The Short Answer
If you are choosing between the two heavyweights of period underwear, Knix is the better investment for both performance and transparency. However, both brands carry significant historical baggage regarding toxic chemicals.
Both companies settled multi-million dollar class-action lawsuits between 2023 and 2024 after independent testing found PFAS (forever chemicals) in their products. While strict 2025 state laws have forced both brands to eliminate intentional PFAS, we still recommend proceeding with caution and considering verified non-toxic competitors instead. Safest Period Underwear
Why This Matters
The period underwear industry exploded on the promise of safe, eco-friendly menstrual care. But in 2020, independent lab testing revealed extremely high levels of fluorineāan indicator of PFASāin the crotch of Thinx underwear. Does Thinx Have Pfas
PFAS are "forever chemicals" that do not break down in the human body or the environment. The skin around the vulva is highly vascular and incredibly absorbent, meaning any chemical coatings in your underwear have a direct, fast-tracked pathway into your bloodstream. Is There Pfas In Period Underwear
Following the Thinx scandal, testing eventually caught up with their biggest competitor. While Thinx's high levels (over 3,000 ppm) suggested intentional chemical waterproofing, Knix was sued for misleading "100% PFAS-free" marketing after trace amounts were discovered in their garments. Is Knix Clean
Now that California and New York have officially banned PFAS in textiles as of 2025, both brands have legally binding agreements to monitor their supply chains. Still, many crunchy consumers are rightfully choosing to spend their money with brands that didn't need a lawsuit to prioritize safety. Period Underwear No Pfas
What's Actually In Period Underwear
Both brands use a multi-layer system to pull moisture away from the body and trap it. Here is exactly what is touching your skin:
- Polyurethane Laminate (PUL) ā This is the ultra-thin plastic layer that makes the underwear leakproof. It is standard across the industry, but manufacturing cleanliness varies wildly between brands.
- Agion Antimicrobial Treatment ā Openly used by Thinx. This is a nanosilver treatment used to control odor, but it can potentially disrupt your natural vaginal microbiome and wash into local waterways. Is Silver In Period Underwear Safe
- Synthetic Fabrics ā Knix heavily relies on nylon and spandex to achieve its famous seamless feel. While highly comfortable, synthetics trap heat and moisture, which can increase your risk of yeast infections compared to organic cotton.
What to Look For
Green Flags:
- Third-Party Lab Results ā The safest brands openly publish their independent fluorine testing results, rather than hiding behind vague marketing terms.
- Organic Cotton Gussets ā The top layer directly touching your skin should be a breathable, natural fiber to support a healthy pH.
- Undetectable Fluorine ā Look for brands that guarantee "undetectable" levels of total fluorine, which is the only reliable way to screen for all 9,000+ types of PFAS.
Red Flags:
- "Stain-Resistant" Claims ā This phrase is historically an industry code word for chemical coatings like PFAS or Teflon.
- Nanosilver Treatments ā Often used to mask the smell of trapped blood, but heavy metals do not belong near your vulva.
- Opaque Supply Chains ā If a brand refuses to explicitly name what materials create their waterproof barrier, it is best to avoid them entirely.
The Best Options
If you are moving away from the drama of Thinx and Knix, there are better options on the market. These alternatives prioritize clean materials without sacrificing leak protection. Is Saalt Period Underwear Clean
| Brand | Product | Verdict | Why |
|---|---|---|---|
| Saalt | Saalt Wear | ā | Consistently tests undetectable for PFAS and uses clean, transparent materials. |
| Aisle | Period Underwear | ā | Features 100% organic cotton options with a verified clean supply chain. |
| Knix | Leakproof Underwear | ā ļø | Better performance than Thinx, but recently settled a massive marketing lawsuit. |
| Thinx | Period Underwear | š« | History of dangerously high PFAS levels and uses controversial silver treatments. |
The Bottom Line
1. Ditch the Thinx. They had the highest recorded levels of forever chemicals in the industry and still use microbiome-disrupting silver. Is Thinx Safe
2. Knix is acceptable but not perfect. They offer superior comfort and leak protection, but their past marketing claims lacked total transparency.
3. Upgrade to verified clean brands. Companies like Saalt and Aisle built their products without toxic chemical shortcuts from day one.
FAQ
Do Thinx and Knix still have PFAS?
Both brands are legally required to test for and eliminate PFAS under their recent lawsuit settlements and the 2025 California textile ban. While intentional PFAS use has stopped, trace contamination can still occur through shared factory machinery. Is There Pfas In Period Underwear
Which brand holds more liquid?
In long-term community testing, Knix consistently outperforms Thinx for heavy flows. Users report that Knix's core absorbs liquid significantly faster and holds up to two tampons' worth without feeling like a bulky diaper.
Why does my period underwear smell?
Thinx users frequently report a "rotting" smell developing after a few months of use. This happens because blood proteins get permanently trapped in the synthetic layers as the chemical antimicrobial treatments gradually wash out.