Traffic hydrocarbons linked to lower IQ's in kids

Here’s a dirty little secret about polluted urban air: It can shave almost 5 points off of a young child’s IQ, a new report suggests.

That’s no small loss, says Kimberly Gray, whose federal agency cofinanced the study, to appear in the August Pediatrics.

Normally, baseline environmental exposures to a pollutant yield at most a subtle change — one that is hard to detect and with impacts that are hard to gauge, says Gray, of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C. But the new study shows that children heavily exposed in the womb to common combustion pollutants known as polycyclic aromatic hydrocarbons had, by kindergarten age, an IQ some 4.5 points lower than that of kids with minimal fetal exposures.

“An IQ change of 4 points is not a subtle effect,” Gray says. It’s in the range of what might be triggered by exposures to high levels of lead or by fetal alcohol syndrome, she explains.

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The children, followed periodically from birth, showed a PAH-linked decline in mental development by age 3.

At age 5, when tests can reliably gauge intelligence, all primarily English-speaking kids were given an IQ test. Those for whom Spanish was their primary language were excluded, Perera explains, because, the IQ test for them is slightly different. Some additional children were dropped from the analysis because a full range of data was not available for them.

Among the remaining 249 kids, PAHs showed an independent effect on IQ even after accounting for other factors that can influence intellect, such as prenatal exposure to lead or tobacco smoke, gender, ethnicity, mom’s IQ, her education and measures of the quality of in-home child care.

The moms’ PAH exposures ranged from 0.5 to 34.5 nanograms per cubic meter of air. Anything above 2.26 ng/m3 — the mean value — was considered high, and kids whose moms had those exposures were compared with kids in the less exposed group. The mean full-scale IQ of kids in the more exposed group was 4.3 points lower than in those in the other half; the mean verbal IQ of the most PAH-exposed kids was 4.67 points lower. Significantly, Perera notes, when all of the kids were compared individually, “we didn’t see any evidence of a threshold” below which PAHs had no effect on IQ.