CHICAGO - Researchers for the first time have linked air-pollution exposure before birth with lower IQ scores in childhood, bolstering evidence that smog may harm the developing brain.

The results are in a study of 249 children of New York City women who wore backpack air monitors for 48 hours during the last few months of pregnancy. They lived in mostly low-income neighborhoods in northern Manhattan and the South Bronx. They had varying levels of exposure to typical kinds of urban air pollution, mostly from car, bus, and truck exhaust.

At age 5, before starting school, the children were given IQ tests. Those exposed to the most pollution before birth scored on average 4 to 5 points lower than children with less exposure.

That's a big enough difference that it could affect children's performance in school, said Frederica Perera, the study's lead author and director of the Columbia Center for Children's Environmental Health.

Michael Msall, a University of Chicago pediatrician not involved in the research, said the study did not mean children living in congested cities "aren't going to learn to read and write and spell."

But it does suggest that you don't have to live right next door to a belching factory to face pollution health risks, and that there may be more dangers from typical urban air pollution than previously thought, he said.

"We are learning more and more about low-dose exposure and how things we take for granted may not be a free ride," he said.

While future research is needed to confirm the results, the findings suggest that exposure to air pollution before birth could have the same harmful effects on the developing brain as exposure to lead, said Patrick Breysse, an environmental-health specialist at the Johns Hopkins Bloomberg School of Public Health.

And along with other environmental harms and disadvantages to which low-income children are exposed, it could help explain why they often do worse academically than children from wealthier families, Breysse said.

"It's a profound observation," he said. "This paper is going to open a lot of eyes."

The study in the August edition of Pediatrics was released today.

In earlier research, involving some of the same children and others, Perera linked prenatal exposure to air pollution with genetic abnormalities at birth that could increase risks for cancer, smaller newborn head size, and reduced birth weight. Her research team also has linked it with developmental delays at age 3 and with children's asthma.

The researchers studied pollutants that can cross the placenta and are known scientifically as polycyclic aromatic hydrocarbons. Main sources include vehicle exhaust and factory emissions. Tobacco smoke is another source, but mothers in the study were nonsmokers.

A total of 140 study children, 56 percent, were in the high-exposure group - meaning their mothers most likely lived close to heavily congested streets, bus depots, and other typical sources of city air pollution; the researchers are still examining data to confirm that, Perera said.

The mothers were black or Dominican American; the results likely apply to other groups, researchers said.

The researchers took into account other factors that could influence IQ, including secondhand smoke exposure, home learning environment, and air-pollution exposure after birth, and still found a strong influence from prenatal exposure, Perera said.

Robert Geller, an Emory University pediatrician and toxicologist, said the study could not completely rule out that pollution exposure during early childhood might have contributed. The researchers said they planned to continuing monitoring and testing the children.