While there is no single known cause for autism, environmental factors have long been thought to contribute to the disorder. Now there is more definitive answers: Mothers who lived in areas with lots of air pollution while they were pregnant may have children who are at increased risk of developing autism.
The U.S-wide study, from the Harvard School of Public Health (HSPH) and published in Environmental Health Perspectives, found that expecting mothers exposed to high levels of fine particulate matter (PM 2.5, or any type of air pollution that’s 2.5 microns in size or smaller) during the third trimester of pregnancy have up to twice the risk of giving birth to an autistic child. The more pollution that the mother is exposed to, the higher the risk of autism.
There have been other studies linking air pollution to autism; in fact, HSPH previously conducted a study looking at a series of hazardous air pollutants and their relation to autism. Their past work did find a link, but the data used to determine a mother’s exposure was cruder than in the current study.
“There have been a couple other groups that have looked at this–those were largely in the California area, another in North Carolina. With respect to those papers, they didn’t compare different time windows of exposure [to air pollution], they were often just looking at the address at birth, they weren’t U.S-wide,” says Marc Weisskopf, associate professor of environmental and occupational epidemiology at HSPH and senior author of the study. “We’re looking at different time periods around pregnancy, and seeing a signal in the third trimester.”
To conduct the study, Weisskopf and his colleagues focused on the children of a group of 116,000 female nurses scattered across the U.S.. Since 1989, these women have been part of the Nurses’ Health Study II, answering questionnaires every few years about their health.
The researchers in the current study gathered data on where the nurses lived during their pregnancies along with data from the EPA and others on levels of fine particulate matter near their residences. Between 1990 and 2002, 245 children of participants in the Nurses’ Health Study were diagnosed with autism spectrum disorder, while 1,522 children were born without the diagnosis.
The findings were disturbing: Mothers exposed to air pollution during pregnancy were significantly more likely to have a child with autism, especially if they were exposed during that key third trimester. Exposure to the same pollution before and after pregnancy made little difference in autism risk. “This suggests a critical window of exposure that can give us some insight into what’s going on with brain development,” says Weisskopf.
Next, Weisskopf and his colleagues want to look into whether there are specific components of PM 2.5 pollution that are related to autism development or whether any particulate matter of that size is dangerous (PM 2.5 is largely the byproduct of vehicle exhaust). Other studies suggest that traffic-related pollutants and exposure to certain metals may be related to autism.
Right now, the researchers have established that air pollution in the third trimester and autism risk are related, but not why or how. Another question for the researchers to look in the future: What exactly is happening inside the mother’s body when she’s exposed to air pollution that leads to a higher risk of autism? Are certain biological pathways initiated? It’s also possible that certain women are more susceptible to having an autistic child because of their genetic makeup.
“This is some of the best evidence to date that the environment can affect the risk of autism. We’ve got a pretty cohesive set of data right now,” says Weisskopf. “It opens up the idea that there may be other environmental factors that we need to look into.”