Underwear from period brand Thinx may contain toxic chemicals in the crotch, it has been reported.

The news comes after Jessian Choy, a reporter for Sierra, the Sierra Club’s magazine, asked nuclear scientist Dr Graham Peaslee of the University of Notre Dame to test unworn Thinx underwear for chemicals. Peaslee discovered that the inside layers of the crotch contained high levels of polyfluoroalkyl substances (PFAS), chemicals that are known to be toxic to humans and which have been linked to cancer, decreased fertility, and more.

Choy reports that according to Peaslee’s particle induced gamma ray emission (PIGE) spectroscopy test, the crotch in her underwear had 3,264 parts per million (ppm), while the one for teens had 2,053 ppm. These numbers, she writes, are “high enough to suggest they were intentionally manufactured with PFAS.” Tens to hundreds of ppm of copper and zinc were also found.

While Thinx did not respond to Choy’s repeated requests for comment, the brand did “vigorously” contest the report in a statement to Fast Company, telling the publication that its products had gone through multiple rounds of testing to ensure they met safety standards.

“Based on these third-party tests, PFAS chemicals were not detected in Thinx products,” CEO Maria Molland said. “Our customers’ health and safety is our number one priority, and we will always work to deliver the safest products available.” Molland also provided Fast Company with copies of the company’s latest lab tests from September 2019, which appeared to confirm the absence of detectable levels of PFAS chemicals.