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 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 non-smokers.

A total of 140 study children, 56 percent, were in the high exposure group. That means their mothers 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 says. The mothers were black or Dominican-American; the results likely apply to other groups, researchers say.

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

Dr Robert Geller, an Emory University pediatrician and toxicologist, says the study can't completely rule out that pollution exposure during early childhood might have contributed.

He also notes fewer mothers in the high exposure group had graduated from high school. While that might also have contributed to the high-dose children's lower IQ scores, the study still provides compelling evidence implicating prenatal pollution exposure that should prompt additional studies, Geller says.

The researchers say they plan to continuing monitoring and testing the children to learn whether school performance is affected and if there are any additional long-term effects.

(China Daily July 22, 2009)