Air Pollution and Autism

New research suggests the dirty air that pregnant women breathe might increase the risk of autism spectrum disorder in their children.

The polluted air women breathe while pregnant, especially in their third trimester, may increase the chance that their children will develop autism spectrum disorder. That’s the conclusion reached by scientists at the Harvard School of Public Health, who made the connection using data from a study of thousands of American women.

The Nurses’ Health Study II is one of the country's biggest and longest running surveys of women’s health. Started in 1989, it investigates risk factors for chronic disease in female nurses across the United States. Researchers have found that it also contains useful data on the participants’ children.

“There’s growing evidence suggesting that the problems underlying autism are starting in utero,” says Marc Weisskopf, an author of the study, which was published last month in Environmental Health Perspectives.

Photo: Stefan Pasch

One in six children in the United States has autism spectrum disorder. Scientists know that autism has many genetic links (another recent study pegged its heritability at just above 50 percent), but a growing body of evidence shows that environmental factors might also play a role.

To be clear, no one knows exactly what causes autism, and these researchers aren’t claiming they’ve found the answer—only an interesting correlation (which, to repeat the old canard, is not causation) worthy of further study.

The Harvard study isn’t the first to suggest a connection between pollution and autism, but it's especially significant for its nationwide scope and for finding a strong link between the disorder and the pollution inhaled during the last trimester of pregnancy, a time when the fetus’ brain is rapidly developing.

Weisskopf and his team looked at the roughly 1,700 kids born to the nurses in the study between 1990 and 2002; about 250 were diagnosed with autism spectrum disorder. Using information on the participants' residence locations, local levels of air pollution, and the women’s socioeconomic status, the researchers estimated how much particulate matter—of 2.5 microns in diameter or smaller—mothers were exposed to before getting pregnant, during each trimester, and after giving birth.

What they discovered was that the higher the mother’s exposure, the higher the risk she would have a child with autism—especially if the mother had been breathing in polluted air between the 27th and 40th week of pregnancy.

During the final trimester, “brain growth…happens in a very timed sequence, so event A has to happen and then that allows B to go forward, C, and D, and E,” says Deborah Cory-Slechta, a professor in the Department of Environmental Medicine at the University of Rochester School of Medicine, who was not involved in the study. “If any of those happen out of sequence or doesn’t occur, it’s like dominoes falling.”

Scientist don’t know exactly how particulate matter might contribute to autism risk, just that there is a correlation between the two. One theory: Airborne particles, such as nitrates, sulfates, metals, soil, and dust, cause an inflammatory response in the mothers’ bodies. Molecules in the blood fight the intruding particles, but while doing so, may also attack healthy cells, including those in the fetus.

Air pollution is one of many potential contributors to autism. If each factor were a player on a baseball team, explains Weisskopf, higher levels of particulate matter would be similar to increasing one player’s batting average, raising the chances for a hit overall. What makes this possibility especially notable, however, is that so many pregnant women are exposed to contaminated air from sources such as power plant emissions, car exhaust, or forest fires.

The more researchers uncover about how dirty air might contribute to the disease, the better regulations lawmakers can put in place to protect those who are vulnerable. Until then, pregnant women may want to avoid polluted areas (by exercising in parks instead of along roads, for example).“If you can make simple choices like that,” says Weisskopf, “I would do it.” And considering air pollution’s known track record with many other serious health conditions, this sounds like good advice for anyone.


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