Autism Risk Much Higher for Children of Pregnant Women Exposed to Air Pollution in 3rd Trimester

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The study is the latest to link environmental factors to autism spectrum disorder. Ali Jarekji/Reuters

Pregnant women who are exposed to high levels of air pollution during the third trimester of pregnancy may be as much as twice as likely to give birth to children with autism, compared with their counterparts who breathed cleaner air, according to a new Harvard study.

The greater the exposure to fine particulate matter spewed by smokestacks, vehicles and fires, the greater the risk for autism, the study found.

The findings, published in Environmental Health Perspectives, add to the growing research linking autism to environmental factors, including proximity to agricultural fields sprayed with a pesticide and to freeways (where vehicles emit lots of fine particulate matter), during pregnancy.

"The evidence base for a role for maternal exposure to air pollution increasing the risk of autism spectrum disorders is becoming quite strong," Marc Weisskopf, an associate professor of environmental and occupational epidemiology at Harvard and senior author of the study, said in a statement. "This not only gives us important insight as we continue to pursue the origins of autism spectrum disorders, but as a modifiable exposure, opens the door to thinking about possible preventative measures."

Weisskopf and his team used data from a major study of 116,430 female nurses in the U.S. that began in 1989 to analyze the outcomes for their children. The team cross-referenced that information with data on the levels of fine particulate matter, known as PM2.5, in the area where those women lived during their pregnancies. The women came from all 50 states.

In that group, the team identified 245 children diagnosed with autism spectrum disorder (ASD), as well as a control group of 1,522 children without ASD during the same period. They looked at their exposure to air pollution before, during and after pregnancy, and found that while exposure prior to and after pregnancy had little relation to a future diagnosis of autism, exposure during late pregnancy made a big difference.

Larger particles of air pollution did not seem to have any impact on the likelihood of an autism diagnosis. But PM2.5—pollution particles 2.5 microns in diameter or smaller—appeared to be the culprit. This fine material poses a greater threat to health because it can become lodged much deeper in the lungs, compared with its larger counterparts. It is frequently linked to asthma diagnosis. Studies have also found links between PM2.5 and neurological disorders.

This year, the U.S. Centers for Disease Control and Prevention estimated that 1 in every 68 children has autism. That rate is much higher than in 2012, when 1 in every 88 was estimated to have autism. In 2000, the rate was roughly 1 in 150 children, or 78 percent lower than the 2012 level.