Scientists know that many chemicals including PAHs can cross the placenta, but they don’t understand how they may interfere with a fetus’ developing brain.
In the sensitive process of brain development, Bellinger said, there are a lot of things that can go wrong.
“Things are changing very rapidly and they're supposed to happen in a particular way, in a particular sequence,” he said. “It's almost like a choreography of events that go into building a brain.” This choreography is made up of hundreds of “performers,” each taking instructions from chemical signals. Pollutants introduce noise to this process, like static over a radio station.
Also, PAHs may affect the fetus’ DNA directly. When a mother breathes in PAHs, those chemicals are changed into byproducts in the bloodstream that can cross the placental boundary and bind directly to a fetus’ DNA, explained Susan Edwards, a graduate student at the Imperial College in London, and the lead author on the Poland study.
The four-point drop in IQs linked to the air pollutants in New York City and Krakow is fairly subtle; parents and teachers wouldn’t notice it because most children would fall within normal ranges.
Nevertheless, “is a three or point IQ-point decrement important? You bet,” said Lanphear, whose research has focused on lead exposure and children’s neurodevelopment.
He said one recent analysis found that for every dollar invested in reducing lead exposure, society would realize a $17 to $220 benefit. “The bulk of the benefit was from increased lifetime earnings by enhancing children's intellectual abilities,” he said.
“At some point, we will cease blaming parents and teachers for children's failure to learn or thrive in academics and focus our attention on reducing their exposure to widespread neurotoxicants,” Lanphear said.
The good news, Perera said, is that levels of PAHs are declining in New York City. Using data from the women’s backpacks, researchers reported that airborne levels dropped by more than 50 percent between 1998 and 2006.
Bellinger said while steps should be taken to reduce children’s exposure to pollutants, other factors remain more important for their cognitive development.
“An important point is that while these exposures are associated with health outcomes, they aren't the major determinants. How parents interact with their children, and the kinds of stimulation they provide, explains a lot more,” he said.
Living in New York City’s Washington Heights, Baldwin, 30, now has five children between the ages of 10 and two. She wore the air-monitoring backpack in 1999 when she was in the third trimester of her pregnancy with her oldest child, Patience.
Like the other children in the group, Patience has been monitored by the team of scientists since she was born. When Baldwin gave birth, a Columbia University researcher was at the hospital to collect the newborn’s cord blood for testing. Her daughter had her first IQ test as part of the study when she was two and has undergone an array of other tests since then, including a recent CT scan of her brain.
There is no evidence that the PAHs affected her daughter’s IQ. Baldwin said the girl has thrived in school and has no learning problems, although she has moderate asthma. She does not know whether her exposure to PAHs during the pregnancy was classified as high or low.
Baldwin said her role in the study has made her “hyper aware” of the risks of pollutants and pesticides. One of her sons was diagnosed with lead poisoning when he was a toddler; he apparently was exposed from peeling paint.
But she tries not to think about what effects her various exposures during her pregnancies might have had on her children. “I don’t like to dwell on something I cannot change,” Baldwin said.