Prenatal exposure to pervasive air pollutants may adversely affect a child's intelligence by preschool, researchers reported today.
In New York City, children exposed to high levels of polycyclic aromatic hydrocarbons in the womb had significantly lower full-scale and verbal IQ scores when they turned 5, according to Frederica Perera, a professor of environmental health sciences at Columbia University, and colleagues.
"The present findings are of concern because verbal and full-scale IQ scores measured ... during the preschool period were shown to be predictive of subsequent elementary school performance in a range of populations," they reported online in the journal Pediatrics.
Polycyclic aromatic hydrocarbons, released by the burning of fossil fuels and tobacco, are ubiquitous in urban areas worldwide. Although previous studies have connected these pollutants with neurodevelopment issues, none has looked specifically at their relationship to intelligence, Perera said.
So she and colleagues followed the development of 249 urban children from the prenatal period to age 5 years.
The youngsters were born to non-smoking, black and Dominican-American women ages 18 to 35 who lived in Washington Heights, Harlem or the South Bronx.
The mothers wore personal air monitors during the third trimester of pregnancy to measure exposure to air pollutants.
Overall, 56.2 percent of the children were exposed to high levels of pollution in the womb, defined as a level greater than 2.26 nanograms per cubic meter. Levels ranged from 0.49 to 34.48 nanograms.
When the children were 5, they were evaluated using the Wechsler Preschool Primary Scale of Intelligence. The mean score was 98.72, which falls in the average range.
Statistically, the scientists controlled for tobacco smoke exposure, the child's gender and gestational age, ethnicity, maternal intelligence and education, and the quality of care at home.
After those factors were accounted for, the full-scale score of youngsters with high exposure was 4.31 points lower than the others' while their verbal score was lower by 4.67 points.
"The decrease in full-scale IQ score among the more exposed children is similar to that seen with low-level lead exposure," Perera said.
Joel Schwartz, an epidemiologist at the Harvard School of Public Health who was not involved in the study, called the difference in IQ points significant from a public health standpoint.
"The key thing to remember is that we are predicting not what happens to a particular child, whose performance can fluctuate from test to test," Schwartz said. "Rather, epidemiology is predicting on average, what would happen to a population if its IQ was four points lower than another population. For population differences this is quite a noticeable effect."
The children will be followed until they are 11 to evaluate more long-term outcomes.
The scientists don't know what mechanism is at work here, but theorized that it might be the result of endocrine disruption, binding of pollutants to various receptors involved in development, DNA damage, epigenetic effects, or oxidative stress.