Women who have higher exposure to fine particulate matter in the air are more likely to have dense breast tissue, a well–established strong risk factor for breast cancer, according to a study by University of Florida researchers that appeared in the journal Breast Cancer Research.

The finding may help explain why previous studies have shown higher density of breast tissue among women living in urban areas, said Lusine Yaghjyan, MD, PhD, MPH, the lead author of the study and an assistant professor in the department of epidemiology in the UF College of Public Health and Health Professions and the UF College of Medicine, part of UF Health.

“Environmental factors such as air pollution may contribute to geographic variation in breast density because urban and rural areas have distinct air pollution patterns and some air pollutants are known to disrupt the body’s endocrine system function,” she said.

Breasts are composed of fat and fibroglandular tissue, which includes breast’s epithelial and connective tissues. Women with dense breasts – as determined by a mammogram – have a higher proportion of fibroglandular elements to fat. 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, according to the Susan G. Komen organization. It can also be harder to spot tumors in mammograms of women with dense breasts.

For the study, the largest of its kind to explore the association between air pollution and breast density, Yaghjyan and her team analyzed data collected from participants in the Breast Cancer Surveillance Consortium, a National Cancer Institute–funded study of women undergoing mammograms at facilities across the country. Nearly 280,000 women age 40 and older with no history of breast cancer and living in locations in the northeastern, southwestern and western United States were included in the UF analysis. The researchers compared participants’ breast density, as defined by the American College of Radiology’s Breast Imaging–Reporting and Data System, across different levels of fine particulate matter and ozone that were measured by the U.S. Environmental Protection Agency in the women’s ZIP codes.

After adjusting for several factors that could influence breast density, such as age, body mass index, race and menopausal status, the team found that women with very dense breasts were about 20 percent more likely to have been exposed to higher levels of fine particulate matter in the air. For every one–unit increase in particulate matter, women had a 4 percent higher chance of having dense breasts.

While more research is needed before establishing a causal relationship between air pollution and dense breasts, Yaghjyan said compounds in particulate matter have the potential to interfere with the normal functioning of the body’s endocrine system.

“Epithelial tissue in breasts is regulated by a variety of hormonal influences, including estrogens and growth factors,” she said. “Chemical components in particulate matter could influence breast density by interfering with normal tissue growth, thus increasing the amount of fibroglandular tissue in the breast and, subsequently, breast density.”

The investigators were surprised to discover that higher ozone exposure was associated with lower levels of breast density. Some previous studies have found that high levels of ozone can lead to cellular death, which may explain why women with greater ozone exposure have less dense breasts, Yaghjyan said, stressing that this study is a first step toward understanding how environmental exposures affect breast density.