PFAS in period underwear: the lawsuits, the testing, and what it actually means

What PFAS are, and why they turned up here

PFAS (per- and polyfluoroalkyl substances) are a large family of synthetic chemicals used to make fabrics resist water and stains. They are called "forever chemicals" because they barely break down, and the broader research literature links some of them to endocrine disruption and other health effects. In period underwear the concern is specific: the fabric sits against absorptive tissue for hours, so a shopper reasonably wants to know whether PFAS were used to build the leakproof layer.

The lawsuits: what was settled, and what was not

Thinx settled a class action for up to $5 million, reported in January 2023 (a $4 million fund plus a $1 million replenishment). The case was about marketing: plaintiffs alleged Thinx sold the underwear as safe, non-toxic, and (for some styles) organic while it allegedly contained PFAS. Thinx did not admit wrongdoing, and the settlement is a resolution of the marketing claims, not a court ruling that the underwear is dangerous.

Knix settled a similar suit for $1.4 million, and agreed to stop describing its products as "PFAS-free," "toxic-chemical-free," or "fluorine-free," and to increase PFAS testing for two years. Knix also did not admit wrongdoing. What both settlements mean: the companies resolved how the products were marketed, and the underlying question of what was in them was argued rather than decided in court.

What the independent testing shows

Separate from the lawsuits, independent labs have tested these products. The controversy started in 2020 when a journalist had Thinx tested at Graham Peaslee's lab at the University of Notre Dame and found fluorine levels high enough to indicate PFAS were intentionally added. A 2025 University of Notre Dame study, published in the journal Environmental Science & Technology Letters, tested 59 reusable period and incontinence products and found signs of intentional fluorination (a marker of deliberately added PFAS) in about a third - 33% - of the period underwear it tested. The researchers concluded PFAS are not necessary to make these products work.

Two caveats on the numbers: different studies report different rates (small commissioned screenings have reported higher figures than the larger peer-reviewed study), because sample sizes and methods differ, and "PFAS detected" is not the same as "intentionally added." Treat the testing as a real, repeated signal that some period underwear contains PFAS - not as a verdict on any one pair you own.

What the two brands say today

The story did not stop in 2023, so here is each brand's own current statement, read on its site on 2026-09-20. Thinx's FAQ: "Per- and polyfluoroalkyl substances, also known as PFAS, are absolutely not part of our product design and we do not treat our finished products with PFAS. We also take measures to help ensure these substances are not added to our products through material, design, production, facility, and supplier controls." Its how-they-work page says "Our underwear is safely made without PFAS." We found no published test results on thinx.com.

Knix publishes a product testing and safety page: it says it began routine testing of its leakproof fabrics and products for certain PFAS in January 2020 and for organic fluorine shortly after, shares sample lab results, states that "neither PFAS nor organic fluorine was detected at the testing limits," and dates its most recent results to August 2026. That is the kind of published testing the shopping advice below asks for. Both statements are the brands' own words; we have not tested either product.

How to shop with this in mind

The ground is shifting in shoppers' favor. Several US states have passed PFAS bans for period products (New York's takes effect in December 2026; California's law bans intentionally added PFAS), so PFAS status is becoming a sellability requirement, not just a consumer concern. Until then: look for brands that publish third-party testing or an OEKO-TEX screening certification, and treat a plain "PFAS-free" claim with mild skepticism unless it is backed by testing - that exact phrase is what Knix agreed to make only with testing behind it.

Sources

Every factual and legal claim above traces to a primary or major-outlet source:

• Thinx settlement (up to $5M, Jan 2023, no admission of wrongdoing): NPR, NBC News, and Top Class Actions reporting.
• Knix settlement ($1.4M, agreed to drop "PFAS-free" claims and add testing, no admission): Top Class Actions and the official Knix marketing settlement notice.
• 2025 testing (33% of period underwear intentionally fluorinated, 59 products): University of Notre Dame, published in Environmental Science & Technology Letters, 2025.
• 2020 origin testing: Sierra magazine and Graham Peaslee, University of Notre Dame.
• State PFAS bans (New York effective Dec 2026; California): NRDC and Bloomberg Law reporting.
• What the brands say today: thinx.com/pages/faq and thinx.com/pages/how-they-work; knix.com/pages/product-testing-safety (all read 2026-09-20).