Women who have higher exposure to fine particulate matter in the air are more likely to have dense breast tissue, a strong risk factor for breast cancer, according to a study by University of Florida researchers. The findings, published in the journal Breast Cancer Research, may help explain why studies have shown higher density of breast tissue among women living in urban areas.

For the study, the largest of its kind to explore the association between air pollution and breast density, the researchers analyzed data from a study of nearly 280,000 women age 40 and older with no history of breast cancer who had mammograms at facilities across the country. They compared participants’ breast density across different levels of particulate matter and ozone that were measured by scientists in the women’s ZIP codes.

After adjusting for factors such as age, body mass index and menopausal status, the team found women with very dense breasts were 20 percent more likely to have been exposed to higher levels of air pollution. For every one-unit increase in particulate matter, women had a 4 percent higher chance of having dense breasts. While the reasons aren’t well-understood, women with very dense breasts may be four to five times more likely to develop breast cancer than women with low breast density, experts say. It can also be harder to spot tumors in mammograms of women with dense breasts.

The researchers said chemical components in the air pollution could be interfering with normal tissue growth. The next step would be to determine how this link between air pollution and breast density could translate into a greater risk of breast cancer.