There’s more evidence that autism in children is linked to toxic exposures by pregnant mothers and developing infants—most recently exposure to air pollutants from traffic. A study published last month by the Archives of General Psychiatry found that children with autism “were more more likely to live at residences that had the highest quartile of exposure to traffic-related air pollution, during gestation..and during the first year of life…compared with control children.” Participants came from California and researchers matched mothers’ addresses from birth certificates as well as questionnaires about residences with air pollution data from the U.S. Environmental Protection Agency (EPA) to determine exposure amounts. Air pollutants to which mothers and babies were exposed included nitrogen dioxide and particulate matter, small toxic particles in the air near roadways that can lodge in the lungs and enter the bloodstream.
Children exposed to the highest levels of particulate matter in the study had about a two-fold risk of autism. There was a similar link found between nitrogen dioxide exposure and autism. “This is a risk factor that we can modify and potentially reduce the risk for autism,” wrote Geraldine Dawson of the University of North Carolina at Chapel Hill, in an email to Reuters Health.
While the researchers controlled for other factors—including a mother’s education and socioeconomic status—the study still cannot lay claim to a direct cause-and-effect relationship between air pollution and autism. But it does provide further evidence that environmental exposures deserve further scrutiny as drivers of autism spectrum disorder, which now impacts one in 88 children according to the U.S. Centers for Disease Control and Prevention.
The study indicates a couple possibilities: that autism is related to immune system malfunction, which is impacted by air pollution; or that children are born with a genetic predisposition to autism that is being triggered by these toxic exposures. “We are not saying that air pollution causes autism,” said lead researcher Heather Volk, assistant professor of preventive medicine at the University of Southern California, in Los Angeles. “But it does appear that this may be one potential risk for autism. We are beginning to understand that pollution affects the developing fetus.”
It’s not the first study to show a relationship between exposure to hazardous air pollutants and autism. A 2006 study published in Environmental Health Perspectives studied 284 children with an ASD born in the San Francisco Bay area, and compared them with 657 controls and looked at birth residences and pollution data to determine exposure to various chemicals of concern: particularly heavy metals, including mercury, cadmium, nickel, tricholoethylene and vinyl chloride. That study concluded that there was “a potential association between autism and estimated metal concentrations, and possibly solvents, in ambient air around the birth residence, requiring confirmation and more refined exposure assessment in future studies.”
In 2010, researchers followed the air pollutant trail to autism again, this time looking at children born in North Carolina and screening for pre-birth exposure to 35 hazardous air pollutants. Again, an association was found, specifically in this case between exposures to methylene chloride, a solvent used in manufacturing; quinolone, a chemical used in petroleum refining and coal mining; and styrene, a pollutant that comes from traffic, and autism rates. Confounding matters, however, findings from the 2010 study negated many of the findings of the earlier California one regarding airborne metals and autism. The study notes: “This suggests that the earlier positive associations for these metals may have been due to confounding by urbanicity, other pollutants, or unknown factors associated with urbanicity or pollutant concentrations.”
Research across the years, however, has increasingly found that exposure to air pollutants holds an association with rising autism numbers, even when it is not always clear exactly why this association exists. Individuals can track their own potential for exposures by checking in with AIRNow, the daily Air Quality Index that’s maintained by a collection of federal, tribal, state and local agencies.