Thermos · Insulated Drink Tumblers
Thermos Guardian Stainless Steel Tumbler
No coating described, and no PFAS statement from the brand.
The product information we reviewed describes stainless steel interior and exterior, with no water-repellent coating named. Water resistance on products like this usually comes from the material and its seals rather than a surface treatment. The brand has published no PFAS statement for it.
The better-graded swap
Ozark Trail
Ozark Trail 18 oz Stainless Steel Insulated Tumbler with Handle and Flip-Straw Lid
80/100 safety
Same rubric that flunked this product. The swap earned Excellent.
Still want this one anyway? Check the price on Amazon. We’d swap it.
Material
Stainless steel interior and exterior
Brand disclosure
Brand is silent on PFAS
Lab data
No public lab test
What Thermos Guardian Stainless Steel Tumbler is made of, ingredients and materials
The Thermos Guardian uses stainless steel for both interior and exterior surfaces, which is chemically inert and does not leach metals into beverages under normal use. Stainless steel is a chromium-iron alloy that resists corrosion and does not require polymer coatings, inks, or adhesives in contact with liquids. This material class avoids the leaching risks associated with ceramic glazes, painted coatings, or epoxy liners found in some competitor tumblers.
What’s concerning
Category baseline
The starting point for this type of product, before anything specific to this product is counted.
No PFAS disclosure
No public statement from the brand about PFAS in this product.
Stainless steel corrosion and pitting
While rare, aggressive acids or prolonged salt water contact can trigger localized corrosion in low-grade stainless steel; the Guardian's grade (likely 18/8 or 304) is resistant but not immune. No specific data on this model's alloy composition is public.
Seal and gasket degradation
Thermos tumblers rely on rubber or silicone seals for insulation and leak-proofing; these materials can harden, crack, or develop odor over years of use. Replacement seals are typically available but not always specified in product documentation.
What’s reassuring
Base material not linked to PFAS
The main material recorded for this product is not one associated with PFAS.
Metal taste or discoloration from interior wear
Stainless steel does not impart taste or color to beverages under normal conditions. Surface discoloration (water stains, mineral deposits) is cosmetic and removable; no health concern is documented.
Lead
The product description indicates stainless steel exterior with no coating or paint layer; no glaze, enamel, or ceramic coating is present. No contamination has been reported for this brand's tumblers.
Source, ATSDR ↗Microplastic shedding or plastic liner leaching
This model has no plastic liner in contact with liquids, eliminating BPA, BPS, and phthalate leaching risks common in plastic-lined alternatives. The seal is isolated from the beverage by the stainless steel barrier.
The full breakdown
The brand's record and disclosure
Thermos has not published a public statement on PFAS use or avoidance in its products, which is typical for legacy drinkware manufacturers and does not indicate a problem but limits independent verification. The brand has no documented recalls related to chemical leaching or coating failures in insulated tumblers. No certifications (such as non-toxic coatings or heavy-metal testing) are advertised for this model, consistent with industry-wide underreporting in this category.
How it compares in its category
Stainless steel tumblers have lower concern among insulated drinkware because they eliminate the need for interior liners or waterproof coatings that may contain PFAS or other additives. Many competing models use plastic liners, ceramic coatings, or powder-coat finishes, all of which carry disclosure or degradation concerns. The Guardian's all-metal construction is a material advantage over hybrid designs.
If you buy it
Wash by hand or in a dishwasher; stainless steel tolerates both without degradation. Do not expose to extreme temperatures beyond normal beverage ranges (boiling water presents no hazard of this kind on record; prolonged exposure to heat above 60C/140F is not required). Inspect the interior and seals annually for any scratches, pitting, or seal wear; replace seals if they harden or crack. Avoid abrasive scrubbing that could damage the passive oxide layer, though minor surface marks do not affect function.
What we don’t know
- Basis
- Model estimate, 72% confidence, not a lab result
- Lab test
- None on record for this exact product
- Brand statement
- Thermos has made none
Formulations change between production runs, so check the current label. How we grade.
Better-graded alternatives in Insulated Drink Tumblers
Klean Kanteen
20oz TKWide with Café Cap, Tofu
- Stainless steel bottle, stainless steel cap
- The brand calls it PFAS-free, which we have not verified
- Powder-coat cap degradation over time
3 hazards checked on this product. See all of them.
Klean Kanteen
16oz Rise Tumbler
- Stainless steel body, powder-coat finish
- The brand calls it PFAS-free, which we have not verified
- Powder-coat finish degradation
3 hazards checked on this product. See all of them.
Klean Kanteen
Insulated TKWide Cap
- Stainless steel bottle, plastic cap and silicone gasket
- The brand calls it PFAS-free, which we have not verified
- Silicone gasket degradation and leaching
3 hazards checked on this product. See all of them.
More insulated drink tumblers grades
How we score, every product runs through the same public 0-100 rubric based on materials, coatings, brand disclosure, lab findings, and recalls. See the full methodology → Spotted a mistake? Report an error.