Causes of Autism During Pregnancy: Air Pollution Doubles the Risk - Study

Prospective parents have a verifiable cause for concern regarding their prenatal child's chances of being born with autism. A new study from the Harvard School of Public Health found that, depending on levels of air pollution, and the kinds of pollutants in the air, an unborn child's risk of developing autism can skyrocket to as much as 50 percent.

The study, which appeared on Tuesday, June 18 in Environmental Health Perspectives, assessed the links between air pollution and autism across the United States. While previous studies have found that exposure to dangerous chemicals and pollutants can affect a developing baby's brain function and also that there are links between exposure to air pollution during pregnancy and autism in children, this is the first nation-wide study to examine the link in such an extensive way. 

Researchers found that women in the U.S. who were exposed to "high levels of air pollution while pregnant were up to twice as likely to have a child with autism as women who lived in areas with low pollution," according to the statement accompanying the study. While this may sound alarming, the overall percentage chance of a child to develop autism is low, so the term "twice as likely" is frightening, but not epidemic, according to researchers.

Researchers looked at data from a long-term study - the Nurses' Health Study II -which was based at Brigham and Women's Hospital and involved the participation of 116,430 nurses going back to 1989. Among those studied in the large group, researchers looked at 325 women who had a child with autism, as well as 22,000 who had a child without the disorder.

Using the EPA's air pollution database, researchers looked for associations between pollutant levels and the time and place of birth and whether the child developed autism. They also adjusted for other possible influencing factors, such as whether the woman smoked during pregnancy, as well as income and education.

The results showed that women who lived in top 20 percent of the locations with the most air pollution, were twice as likely to have a child with autism as those who lived in the top 20 percent of locations with the cleanest air. Diesel particulates and mercury in the air were associated with the highest level of risk, but other types of air pollution - from lead, manganese, methylene chloride, and combined metal exposure - were also associated with a 50 percent increase in risk. 

According to lead author of the study, Andrea Roberts, prospective parents should not take alarm by the new findings, as the overall risk of developing autism is still low. "Let's say a woman's risk for having a child with autism is one in 100, women who live in the most polluted cities have a risk that is about one in 50, which means that 49 children would not have autism," Roberts said to Health Day. According to the U.S. Centers for Disease Control and Prevention's Autism and Developmental Disabilities Monitoring (ADDM) Network, one in 88 children was identified with some form of autism in 2008, as measured on a spectrum between mild and severe.

While the correlation is clear, the way that air pollution can affect a developing brain is still unknown, and there are other factors that can increase or decrease the risk of autism.

"There [are] many genes, probably hundreds, and many environmental factors, probably hundreds, that increase risk of autism," said Alycia Halladay, senior director of environmental and clinical sciences at Autism Speaks, to Health Day. "The real message is that a lot of things cause autism, namely genetics and the environment and their interaction."