At first glance, Jacob Sanchez seems like a typical 10-year-old boy. He has a sweet face, with huge hazel eyes and chubby cheeks, and would love nothing more than to binge on popcorn and Minecraft. But talk to him for a few minutes and you’ll hear an odd inflection in his voice. He might stand too close, or make repetitive guttural sounds.

Jacob has a high-functioning form of autism, and nobody knows how he got it. His mother thinks about it all the time. She wonders whether it was the mercury in the canned tuna she gorged on while pregnant. She worries about the soot-filled smoke from the chimney next door. School vaccines are on her mind, too. And then there’s the Cross Bronx Expressway.

As if it’s not stressful enough to live near the noise and hassle of highways, scientists are raising more fears for families near them — that air pollution may cause autism. Numerous studies have shown that pregnant women exposed to high levels of pollution have a higher risk of giving birth to a child with autism. Now, scientists have begun to unravel just how pollution might render children vulnerable to the disorder, even in the womb. Some think pollution nudges developing fetuses that already have genetic risk factors toward the autism spectrum. Others suspect that pollution triggers an immune response in pregnant women that makes brain development go awry.

This isn’t the first time pollution has been on the suspect list for autism. But evidence has mounted over the past year, and understanding the relationship between autism and air pollution is crucial — it could light the way toward earlier diagnosis and even intervention, whether through environmental policies that impose stricter regulations on vehicle emissions or filters that remove the chemicals most strongly linked to the disorder, for example. Evidence of an air pollution-autism link “is really getting stronger,” says Marc Weisskopf, an associate professor of environmental and occupational epidemiology at the Harvard School of Public Health.

Studies have even pinpointed when a fetus is most vulnerable to pollution’s harmful effects: the third trimester.

But of course, it’s early yet. To confirm the link between pollution and autism, scientists need to do more studies with more mothers. Since pollution tends to occur in high-poverty areas, other factors — like poor nutrition — might also play a role. And even if pollution does turn out to contribute to autism, it might barely do so, a tiny speck in a constellation of causes.

Some studies have shown higher rates of autism in kids whose mothers lived in highly polluted areas while pregnant, but only in limited geographic areas. Others have even pinpointed when a fetus is most vulnerable to pollution’s harmful effects: the third trimester. One study published in the journal Epidemiology found autism rates to be a higher among samples of children in North Carolina and the San Francisco Bay Area whose mothers inhaled high levels of pollutants during this period compared to those who did so earlier or after giving birth. That matches up with previous studies showing that fetuses’ nervous systems undergo a huge amount of development during the last stretch of pregnancy. Anything that goes amiss during this critical window could have lifelong effects.

Now that we know pollution could cause autism — and when — the next question is how? The answer could yield ways to diagnose and treat the disorder early on. Some scientists think that variants of certain genes make kids susceptible to autism — and pollution pushes them over the edge. University of Southern California researchers zeroed in on a gene called MET, important for brain development, and a study found that rat pups with a variant of MET showed even less MET if their mothers had been exposed to a chemical found in car exhaust while pregnant. Sure enough, when the researchers looked at humans, kids whose mothers had lived in polluted neighborhoods and had the high-risk variant were more likely to develop autism.

Other researchers think that the exhaust pregnant mothers inhale triggers inflammation, altering levels of immune molecules in the mother’s bloodstream and the placenta. Besides fighting infection, these molecules activate pathways crucial to nervous system development, which could be impaired when they’re not present at just the right amount. Scientists have already uncovered clues that inflammation may contribute to autism. Women who get the flu during pregnancy double their risk of giving birth to a child with autism, and kids with autism often suffer recurring ear infections. USC researchers showed that inhaling a chemical found in car exhaust not only triggered inflammation in mice, but also impaired the ability of their neurons to mature. Once scientists identify the key molecules involved in these immune pathways, they could target them to mitigate autism symptoms, or use them as markers for diagnosing the disorder.

But as scientists continue to explore the air pollution-autism link, the most effective solution remains tackling pollution itself, says Amy Kalkbrenner, an assistant professor in environmental health sciences at the University of Wisconsin-Milwaukee. “You can’t escape the air.”