Prenatal exposure to pervasive air pollutants may adversely affect a child's intelligence by preschool, researchers found.

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, DrPH, of 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 Pediatrics.

Polycyclic aromatic hydrocarbons, released by the burning of fossil fuels and tobacco, are ubiquitous in urban areas worldwide, they said. Previous studies have connected these pollutants with neurodevelopment issues, but none has looked specifically at their relationship with intelligence, they said.

So Dr. Perera and colleagues followed 249 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% of the children had been exposed to high levels of pollution in the womb, defined as a level greater than 2.26 ng/m3. Levels ranged from 0.49 to 34.48 ng/m3.

When the children were 5, they underwent testing with the Wechsler Preschool Primary Scale of Intelligence. The mean score was 98.72, which falls in the average range.

After controlling for tobacco smoke exposure, the child's gender and gestational age, ethnicity, maternal intelligence and education, and the quality of the home caretaking environment, high exposure in utero was associated with lower full-scale (P=0.007) and verbal (P=0.003) IQ scores.

The full-scale score was lower by 4.31 points and the 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," Dr. Perera said.

Joel Schwartz, PhD, an epidemiologist at the Harvard School of Public Health who was not involved in the study, said the difference in IQ points is significant.

"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," Dr. 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 mechanisms underlying the association remain unknown, but fetal toxicity may be caused by endocrine disruption, binding of pollutants to various receptors involved in development, DNA damage, epigenetic effects, or oxidative stress, the researchers said.

A causal link was not confirmed, but if one were confirmed, it would be necessary to seek ways to lower exposure to polycyclic aromatic hydrocarbons.

"Fortunately," Dr. Perera said, "airborne polycyclic aromatic hydrocarbon concentrations can be reduced through currently available controls, alternative energy sources, and policy interventions."

The authors acknowledged some limitations of the study, including the lack of applicability of the findings to individuals with other racial, ethnic, cultural, or socioeconomic backgrounds and the lack of direct postnatal exposure data.