Two new studies published recently bring grim news to new or soon-to-be new moms in urban areas. According to the studies, baby's brains may be damaged by common air pollutants breathed by their mothers while the babies are still in the womb.

The studies involving more than 400 pregnant women in two cities found that 5-year-olds exposed in the womb to above-average levels of polycyclic aromatic hydrocarbons, or PAHs, score lower on IQ tests. The compounds, created by the burning of fossil fuels (in homes, factories, or from cars), are prevalent in urban environments.

One half of the study monitored almost 250 women and children in New York City for the effects of environmental contaminants. Across the Atlantic, in Krakow, Poland, another 214 children and moms participated in a parallel study.

The findings for both studies are as similar as they are disturbing: The children whose mothers had above-average exposure to PAHs scored about four points lower on IQ tests than children whose mothers had below-average exposure.

Recruited for the study between 1998 and 2006, the pregnant women in New York City and Krakow carried backpacks for 48 hours that contained equipment for measuring PAHs. Then their children were divided into high and low exposure groups — those above the median and below — and when they reached the age of 5, they underwent standardized tests to measure their cognitive skills.

In Krakow, most of the compounds come from coal burning for home heating and factories, while in New York City, exhaust from cars, trucks and buses was the major source.

Researchers note that the difference in IQs is small, but health experts say it is enough to hamper school performance and perhaps lifelong learning. And the deficit is similar to that linked to low-level exposure to lead, a well-documented cause of reduced IQs in children.