According to new research out of the University of Pittsburgh Graduate School of Public Health, kids that have been exposed to two air pollutants, chromium and styrene, may have an increased risk of developing autism.

If children are exposed to chromium and styrene while in the womb or during their first two years of life, they may have an increased risk by up to 65 percent, the study found. The toxins could possibly trigger a child’s genetic predisposition to the neurodevelopmental disorder. Styrene in particular, which is found in car exhaust, was shown to double the risk.

The researchers interviewed 217 families of children who had autism spectrum disorder (ASD), then compared them with two different sets of families who had kids without ASD. All of the children — whether autistic or not — had been born within the same time period and in the same six counties in Pennsylvania. The researchers used the U.S. National Air Toxics Assessment to get an idea of what potential pollutants to which the kids may have been exposed, and found that styrene, chromium, and cyanide were all seemingly associated with ASD.

But the researchers are wary of making any clear-cut conclusions about autism and its potential link to air pollution. Why one child may develop autism, while another exposed to the same chemicals would not, are questions that are still not fully answered. “These findings are preliminary,” Evelyn Talbott, lead author of the study and a professor of epidemiology, said according to HealthDay. “We don’t know what causes autism. We have little information on risk factors. This is just one more piece of the puzzle.”

Researchers currently agree that it’s very difficult to pinpoint the factors that may lead to autism, but it’s most likely a combination of genetics and environment. “More and more, people are believing in gene/environment interactions,” Talbott said. “We do know that about 10 percent of autism spectrum disorders run in families.”

Last year, another study out of the Harvard School of Public Health pinpointed a similar association between autism and air pollution. This 2013 study was the first national study to fully attempt to examine this 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,” Andrea Roberts, an author of the study and a research associate at Harvard, told the Huffington Post. “All of the chemicals studied are known neurotoxins. 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 researchers remain wary, noting that there is not enough evidence to draw any conclusions about these associations. To what extent inheritance, genes, upbringing, and environment all play a role in the development of autism has yet to be fully understood.