Nov. 26, 2012 -- Being exposed to high levels of air pollution from traffic may raise the risk of autism, researchers say.
"Children exposed to higher levels of traffic-related pollutants during pregnancy or during the first year of life were at increased risk of autism compared to children exposed to the lowest level," says Heather E. Volk, PhD, MPH, assistant professor of research at the University of Southern California Keck School of Medicine.
The risk differed depending on timing.
During pregnancy, the highest exposures to pollution were linked with a two-times-higher risk of autism, she says. High levels during the child's first year tripled the risk.
The study is published online in the Archives of General Psychiatry.
Air Pollution & Autism Risk
Autism is a diverse disorder marked by problems in communicating and interacting socially. It now affects about 1 in 88 children in the U.S.
Researchers have been looking at the potential role of air pollution in autism only for about three years, Volk says.
Air pollution has been linked with a variety of ill health outcomes, she says, including babies being born small for their gestational age. "When you think about the birth outcome literature, looking at air pollution [and autism risk] makes some sense," she says.
In 2011, Volk's team reported a higher risk of autism for children whose families lived within about 1,000 feet of a freeway.
For the new study, she looked at data from 279 children with autism and a comparison group of 245 children without it.
At the time they started, the children were ages 2 to 5 years.
Volk used the mothers' addresses to estimate exposure to pollution during each trimester of pregnancy and during the child's first year of life.
They used information from the EPA and did traffic modeling to figure out how much traffic-related air pollution was at each location. They also looked at exposure to particulate matter and nitrogen dioxide.
In her previous study, Volk says, she just looked at how far people lived from roads. The new study went further.
"Now we consider how busy the road was, traffic density, volume of traffic, and how often the road is traveled," she says.
The risk of autism was higher for those exposed to more pollution, either before birth or during their first year.
Based on the findings, however, Volk says she can't say that living in a specific area is worse than another.
For instance, those who live in a rural area might be close to a very busy high-traffic intersection, increasing pollution exposure.
She found a link or association. It does not prove cause and effect.
The link held after she considered other factors that affect risk, such as prenatal smoking, the mother's age, race, and ethnicity.
Pollution & Autism Risk: How to Explain It?
Exactly why pollution is linked to autism risk is not certain.
However, some pollutants have been shown to limit the action of a gene important in early brain development. Expression of this gene has been found to be reduced in autistic brains.
Air pollution can also cause inflammation, and that may play a role, Volk says.
The study was funded by the National Institute of Environmental Health Sciences and the University of California Davis MIND Institute, dedicated to research on autism and other neurodevelopmental disorders.
Volk reports receiving support from Autism Speaks to present research findings at a medical meeting.
It's too soon to make specific recommendations for those in high-pollution areas, Volk says.
However, she says, they may want to follow the same recommendations now in place for those who have respiratory disease, such as avoiding outdoor activity on high-pollution days.
"I think we need more study to see when a woman might be most vulnerable," Volk says.
Air Pollution & Autism Risk: Perspective
The study builds on other research, says Andrew Adesman, MD, chief of developmental and behavioral pediatrics at Steven & Alexandra Cohen Children's Medical Center of New York in New Hyde Park.
He reviewed the findings.
Researchers now suspect that genetics and environmental factors often interact in the cases of autism, he says. Air pollution may be one of those factors, but is likely to be one of many risk factors, if it bears out.
"Although this study provides further support for the notion that exposure to traffic-related air pollution is a risk factor for autism, most children with autism do not live near highways," he says. "Undoubtedly, autism has many different causes and risk factors -- and only some of these are known at this time."