As autism rates increase nationwide, so, too, do theories about the disorder's cause. Genetics has been identified as a major culprit, and some researchers think it's linked to bacteria in the gut.

A new study from the Harvard School of Public Health, published yesterday in the journal Environmental Health Perspectives, however, adds to a growing body of evidence that says part of the problem may come from outside the body. Small particulate matter that pollutes the air, say some scientists, might be to blame.

"We know that autism spectrum disorder is a spectrum. It's a multifactorial disorder with lots of contributors to its origins," Marc Weisskopf, an epidemiologist at Harvard University and lead author of the study, told VICE News. "We know genetics is a very important component. One of the things that this study shows is not only that air pollution may play a role, but also just in general that the environment is an important player in autism."

The researchers surveyed women from all 50 states who were part of the Nurses' Health Study II, a survey of more than 100,000 women that began in 1989, which provided extensive data on where the women lived and when they gave birth, among other life details. The researchers examined 1,774 children born between 1990 and 2002, 252 of whom had been diagnosed with autism.

They then used monthly air pollution data collected from monitoring stations set up across the country to build models showing just how much air pollution a woman was exposed to during each month of pregnancy and in the months immediately before and after.

They found that women living with the highest amount of air pollution during pregnancy had twice the risk of giving birth to an autistic child than women living with the lowest levels. The effect was greatest with higher exposure during the third trimester.

"The third trimester, there's a lot of neuronal growth and migration going on — brain-building, essentially," Weisskopf told VICE News. "It's definitely a period of very active brain growth that if you disrupt in the wrong way could easily could have some important implications."

Small particles in the air come from a variety of sources, including car exhaust, power plants, and wood smoke. The size of pollutants, not just the type or origin, impact the body in different ways. Those larger than 2.5 microns had very little effect because the trachea is adept at blocking larger particles from entering the respiratory system. Particles smaller than 2.5 microns — about 30 times smaller than the width of a human hair — aren't filtered out and can end up in the bloodstream, which can lead to serious heart and lung problems and premature death.

The known dangers of fine particulate matter in the air, coupled with the researchers' ability to link higher autism risk to a specific trimester of pregnancy, makes it easier to a draw a clear link between autism and the air.

"That specificity in our findings really goes a large step further than what's come before to really solidify and strengthen our level of confidence that this is something specifically related to the air pollution," Weisskopf told VICE News.

What role air pollution might be playing is not entirely clear. It may be that biological responses to the pollution are exacerbating an underlying genetic risk. Or the pollution might, in some cases, be causing the disease itself.

But the new study is the latest in a collection of research showing a link between exposure to small particulate matter in the air and having a child with autism. A 2010 study led by researchers in Los Angeles found that, after adjusting for socio-demographic factors and whether or not a subjected smoked, mothers living near a freeway, who would be exposed to high levels of air pollution, were more likely to have an autistic child than mothers who did not. In October, researchers at the University of Wisconsin-Milwaukee found similar results for children in North Carolina.

The Harvard study is the first to look at the link in children nationwide.

"This paper is an important contribution because it shows such similar results in a different study sample," Amy Kalkbrenner, an environmental epidemiologist at the University of Wisconsin-Milwaukee and lead author of the North Carolina autism study, told VICE News. "Consistency of this degree is rare in questions of human health impacts of environmental chemical exposures, which are notoriously tricky to study."

Air pollution is likely a small piece of the overall autism picture, Weisskopf said, meaning cleaner air doesn't necessarily greatly reduce the chances of having an autistic child. But if researchers can figure out why air pollution increases the risk at all, then they may have a better understanding of the disease and its disparate causes.

"If they're all doing it eventually through one pathway and it's inflammation, well, then we might actually be able to halve the risk," Weisskopf told VICE News. "But that's a long way down the line."