A first-of-its-kind study of 60 pregnant women in poor areas of New York City used backpacks to monitor the women's exposure to airborne carcinogens and then tested their babies' umbilical-cord blood after birth. Babies whose moms were exposed to higher pollution levels had 53 percent more aberrations in their chromosomes. Other studies have shown that these types of chromosomal changes increase the risk of cancer.

''This finding shows the process can begin as early as the womb as a result of air pollution,'' said study author Frederica Perera, the director of Columbia University's Center for Children's Environmental Health. ''We know that these pollutants make their way across the placenta.''

Perera's study didn't determine what parts of the babies' genes changed or if they all changed in the same areas.

The peer-reviewed study - funded by the National Institute of Environmental Health Sciences and published in this month's Cancer Epidemiology Biomarkers and Prevention journal - links in-the-womb chromosome damage to elevated exposure to polycyclic aromatic hydrocarbons.

There are more than 100 PAHs, which are the byproducts of combustion, including car and truck exhaust, power plant emissions, tobacco smoke and even the smoke from grilling meats. Fifteen of the most common PAHs are listed as carcinogens in the official government list of cancer-causing agents.

PAHs get into the air usually as ultra-small particles - not smog - that can travel hundreds of miles and then lodge in the lungs, said Janet Arey, a University of California at Riverside atmospheric chemistry professor who has studied PAH affects for the Environmental Protection Agency. In many places, including Southern California, the highest levels are closest to traffic congestion.

The finding comes on the heels of a 2004 Canadian study that exposed mice to similar air pollutants and found that they caused an increase in genetic mutation.

The new Columbia University research ''is an important study that points out that air pollution seems to be causing harm to DNA,'' said biology professor James Quinn of McMaster University, the author of the 2004 research. The next big question, he said, is whether these changes are passed to future generations.

The problem the studies have found is more of a general risk, rather than specific concerns for individuals.

''This is not something for pregnant women and new mothers to be alarmed about,'' said Perera, a professor of environmental health. ''This doesn't mean that their child is going to get cancer.''

But Perera and Kenneth Olden, the director of the National Institute of Environmental Health Sciences, said the new study should make federal officials look toward better pollution-prevention methods.

The issue is timely, because President Bush's proposed revisions of portions of the 1970 Clean Air Act, which regulates emissions from all major polluters except power plants that existed before it went into effect, are up for a key committee vote today in the Senate.