On Nov. 5, 2026, the National Institutes of Health (NIH) released a new study that shows 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. 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.

The study found that women with higher blood plasma levels of two of these substances, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), were associated with a higher incidence of ER-negative breast cancer. No clear association was observed between the two chemicals and breast cancer risk overall.

ER-negative tumors lack receptors that allow hormone estrogen to help cancer cells grow. Compared with breast cancers that respond to hormone estrogen, ER-negative breast cancers tend to be more aggressive and have worse outcomes. Scientists know less about what raises the risk of this type of breast cancer.

“Our findings, which we confirmed using a meta-analysis of prior research, indicate that certain legacy PFAS, including PFOA and PFOS, are associated with a higher risk of ER-negative breast cancer, a more aggressive tumor type with few known risk factors,” explained NIH Stadtman Investigator Alexandra White, Ph.D., who led the study. The findings are reported in the Journal of Clinical Oncology. 

NIH researchers and their collaborators measured 17 PFAS in blood, collected years before women were diagnosed with cancer. Women in the group with the highest PFOA levels had a 67% higher rate of ER-negative breast cancer than those in the lowest group. For PFOS, the rate was 93% higher, or nearly twice as high. These numbers describe relative differences between groups, not an individual woman’s chance of getting breast cancer. 

The research drew on the Sister Study, led by NIH’s National Institute of Environmental Health Sciences (NIEHS). The larger study enrolled 50,884 women ages 35–74 in the United States and Puerto Rico from 2003 to 2009. None had breast cancer when they joined the Sister Study, but each had a full or half-sister who had been previously diagnosed with breast cancer. For the PFAS study, researchers included 1,609 women who developed breast cancer and a comparison sample of 2,269 women. 

Because the strongest associations involved tumors without hormone receptors, the authors suggest that PFAS may affect cancer risk through other non-hormonal biologic pathways. Possible pathways include changes in gene activity or immune system. However, this study did not test those explanations. The authors say future studies should evaluate people with higher PFAS exposure and in groups more affected by ER-negative breast cancer.