In Krakow, most of the compounds come from coal burning for home heating and factories, while in New York City, exhaust from cars, trucks and buses is the major source.
The results in Poland, published in April, bolster the New York City findings, which were published a year ago, because they found the same effect on IQs of children living in different conditions, in different parts of the world.
While the New York City group – from Harlem, the South Bronx and Washington Heights – included only African-American and Dominican women, all the women in Poland were Caucasian.
“We’re confirming a finding in a different population, a different setting, even a different gradient of exposure, and that reinforces the initial finding in New York,” Perera said. “The study provides further evidence that prenatal exposure to air pollution adversely affects children’s cognitive development.”
In Krakow, the women on average were exposed to eight times higher concentrations of PAHs than the women in New York City. (In Krakow, 17.96 nanograms per cubic meter; in New York City, 2.26)
Some experts say the findings raise questions because the IQ losses were similar in the two groups despite the eightfold difference in their average level of exposure.
“It is a striking finding given the many differences in the two cohorts,” said Jennifer Adibi, an epidemiologist at University of California, San Francisco who studies how environmental exposures affect fetal development. “We would want to see this association replicated in other cohorts before we attribute a causal relationship to it.”
Adibi added that many factors are different in New York City and Krakow - such as diet, lifestyle, ethnicity and other contaminants.
“Direct comparison between the two studies, given their differences and the sample sizes, is extremely difficult,” added Negin Martin, a research fellow at the National Institute of Environmental Health Sciences who studies how pollutants affect brain development.
Nevertheless, Martin said it is notable that the studies were consistent; they each found “a measurable and statistically significant decrease in IQ,” she said. When the scientists compared only the 150 women who had similar exposures in the two cities, IQ differences were still found.
The researchers also adjusted their data to consider some factors that could influence a child’s intelligence and skew the results, such as maternal education and second-hand smoke. One possible confounding factor, however, is that the children were not tested for lead exposure when they were toddlers. They were checked only for prenatal exposure, which is normally less of a problem. Accounting for such factors is “a pretty big challenge, and it's one that Columbia's group does very well,” said Bellinger, who was not involved in the study.
Researchers have now linked several environmental contaminants to effects on developing brains.
In earlier work, the Columbia University team found lower IQs in children whose pregnant mothers were exposed to high levels of a pesticide, chloropyrifos, which was widely used in households until it was banned for residential use in 2001. Similar results were reported in children with prenatal exposure to secondhand smoke. And for more than two decades, scientists in the Faroe Islands, in the North Atlantic, have reported reduced IQs in children exposed in the womb to mercury in seafood.
Lanphear said PAHs, lead, mercury, tobacco smoke are systemic poisons that can damage multiple organ systems.
“It is well known that airborne pollutants – a soup of various types of toxicants – are associated with low birth weight, diminished lung function, lung cancer and asthma. So while these studies [of children’s IQ] are novel, the results shouldn't surprise us,” he said.