The latest statistics from the Centers for Disease Control and Prevention (CDC) estimate 1 in 68 children are diagnosed with autism spectrum disorder. Now a new study from the Harvard School of Public Health finds women exposed to high levels of fine particulate air pollution during pregnancy face up to twice the risk of having a child with autism.

The study focused specifically on fine particulate air pollution, made of components like acids, chemicals, metals, and soil or dust particles smaller than 2.5 microns (PM2.5).

According to the Environmental Protection Agency (EPA), the size of particles is directly linked to their potential for causing health problems because of the ability to pass through the throat and nose and enter the lungs.

Results showed the greater the exposure to the pollution, the greater the risk of autism, researchers said, with the highest risk occurring during the third trimester of pregnancy. This is the first national study exploring the link between airborne particulate matter and autism.

The study population was composed of children of participants in the Nurses’ Health Study II, a group of more than 116,000 female U.S. nurses in all 50 states who have been the platform for various forms of women’s health research since 1989. The researchers collected data on where participants lived during their pregnancies as well as information from the EPA on levels of fine particulate matter air pollution in locations across the U.S.

“Particulate matter air pollution does vary across the country both in time and space. More urbanized areas tend to have higher levels because of traffic, but rural areas can have higher levels than one might think because regionally transported components can come from far away,” senior study author Marc Weisskopf, an associate professor of environmental and occupational epidemiology at Harvard, told FoxNews.com.

Researchers found exposure to PM2.5 was significantly associated with autism during pregnancy, but not before or after. Little association was found with air pollution from larger-sized particles.

“I think the evidence base is getting quite strong for an increased risk of autism with higher maternal exposure to air pollutants,” Weisskopf said. "This not only gives us important insight as we continue to pursue the origins of autism spectrum disorders, but as a modifiable exposure, opens the door to thinking about possible preventative measures."

Weisskopf said he hopes further research will discover not only what particular components of PM2.5 are responsible for the increased autism risk, but also the biological mechanisms the particles are triggering that lead to autism.

Pregnant women can’t avoid at least some level of exposure to PM2.5, according to Weisskopf, but he suggests keeping away from cities and roadways that have consistently high levels of pollution.

The study was published in Environmental Health Perspectives.