Air pollution might also be linked to autism risk, but the details are hazy. At least 14 studies have suggested an association with autism, and air pollution is known to trigger inflammation, but analyses of individual airborne chemicals have been inconsistent. Researchers are also confused by the fact that cigarette smoking, which contains many of the same chemicals as air pollution, is not associated with the condition.
Certain pesticides, such as chlorpyrifos, can disrupt sex-hormone pathways implicated in animal models of autism. But again, studies linking pesticides to autism have been mixed, and questions about causation are unresolved. More answers may emerge, however, as researchers uncover new ways to study interactions between fetuses and the outside world. In addition to Arora’s work on baby teeth, researchers are investigating what kinds of chemical stories meconium, a newborn’s first feces, can tell.
Princeton University neuroscientist Sam Wang has long been interested in autism’s potential environmental causes, but he says he finds the research intimidating. “It’s like the sands of the seas,” he says. “It’s this enormous literature, and people who work in it have all these different perspectives.”
Several years ago, in an attempt to bring clarity to the issue, Wang perused about 100 studies and then ranked dozens of associations between autism and both genetic and environmental factors by their relative risk ratios. He described his findings in a 2014 op-ed in The New York Times.
What came out on top in Wang’s analysis of environmental factors was birth—in particular, rare birth injuries to the cerebellum, a brain region that coordinates muscle movements, among other functions. “If it’s a difficult birth, or there’s a bleed on the cerebellum, then that increases the risk of autism dramatically,” by a whopping 3,800 percent, he says. “It’s bigger than any other risk factor, other than sharing your entire genome with a person with autism.” Wang’s research supports the link, too: He has shown that mice with early damage to the cerebellum later have serious cognitive and behavioral problems that mimic autism traits.
The timing of birth also made Wang’s list: Babies born at least nine weeks premature seem to have higher odds of autism, he found.
When Noelle Mathias found out she was pregnant with her eldest daughter, Elena, in 2008, she took good care of herself. Mathias exercised, ate well, and didn’t drink alcohol or smoke. “As far as I knew, it was a normal pregnancy,” she recalls. But her water broke early at 36 weeks, and Elena was born less than 24 hours later. When Elena was 2, Mathias and her husband noticed she wasn’t responding to her name. They had Elena evaluated, and soon after, the girl was diagnosed with autism.
It’s impossible to know whether Elena’s preterm birth played a causal role in her diagnosis. Is being born early itself the issue, or might an underlying genetic susceptibility or environmental insult increase the odds of both preterm birth and autism?