This is the first story in a series, “Fighting ‘Forever Chemicals’: Women face pervasive PFAS risks.”

Jessian Choy had worn Thinx menstrual underwear for years before she learned they contained “forever chemicals.” 

“I had always known that anything water, grease and stain resistant could have toxic PFAS chemicals in them because of my day job at the time,” said Choy, who was working in San Francisco’s Department of the Environment when she found this out.

“But,” she said, “my only vice at the time was … the Thinx underwear and I just didn’t want to know what was in it.”

Forever chemicals, also known as per- and polyfluoroalkyl substances (PFAS), are a pervasive group of compounds that have been linked to a number of cancers and other illnesses. The toxic substances have become widespread in the air, soil and water via industrial discharge and are found in a number of common household items, from cookware to dental floss to stain-resistant furniture.

And many of the products in which they have been detected — including waterproof makeup, workout leggings and period products — are primarily marketed toward women.

Thinx denies that its products contain the substances, but settled a class-action lawsuit over allegations that they do last year.

Found in ‘essential’ products

Choy writes a column at Sierra Magazine, a publication of the environmental nonprofit Sierra Club. When a reader asked her to recommend the most eco-friendly period products, she started digging into the issue.

Choy said she felt she couldn’t recommend the undergarments without finding out if her suspicions that they contained the toxic chemicals were correct. 

She reached out to Graham Peaslee, a professor at the University of Notre Dame who was already well-known for discovering the presence of PFAS in fast food packaging. 

Peaslee agreed to test the Thinx briefs and BTWN Shorty underwear for teens for her — she sent unused pairs— and found that they contained enough PFAS to suggest they were made with the substances on purpose. Thinx has continued to maintain that its products do not contain PFAS after Choy wrote about Peaslee’s findings. 

In 2020, the company provided journalists with tests conducted by a different third party that did not find the substances in their product. 

“Our product safety testing is conducted by third party facilities to ensure our products meet the robust European safety standards of REACH and OEKO-TEX,” then-Thinx CEO Maria Molland said in a statement at the time. “Based on this outside expert testing, PFAS chemicals were not detected in Thinx products.”

Peaslee said that the day after Choy published the findings, Thinx also told him that its products didn’t contain PFAS. Thinx’s findings, he said, only looked for a subtype known as “long-chain PFAS,” whose chemical structure contains more carbon atoms. During his assessment, Peaslee said he found short-chain PFAS, which have fewer carbon atoms. 

The scientist said he told the company as much. In response, he said they called him back the next day and asked him to tell the public their product was safe — an apparent attempt to blunt the negative publicity from an article that Choy wrote about his findings. 

“They called me back and said, ‘Well, this thing’s getting out of hand, can you just issue a statement saying they’re safe to wear?’” Peaslee said. “And I was like, ‘I’m sorry, did you not listen to a word I said yesterday? I wouldn’t put my damn daughter in these things.’”

Allegations of PFAS in Thinx later spurred a class-action lawsuit claiming that the company “misrepresented the true nature of Thinx Underwear” by calling it free of harmful chemicals. The lawsuit was settled in 2023 for $5 million, enabling consumers to get back $7 per pair for every pair of Thinx underwear they purchased, up to three pairs. The company will also have to “take measures” aimed at making sure PFAS aren’t intentionally added to the underwear under the settlement. Nonetheless, Thinx still says its products are safe. 

“We stand by the quality, safety and efficacy of our products. The lawsuit is related to how products were marketed and was not about injuries or harm caused by the products,” Thinx spokesperson Felicia Macdonald shared in a written statement with The Hill last year. Macdonald is no longer with Thinx.

“We have resolved this matter so that we can focus our attention on doing what the brand does best — bringing innovative, safe and comfortable leak protection underwear to consumers,” she said. 

Thinx declined to comment to The Hill on Peaslee’s account or say whether it had tested for short-chain PFAS.

Peaslee is not the only scientist to have found PFAS in period underwear — and Thinx is not the only brand found to contain them. In May 2022, a team of researchers at the Massachusetts-based Silent Spring Institute published a study in Environmental Science & Technology looking at the presence of PFAS in underwear and several other consumer items.

Among those products was menstrual underwear, which the scientists said they “selected because of presumed use by children or adolescents.” The companies marketing the menstrual underwear all promoted their products as leak-proof, per the study. 

While analyzing six different period underwear products, however, the researchers directly identified the substances in one pair and found compounds that can react to become PFAS in that same pair and another. 

Research released in August from Peaslee’s lab also found indicators of PFAS in some period products, including wrappers for several pads and some tampons and outer layers of menstrual underwear.  

In a written statement accompanying the research, Peaslee noted that although “feminine products are essential,” putting PFAS in their layers or wrapping is not, since “plenty of them are made without relying on these compounds.”

Ruthann Rudel, director of research at the Silent Spring Institute, said she thinks period underwear is helpful but that it’s worth advancing technologies that would rid these products of toxic chemicals.

Worn close to the skin

Menstrual underwear is far from the only product to contain PFAS that is geared mostly toward women. 

The compounds are common ingredients in North American cosmetics — many of which may contain high levels of them. A 2021 study in Environmental Science & Technology Letters, co-authored by Peaslee, tested 231 makeup products and found that 63 percent of the foundations, 58 percent of the eye products, 55 percent of the lip products and 47 percent of the mascaras it looked at contained high levels of fluorine. 

Researchers often test for fluorine to screen for the presence of PFAS, as the substances contain at least one fully fluorinated carbon atom — or four fluorines attached to one carbon. (The atom fluorine is different from the additive in toothpaste and water known as “fluoride,” which actually stands for sodium fluoride and is recognized by the Food and Drug Administration as a safe anti-cavity agent in small doses.) 

After identifying the samples with the highest fluorine content, scientists sometimes then perform targeted analyses for PFAS compounds. In Peaslee’s 2021 cosmetics study, researchers performed targeted tests on 29 of the 231 samples and confirmed detectable levels of PFAS in all of them.

Separately, the Environmental Working Group has identified 300 cosmetic products from 50 different popular brands that contain PFAS in its Skin Deep database. The advocacy organization found that 200 of these products contain PTFE, which is also used in Teflon pans. 

PFAS are also allegedly found in a variety of workout wear from name brands using “a third-party EPA-certified laboratory,” according to a report published by consumer activist blog Mamavation in 2022 based on testing by “a third-party EPA-certified laboratory.”  Mamavation declined to name the lab on the record, but The Hill was able to verify its existence.

The nonprofit site Environmental Health News and Mamavation identified PFAS in leggings and yoga pants as well.

It’s not entirely clear what impact wearing products containing the substances could have on the body. Linda Birnbaum, former head of the National Institute of Environmental Health Sciences and the National Toxicology Program, said PFAS “can be irritating on the skin at high concentrations,” but that she’s not aware of much research indicating they are altering skin. 

Research on the impacts of absorption of PFAS through the skin is limited. However, one 2020 study on mice showed that a type of PFAS known as PFOA was harmful to the animals’ immune systems when it was exposed to their skin, similar to impacts of ingesting the substance — which is known to be hazardous. 

Loreen Hackett, a longtime anti-PFAS activist from Hoosick Falls, N.Y., has been scrutinizing every product that enters or comes into contact with her body for years.

“You got to look at leggings … and all this other shit. But we have to look more,” she said. 

“Do [men] care if it’s in their shaving cream? I don’t know,” she continued. “But women use 10 times more personal care products than men. So of course it’s going to affect us more.” 

“How many guys do you see really lotioning their hands up?” Hackett asked.