Women exposed to high levels of fine particulate matter specifically during pregnancy - particularly during the third trimester—may face up to twice the risk of having a child with autism than mothers living in areas with low particulate matter, according to a new study from Harvard School of Public Health (HSPH). The greater the exposure, the greater the risk, researchers found. It was the first U.S.-wide study exploring the link between airborne particulate matter and autism.

“Our data add additional important support to the hypothesis that maternal exposure to air pollution contributes to the risk of autism spectrum disorders,” said Marc Weisskopf, associate professor of environmental and occupational epidemiology and senior author of the study. “The specificity of our findings for the pregnancy period, and third trimester in particular, rules out many other possible explanations for these findings.”

The study appears online December 18, 2014 in Environmental Health Perspectives and will be available at http://ehp.niehs.nih.gov/1408133.

Prior studies have suggested that, in addition to genetics, exposure to airborne environmental contaminants, particularly during pregnancy and early life, may affect risk of autism. This study focused specifically on the pregnancy period.

The study population included offspring of participants living in all 50 states in Nurses’ Health Study II, a cohort of more than 116,000 female U.S. nurses begun in 1989. The researchers collected data on where participants lived during their pregnancies as well as data from the U.S. Environmental Protection Agency and other sources on levels of fine particulate matter air pollution (PM2.5) - particles 2.5 microns in diameter or smaller—in locations across the U.S. The researchers identified 245 children who were diagnosed with autism spectrum disorder (ASD) and a control group of 1,522 children without ASD during the time period studied.

The researchers explored the association between autism and exposure to PM2.5 before, during, and after pregnancy. They also calculated exposure to PM2.5 during each pregnancy trimester.

Exposure to PM2.5 was significantly associated with autism during pregnancy, but not before or after, the study found. And during the pregnancy, the third trimester specifically was significantly associated with an increased risk. Little association was found between air pollution from larger-sized particles (PM10-2.5) and autism.

“The evidence base for a role for maternal exposure to air pollution increasing the risk of autism spectrum disorders is becoming quite strong,” said Weisskopf. “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.”

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Other HSPH authors of the study included lead author Raanan Raz, visiting scientist in the Department of Environmental Health; Andrea Roberts, research associate in the Department of Social and Behavioral Sciences; Kristen Lyall, visiting scientist; Jaime Hart, instructor, Department of Environmental Health and Department of Epidemiology at HSPH and assistant professor of medicine, Channing Division of Network Medicine, Brigham and Women’s Hospital and Harvard Medical School; Allan Just, research fellow in the Department of Environmental Health; and Francine Laden, Mark and Catherine Winkler Associate Professor of Environmental Epidemiology.

Funding for the study came from the Environment and Health Fund (Israel), NIH grants P30 ES000002, R01 ES017017, and UM1 CA176726, US Department of Defense grant W81XWH-08-1-0499, grant 1788 from the Autism Speaks Foundation.

“Autism Spectrum Disorder and Particulate Matter Air Pollution before, during, and after Pregnancy: A Nested Case-Control Analysis within the Nurses’ Health Study II Cohort,” Raanan Raz, Andrea L. Roberts, Kristen Lyall, Jaime E. Hart, Allan C. Just, Francine Laden, Marc G. Weisskopf, Environmental Health Perspectives, online December 18, 2014.

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Harvard School of Public Health brings together dedicated experts from many disciplines to educate new generations of global health leaders and produce powerful ideas that improve the lives and health of people everywhere. As a community of leading scientists, educators, and students, we work together to take innovative ideas from the laboratory to people’s lives - not only making scientific breakthroughs, but also working to change individual behaviors, public policies, and health care practices. Each year, more than 400 faculty members at HSPH teach 1,000-plus full-time students from around the world and train thousands more through online and executive education courses. Founded in 1913 as the Harvard-MIT School of Health Officers, the School is recognized as America’s oldest professional training program in public health.

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Marge Dwyer
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Harvard School of Public Health