For the new study, published in the Archives of General Psychiatry, researchers in California analyzed some 500 children living in that state: roughly half had autism and half did not. The kids’ mothers gave an address for each and every home in which they had lived during pregnancy and the child’s first year of life. Researchers took that information — along with data on traffic volume, vehicle emissions, wind patterns, and regional estimates of pollutants like particulate matter, nitrogen oxide, and ozone — to estimate each child’s likely pollution exposure. According to the study, children in the top 25% of pollution exposure (using one of two different pollution scales) were far more likely to be diagnosed with autism than kids in the bottom 25% of the pollution scale.

The researchers stress, however, that their study does not definitively prove that pollution is the root cause of autism.

“We’re not saying that air pollution causes autism. We’re saying it may be a risk factor for autism,” says Heather Volk, lead author on the new study and an assistant professor of preventive medicine at the University of Southern California. “Autism is a complex disorder and it’s likely there are many factors contributing,” she says.

In particular, she says, genetic differences may leave some children more susceptible than others to the effects of damaging environmental stimuli such as air pollutants. Still, changes in air pollution over time cannot completely explain the entire disturbing rise in autism prevalence over the past two to three decades. According to the Centers for Disease Control and Prevention (CDC), 1 in 88 U.S. children has now been diagnosed with an autism spectrum disorder. And the pollutants that Volk and colleagues analyze are not necessarily more common today than they were 30 years ago.

Even so, the latest study findings suggest that air pollution may be one of the best characterized environmental risk factors for autism. In an earlier study published in 2010, Volk and colleagues showed that kids with autism were much more likely than kids without the disorder to have been born to mothers living within 1,000 feet of a freeway. Other researchers have shown that kids with autism are also unusually likely to have exposure to high levels of diesel exhaust particles and metals (mercury, cadmium, and nickel) and to other air-pollutant chemicals, such as those used to make rubber, plastics, and dyes.

These associations continued to remain strong even after researchers adjusted for other characteristics, like poverty, that may also be connected to pollution. Unlike asthma, for example, autism rates are not consistently higher among lower income populations. In Volk’s study, the links between air pollution and autism risk were virtually unchanged after accounting for parents’ race and ethnicity, educational attainment, and smoking status, as well as for the area’s population density.

The new study builds on previous research into the relationship between air pollution and autism with two new insights. One is the focus on nitrogen dioxide (a common by-product from motor vehicle engines and power plants) and to particulate matter (dust, soot, and smoke in the air). The other is the study’s detailed, fine-grained data on individuals’ exposure to air pollution. For their research, Volk and her colleagues recorded information not just about proximity to major roadways, but also about whether homes were downwind or upwind from the roads, and the traffic load on these roads, in addition to regional-level data from the Environmental Protection Agency on other pollutant compounds. The researchers also looked at exposure throughout pregnancy and during the first year of life, and did not limit their measurements to a single point in time.

Because conditions like autism are likely the result of a complex interaction between genetic and environmental factors, however, the new findings may ultimately raise as many questions as they answer. It’s not clear how or why the chemicals we breathe may affect development and autism risk, for example, although the researchers suggest that pollutants may impact both neurological development and inflammation, which can damage the lining of blood vessels in the brain and compromise the blood-brain barrier.

As research into the role that air pollutants can play in autism continues to grow, say the scientists, anti-pollution laws may be passed that could ultimately affect rates of the disorder.