Traffic hydrocarbons linked to lower IQs in kids

Prenatal exposures to common air pollutants correlate with drop in intelligence scores

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.

Five years ago, molecular epidemiologist Frederica Perera of Columbia University and her colleagues reported that a fetus appears especially vulnerable to the carcinogen benzo(a)pyrene and related PAHs. These hydrocarbons are emitted by everything from cars and trucks to industrial boilers, coal-fired power plants — even cigarettes.

Animal studies suggested that a mom’s exposure to such PAHs should be 10 times that of the fetus. In that earlier study, however, Perera’s team found that the mean level of genetic alterations — chemical groups that attach to genes and are called DNA adducts — in a child’s blood at birth was virtually identical to mom’s. A second testing method showed DNA adduct levels to be actually higher in newborns than in moms.

For their new study, Perera and colleagues recruited nearly 400 moms-to-be between 1998 and 2003 from inner-city neighborhoods in New York: Washington Heights, Harlem or the South Bronx. Each woman got a backpack fitted with a PAH sampler to wear continuously for two days during her last trimester of pregnancy. Traffic, which doesn’t vary much by season, was the primary source of PAHs that these women encountered — indoors and out, notes Perera.

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.

The same no-threshold effect is seen with lead. And there, society can incur huge costs associated with even seemingly small drops in IQ, notes Bruce Lanphear, a pediatric epidemiologist at Simon Fraser University in Burnaby, Canada.

“For any individual child, unless it’s your own, you might consider a 5-point drop in IQ as no big deal,” he acknowledges. But extrapolate that across a population the size of the United States, “and a downward shift in IQ by 5 points will increase by 3.5 million the number of children who meet the criteria for mental retardation.” It also increases the number of children needing more, costlier help in school.

“These numbers are not small at all,” he says. In fact, he argues, the new data point to yet another area where investing in pollution prevention could yield, socially and economically, a huge bang for the buck.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

More Stories from Science News on Humans