An intrepid reporter for Sierra, the Sierra Club’s magazine, has uncovered something disturbing . She asked nuclear scientist Graham Peaslee of the University of Notre Dame to test unworn Thinx menstrual underwear, and he discovered polyfluoroalkyl substances (PFAS) on the inside layers of the crotch. These are chemicals known to be toxic to humans, even at very low levels, and have been linked to cancer and fertility problems. That’s right, they were found in the exact undergarments closest to the wearers’ vagina—a particularly absorbent part of a woman’s body.

Thinx vigorously contested the Sierra report, telling Fast Company in a statement that its products had gone through multiple rounds of testing to ensure they met or exceeded 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 copies of the company’s latest lab tests, from September 2019, which appeared to confirm the absence of detectable levels of PFAS chemicals.

Thinx’s internal testing seem to contradict the findings from Peaslee’s lab described in Sierra. We’ve reached out to the publication to see if they might be able to explain the discrepancy between the results.

Sierra journalist Jessian Choy has been interested in the chemicals used in menstrual products for some time. She has explored how toxic chemicals in single-use menstrual pads can be absorbed through the skin, for instance. She then turned her attention to period underwear designed to absorb leaks, which became popular over the last decade thanks to startups like Thinx, Lunapads, and Knix. These brands claim to create more comfortable and effective versions of the product. (The flood of new menstrual panties on the market was part of a wider wave of innovation around menstrual products, which I covered in-depth in 2016.)

Choy was curious to learn more about the materials that brands were incorporating into their menstrual underwear. So she mailed unused underwear from Thinx and Lunapads to Peaslee, a scientist who has previously uncovered harmful chemicals in everyday products. For instance, his lab discovered PFAS chemicals in fast-food wrappers in 2017.

Peaslee conducted a particle induced gamma ray emission spectroscopy test, which has been used to study materials at a microscopic level. He found that Lunapads were completely free of PFAS. On the other hand, Thinx’s main line of menstrual briefs had PFAS levels of 3,264 parts per million. Its BTWN brand briefs, marketed at teens, had 2,053 parts per million.

Thinx describes both of these products as part of the brand’s “organic cotton” line. And while there is no language in Thinx’s marketing that explicitly says its products are PFAS-free, the term “organic” would reasonably suggest that they do not contain harmful chemicals. Meanwhile, there’s evidence that exposure to even very low levels of PFAS is harmful to human health. Research has shown that PFAS can lead to decreased fertility and lower responses to vaccines, and in some cases, increased cancer risk.