London radiologist Nick Perry has been reading breast X-rays for several decades. For a long time, he's noticed something different about his patients who lived in the city. His urban-dwelling patients seemed to have denser breast tissue than women who lived further out in the suburbs or rural areas. Their X-rays appeared cloudier.

Perry says breast density does more than just make mammograms harder to read. He says breast density is a marker for a risk factor for developing breast cancer.

So, he decided to study women who come to the London Breast Institute, where he's the director. Perry and his colleagues examined mammograms from close to 1,000 patients. They assessed the breast density represented on the x-rays and sorted the patients by their postal codes.

They found that Perry's premise was correct.

"Women with more central addresses had denser breast tissue," Perry reports. "And not only that, in areas that were outside of London ? still built up to some extent, but diminishing areas of population density ? we found there was a definite gradient." In other words, the further from the center of the city and the less populated areas became, women from those areas had less dense breasts ? as shown by mammograms.

Furthermore, for subjects between the ages of 45 and 54, women who lived in central London were twice as likely to have denser breasts than women who lived in less populated areas. This put these women at higher risk for developing breast cancer.

Perry says he's not sure why city dwellers have denser breasts. He suggests it could be a combination of stress from city living and pollution. He points to recent research that associates stress with breast cancer risk. "We know that estrogenic activity has been recorded in association with fossil fuel emissions. And indeed, the very latest study shows that the women who have their first birth ? birth of her first child ? living in areas of higher exposure to traffic emissions, run over twice the risk of developing postmenopausal breast cancer."

Perry says since he made his findings public, he's been approached by radiologists from all over the world who say they've noted similar trends in their patient populations. He says he'd like to research this phenomenon further.

Perry presented his findings at the annual meeting of the American Radiologic Society in Chicago last week.