Living in an area with high levels of air pollution may increase a woman's chances of having a child with autism, according to the first national study to date that investigates the possible link.

"Women who were exposed to the highest levels of diesel or mercury in the air were twice as likely to have a child with autism than women who lived in the cleanest parts of the sample," study author Andrea Roberts, a research associate with the Harvard School of Public Health, told The Huffington Post.

Earlier studies have established a potential connection between air pollution and autism risk, but have concentrated on a few individual states. The latest study, published in the journal Environmental Health Perspectives on Tuesday, draws on a large sample of women across the whole country.

Researchers crossed U.S. Environmental Protection Agency data on the level of air pollutants from year to year with data from the Nurses' Health Study, one of the longest running investigations of women's health in the U.S. They looked for associations between levels of pollutants in the time and place that a woman was pregnant and whether that woman went on to have a child with a diagnosed autism spectrum disorder.

The researchers split up the locations into fifths, and women who lived in the most polluted sections -- those with the highest levels of diesel particulates or mercury in the air -- were twice as likely to have a child with autism compared to those in the cleanest sections. Other types of air pollution, including lead, manganese and other hard metals, were also linked to a greater risk of autism, although the risk was not quite as high.

"All of the chemicals studied are known neurotoxins," Roberts said. "They are also known to pass from mother to baby while a woman is pregnant. It's very plausible that the 'stuff' the mother is taking in through the air is affecting her baby's brain development."

But the researcher cautioned against reading too much into the results, particularly with regard to mercury. Many parents continue to worry that the form of mercury sometimes used in vaccines is linked to autism -- although that claim has been repeatedly disproven by research efforts.

"Our data is not good enough to know which thing in the air might actually be causing this [link], if it even is something in the air that is increasing the risk of autism," Roberts said. "It might be that it's actually diesel that's [behind] the problem. We are skeptical about the possibility of mercury being causal in this way, through the air."

Alycia Halladay, the senior director for environmental and clinical sciences for the advocacy group Autism Speaks, said that "in terms of how [air pollution] is linked to autism, that's not something we're able to specifically define."

"We can make a lot of hypotheses, but we don't really know," added Halladay, who did not work on the study.

The next step, which would represent a crucial step forward, is to look at blood samples of both mothers and their babies to see what chemicals are actually being absorbed, Roberts said. Only then can researchers assert with any real certainty that the link exists, and begin to understand which specific air pollutants are the most damaging.

The cause of autism is not yet known, although researchers suspect that a combination of genetic and non-genetic factors, such as parental age and complications during pregnancy could all play a role.

For now, there are few specific steps women living in high-pollution areas can take in order to mitigate their risk.

"There are things any doctor or health care professional would tell you to do," Halladay said. Those include eating well, taking a prenatal vitamin, getting regular prenatal health care check-ups and staying away from sick people or viruses as much as possible.