Women with higher blood levels of two common per- and polyfluoroalkyl substances, also familiarly known as PFAS, were more likely to develop estrogen receptor-negative, or ER-negative, breast cancer, according to a new study by the National Institutes of Health (NIH), part of the U.S. Department of Health and Human Services (HHS). PFAS are a large group of chemicals used in common consumer products. Often referred to as "forever chemicals," PFAS do not break down easily in the environment and can remain in the body for years.