In a finding that points to a link between environmental toxins and autism, a new study shows that children who were exposed to the highest levels of traffic-related air pollution during gestation and in early infancy were three times more likely to be diagnosed with the neurodevelopmental disorder than were those whose early exposure to such pollutants was very low.

The study, published Monday in the Archives of General Psychiatry, found that early exposure to high levels of air pollution in general was linked to an increased likelihood of autism in a group of more than 500 children followed for several years from birth. The researchers gathered regional air quality data and used detailed calculations to estimate the air quality around the residence in which a child’s mother spent her pregnancy and the resulting child spent his or her first year.

Their findings suggest that the link between air pollution and autism is evident largely at the highest levels of exposure, and slightly higher when the exposure comes later in a woman’s pregnancy. The strongest link was found between exposure to nitrogen dioxide -- a pollutant found plentifully around freeways -- and autism, while exposure to particulates was less strongly linked to autism.

The researchers found that the link between traffic-related air pollutants held steady after they took into account parental education, ethnicity, whether a mother smoked during pregnancy, or how densely populated the region was. That suggests that the finding is not just a backhanded way of capturing some link between autism and the socioeconomic or demographic factors in which a child is raised.

The study refines earlier findings by the same authors that linked autism to a child’s living near a freeway.

While the authors caution that this link is not proof that air pollution causes autism, they do suggest there are several ways in which air pollutants could influence the development or function of a child’s brain in ways that could result in the sorts of neurodevelopmental problems seen in autism. Diesel exhaust particles and polycyclic aromatic hydrocarbons, which are present in traffic pollutants, have been shown to interfere with gene expression important in healthy brain development. Other research suggests that traffic-related air pollutants induce inflammatory reactions and oxidative stress in the brain and throughout the body.

The article was one of three studies published in the Archives of General Psychiatry probing autism’s possible origins and its effects on the brain. Those come at a time when the diagnosis of autism spectrum disorder in U.S. children has skyrocketed. In the last six years alone, it has increased 78%.

Collectively, the studies “point to an urgent need for more research on prenatal and early post-natal brain development in autism, with a focus on how genes and early development combine to increase risk” for autism, wrote Geraldine Dawson, chief science officer of Autism Speaks and a professor of psychiatry at the University of North Carolina at Chapel Hill.