Children exposed to higher levels of air pollution early in life may have an increased risk of autism, a new study suggests.

In the study, children living in areas with high levels of traffic-generated air pollution during their first year of life were three times more likely to have autism than children living in areas with low levels.

There was also a link between exposure to air pollution in the womb — particularly during late pregnancy — and an increased risk of autism, according to the study.

The findings held even after the researchers accounted for factors possibly related to autism risk, including a child's gender and ethnicity, their parent's education and smoking in pregnancy.

The results add to previous findings linking exposure to air pollution with autism risk

However, the new study shows only an association, not a cause-effect link, and the researchers did not gather information on the children's nutrition, or their exposure to indoor pollutants and secondhand smoke, which could affect the results.

“Although additional research to replicate these findings is needed, the public health implications of these findings are large because air pollution exposure is common and may have lasting neurological effects,” the researchers write in the Nov. 26 issue of the journal Archives of General Psychiatry.

Autism and air pollution

The study included 524 children between ages 2 and 5 living in California, including 279 children with autism. Most children lived in areas around Sacramento, Los Angeles and San Francisco.

Heather Volk, an environmental epidemiologist at the University of Southern California, and colleagues used the children's addresses to estimate their level of exposure to air pollution from traffic during pregnancy and the first year of life. The researchers also used information from air-pollution monitoring systems throughout the area that took into account road traffic volume and other measures.

Children living in areas estimated to have the highest levels of traffic-generated air pollution — about 32 parts per billion (ppb) and above — were more likely to have autism than children living in areas estimated to have levels of 10 ppb and below.

A person living at least 500 meters from a freeway in Southern California would be exposed to air pollution levels one-half to one-third of the levels seen in the highest pollution group, Volk said.

Children exposed to air pollution levels between 10 and 32 ppb were not at increased risk of autism compared with children exposed to lower levels.

Exposure to high levels of particulate matter, or particles in the air, was also associated with an increased risk of autism. Such particles are small enough to be inhaled into the lungs, according to the Environmental Protection Agency.

Volk noted that air-quality measures do not necessarily reflect the amount of traffic-generated air pollution on a local level, and so people living in areas with good air quality might still be at increased risk.

Potential for brain damage

If the link is real, exposure to air pollution is not likely to account for many cases of autism, said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Steven & Alexandra Cohen Children's Medical Center of New York in New Hyde Park, N.Y., who noted that many children don't live within 1 to 3 miles of a major highway.

“As much as it would be perhaps attractive to find a single cause for autism, the reality is there are many different causes,” including genetic factors, said Adesman, who was not involved in the study. Many lines of research are needed to investigate these causes, Adesman said.

It's not known how air pollution might increase autism risk, but previous studies suggest it's biologically plausible.

Particles called polycyclic aromatic hydrocarbons that are present in diesel exhaust have been shown to affect brain function through interactions with genes, the researchers said. In addition, research suggests air pollution may trigger inflammation that impairs the the tissue layer separating the brain from its blood vessels.

Pass it on: Researchers have found a link between exposure to high levels of air pollution early in life and an increased risk of autism.

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