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"The study does not prove that pollution increases risk for autism. It found an association," cautioned lead author Andrea Roberts, a research associate at the Harvard School of Public Health in Boston. "It adds to the weight of the evidence that there may be something in air pollution that increases risk for autism."
Researchers compared exposure to air pollution among 325 women who had a child with autism and 22,000 women who did not. The women were participants in the Nurses' Health Study II. Pollutants measured included diesel particulate matter, lead, manganese, mercury, methylene chloride, and a combined measure of metal exposure.
Twenty percent to 60 percent of the women lived in areas considered highly polluted. And the study showed that: those women who lived in the 20 percent of locations that had the highest levels of diesel particulates or mercury in the air were twice as likely to have a child with autism, compared to those who lived in the 20 percent of areas with the lowest levels of these pollutants.
In addition, those who lived in the 20 percent of locations with the highest levels of lead, manganese, methylene chloride, and combined metal exposure were about 50 percent more likely to have a child with autism than those who lived in the 20 percent of areas with the lowest concentrations.
The findings held even after the researchers took into account other factors known to affect autism risk, such as income, education and smoking during pregnancy. Overall, the association was stronger for boys than it was for girls, but the number of girls included in the new study was too low to draw any firm conclusions.
The findings, which were published June 18 online in Environmental Health Perspectives, do add to a growing body of research that suggests the air women breathe while pregnant is one of many factors linked to autism risk. Previous studies have shown that pregnant women who live in polluted areas or close to freeways are more likely to have a child with autism, but the studies were done regionally. The new data is nationwide.
Exactly how, or even if, air pollution affects the developing brain is murky. "By definition, pollution is stuff that is not good for us," Roberts said.
Still, the overall increase in autism risk that may be attributed to pollution is 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.
"Even if the risk is doubled, it's still low," she explained.
The U.S. Centers for Disease Control and Prevention now estimates that about one in 50 children aged 6 to 17 in the United States has an autism spectrum disorder, the name for a larger group of disorders that can range from the mild to the severe, and affect social and communication skills.
Other experts also urged caution in interpreting the new findings.
"There many genes, probably hundreds, and many environmental factors, probably hundreds, that increase risk of autism," said Alycia Halladay, senior director for environmental and clinical sciences at the advocacy group Autism Speaks. "The real message is that a lot of things cause autism, namely genetics and the environment and their interaction."
Laura Anthony, the associate director of the Center for Autism Spectrum Disorders at Children's National Medical Center in Washington, D.C., said that these risks accrue during pregnancy, delivery and within the first month of a newborn's life. "Everything points to that as the critical period. This is the time when the brain is most sensitive because it is still developing," she said.
The new findings don't mean that pregnant women should head for the hills to avoid smog, Anthony added. "Even if you live someplace rural, you may be exposed to pollution while driving or you could live in a rural place right next to a plant [or factory]," she said. "We all need to campaign for cleaner air for a lot of reasons."
Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Steven and Alexandra Cohen Children's Medical Center in Lake Success, N.Y., said that the new findings do add weight to previous studies that looked at the connection between prenatal exposure to airborn pollutants and later autism.
"While they do validate and affirm what other studies have found, there are many risk factors and genetic causes identified with autism," Adesman said. "Even with the strength of this study, parents can't presume that most cases of autism are due to airborne contaminants. It's easier said than done to suggest that she move or not breathe the air."
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