Two synthetic chemicals known as PFAS were associated with a higher risk of aggressive breast cancer in research from the National Institutes of Health. The findings focused on perfluorooctanoic acid, or PFOA, and perfluorooctanesulfonic acid, or PFOS.

PFAS are often called forever chemicals because they break down extremely slowly and can accumulate in the environment and human body. They have been used in nonstick cookware, upholstery, clothing, food packaging and personal care products.

The study, published in the Journal of Clinical Oncology, linked higher blood levels of PFOA and PFOS to a greater likelihood of developing estrogen receptor negative, or ER-negative, breast cancer. The researchers found no clear association with breast cancer overall.

ER-negative breast cancers tend to be more aggressive and have worse outcomes than cancers that respond to estrogen, according to a Department of Health and Human Services press release. Relatively few risk factors beyond genetic predispositions are known for this subtype.

Researchers used data from the Sister Study, a large United States research project examining environmental and genetic factors related to breast cancer. Every participant had a sister diagnosed with breast cancer at enrollment, but participants did not have the disease themselves.

The team measured 17 PFAS in blood samples collected before any breast cancer diagnosis. Samples came from 1,609 women who later developed breast cancer and a comparison group of 2,269 women.

Researchers then examined the relationship between PFAS concentrations and breast cancer risk according to tumor type. Women in the highest PFOA exposure group had a 67% higher rate of ER-negative breast cancer than those in the lowest group.

The rate was 93% higher among women in the highest PFOS exposure group compared with those in the lowest group. Those figures describe relative differences between the groups, not an individual woman’s chance of developing breast cancer.

“This study, along with the accompanying meta-analysis of previous studies, provides evidence that two PFAS are related to a higher ER-negative breast cancer risk,” study leader Alexandra White, Ph.D., a researcher at the National Institutes of Health, told Fox News Digital.

The research mainly examined older PFAS chemicals that have largely been phased out. White identified limited knowledge about the replacement chemicals as a limitation, noting that experts know very little about their possible health effects.

“They tend to be shorter-chain compounds and may be less persistent in our bodies and the environment, but we know little about how they may influence health outcomes , such as breast cancer,” White said.

The findings establish an association rather than proving that PFOA or PFOS caused the cancers. The study’s authors said additional research is needed to clarify the potential relationship between exposure and disease.

HHS Secretary Robert F. Kennedy Jr. said “women deserve clear answers about how chemicals in their environment may affect their health,” while calling for further investigation. He pledged to pursue evidence, confront environmental health risks and provide women with information to protect their health.

Dr. Tiffany Troso-Sandoval, a New York City based breast and gynecological cancer specialist, said forever chemicals can accumulate in the body and increase cancer risk. She worked at Memorial Sloan Kettering for more than 25 years.

Troso-Sandoval said the notable finding was that the association appeared only with ER-negative breast cancer, an aggressive subtype. She also stressed that higher PFAS levels among patients do not demonstrate that the chemicals caused their cancers.

She suggested PFAS could damage DNA, disrupt the body’s ability to repair that damage or interfere with the destruction of cancerous cells. First author Che-Jung Chang, Ph.D., said exposure is potentially modifiable and that people can take steps to reduce it.

Troso-Sandoval advised people to review products they use frequently and replace those first. She recommended considering stainless steel cookware, avoiding heating food in plastic containers and reviewing cosmetics and personal care products that may contain potentially harmful compounds.