Dense breasts have less fatty tissue and more non-fatty tissue compared to breasts that aren't dense. Dense breasts have more gland tissue that makes and drains milk and supportive tissue (also called stroma) that surrounds the gland. Breast density can be inherited, so if your mother has dense breasts, it's likely you will, too.
Research has shown that dense breasts:
- can be twice as likely to develop cancer as nondense breasts
- can make it harder for mammograms to detect breast cancer; breast cancers (which look white like breast gland tissue) are easier to see on a mammogram when they're surrounded by fatty tissue (which looks dark)
One way to measure breast density is the thickness of tissue on a mammogram.
The Breast Imaging Reporting and Database Systems, or BI-RADS, which reports the findings of mammograms, also includes an assessment of breast density. BI-RADS classifies breast density into four groups:
- Mostly fatty: The breasts are made up of mostly fat and contain little fibrous and glandular tissue. This means the mammogram would likely show anything that was abnormal.
- Scattered density: The breasts have quite a bit of fat, but there are a few areas of fibrous and glandular tissue.
- Consistent density: The breasts have many areas of fibrous and glandular tissue that are evenly distributed through the breasts. This can make it hard to see small masses in the breast.
- Extremely dense: The breasts have a lot of fibrous and glandular tissue. This may make it hard to see a cancer on a mammogram because the cancer can blend in with the normal tissue.
Still, no one method of measuring breast density has been agreed upon by doctors. Breast density is not based on how your breasts feel during your self-exam or your doctor's physical exam.
While having dense breasts increases the risk of breast cancer, researchers aren’t sure why some women have dense breasts and other women don’t. Researchers wondered if environmental factors could contribute to breast density.
A study suggests that there may be a link between breast density and living in areas with high levels of fine-particle air pollution.
The research was published online on April 6, 2017 by the journal Breast Cancer Research. Read “Association between air pollution and mammographic breast density in the Breast Cancer Surveillance Consortium.”
To do the study, the researchers looked at information on breast density for women having screening mammograms between 2001 and 2009 at facilities that are part of the Breast Cancer Surveillance Consortium. The Breast Cancer Surveillance Consortium was established in 1994 to enhance the understanding of breast cancer screening practices in the United States and their relation to changes in stage at diagnosis, survival, or breast cancer mortality. The database includes information from 1994 to 2009 on more than 2.3 million women who had 9.5 million mammograms.
The study included 279,967 women who were age 40 and older and who had a zip code in the database. The researchers used the BI-RADS breast density classification in each woman’s records:
- 10.8% had mostly fatty breasts
- 41.7% had scattered density
- 39.4% had consistent density
- 8.2% had extremely dense breasts
To determine the women’s exposure to air pollution, the researchers matched the women’s zip codes to air pollution estimate grids from the U.S. Environmental Protection Agency (EPA). The researchers looked at the women’s exposure to air pollution particulate matter that was smaller than 2.5 micrometers in size, as well as ozone. About 60% of the women lived in urban areas and about 40% lived in rural areas.
A micrometer is one millionth of a meter. Air pollution particulate matter that is smaller than 10 micrometers can enter the lungs, and studies have linked these tiny particles to heart disease, stroke, lung cancer, asthma, and other diseases.
Ozone is created by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight. Car exhaust, gasoline vapors, chemical solvents, and emissions from industrial plants are major sources of nitrogen oxides and volatile organic compounds. Breathing ozone can cause or worsen a variety of health problems, including asthma, emphysema, and bronchitis.
For 94% of the women in the study, the researchers were able to obtain air pollution estimate grids for the year before their mammogram.
The researchers found that women with extremely dense breasts were exposed to higher levels of air pollution particulate matter that was smaller than 2.5 micrometers.
Women with extremely dense breasts were about 20% more likely to have been exposed to higher levels of air pollution particulate matter that was smaller than 2.5 micrometers than women who had breasts with scattered density. Women with mostly fatty breasts were about 12% less likely to have been exposed to high levels of air pollution particulate matter that was smaller than 2.5 micrometers.
The researchers said that a one unit increase in air pollution particulate matter that was smaller than 2.5 micrometers was linked to:
- a 4% higher risk of having consistently dense breasts
- a 2% lower chance of having fatty breasts compared to breasts with scattered density
When the researchers looked at ozone exposure, they found the opposite relationship. Women exposed to higher levels of ozone were less likely to have extremely dense breasts.
"Our findings suggest that previously reported geographic variation in breast density could, in part, be explained by different air pollution patterns in urban and rural areas," said Lusine Yaghjyan, professor of epidemiology at the University of Florida and lead author of the study. "Breast density is a well-established and strong breast cancer risk factor so future studies are warranted to determine if the observed associations are causal, which if confirmed may have implications for risk prevention.
"We found a positive association between fine particle concentration exposure and breast density but an inverse association between ozone exposure and breast density," she continued. "This is an intriguing result that warrants further investigation to unpack any possible biological mechanism that might cause ozone exposure to reduce a woman's chance of having dense breasts."
Many of the chemicals that make up air pollution smaller than 2.5 micrometers are known to be hormone disruptors. Hormone disruptors can affect how estrogen and other hormones act in the body by blocking them or mimicking them, which throws off the body’s hormonal balance.
While this study doesn’t prove that exposure to high levels of very fine air pollution particles caused a woman to have dense breasts, it suggests that there may be some type of link, perhaps in combination with exposure to other chemicals.
If you suffer from asthma or have difficulty breathing because of allergies or other conditions, you may want to monitor your local air quality. The EPA has an online Air Quality Index tool called AirNow that allows you to check the Air Quality Index for your area. If the air quality is unhealthy, very unhealthy, or hazardous, you may want to avoid going outside and keep your windows and doors closed until the air is cleaner.
For more information on breast density, visit the Having Dense Breasts page in the Breastcancer.org Lower Your Risk section.