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 exhausts.

The children were given IQ tests at age 5, before they started school. Those exposed to the most pollution before birth scored on average four to five points lower than children with less exposure.

Mothers-to-be should try to avoid heavily congested streets, bus depots and other typical sources of city air pollution. [China Daily]

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

Dr Michael Msall, a University of Chicago pediatrician not involved in the research, says the study doesn't mean that 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 says.

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

While future research is needed to confirm the new 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, says Patrick Breysse, an environmental health specialist at Johns Hopkins' school of public health.

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

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

The study in next month's edition of Pediatrics was released on Monday.