Pregnant women who live in smog-filled areas may be twice as likely to have
children with autism, a new study suggests.
"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
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
What the study found
Twenty percent to 60% of the women lived in areas considered highly polluted.
And the study showed that: those women who lived in the 20% 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% of
areas with the lowest levels of these pollutants.
In addition, those who lived in the 20% of locations with the highest levels
of lead, manganese, methylene chloride, and combined metal exposure were about
50% more likely to have a child with autism than those who lived in the 20% 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
The findings 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
"Even if the risk is doubled, it's still low," she explained.
Campaign for cleaner air?
The US 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, DC, 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
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 behavioural paediatrics at
Steven and Alexandra Cohen Children's Medical Center in Lake Success, NY, 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."
Learn more about autism at the US Department of Health and