Meanwhile, autism odds were about 50 percent greater for women living in areas with the highest levels of other pollutants including lead, manganese, methylene chloride and combined metal exposure as opposed to those residing in low exposure locations, according to the study published online Tuesday in the journal Environmental Health Perspectives.

“Our findings raise concerns since, depending on the pollutant, 20 percent to 60 percent of the women in our study lived in areas where risk of autism was elevated,” said Andrea Roberts, a research associate at the Harvard School of Public Health and the study’s lead author, in a statement.

This is not the first study to suggest a link between autism and exposure to air pollution, but previous research relied on data from just a handful of locations within the United States while the new study provides evidence from a nationwide sampling.

Scientists behind the current findings said further research should examine the blood of pregnant women or newborn babies to measure exposure to pollutants which would allow for a better understanding of how these factors impact autism odds and potentially point to interventions.