Is Polyester Safe in a Sauna?

What polyester actually is

Polyester is PET, polyethylene terephthalate. Same polymer family as a clear drinks bottle.

It is made by a polymerization reaction that needs a catalyst, and the standard catalyst across most of the industry is antimony trioxide. Residual antimony stays in the finished fiber. This is not a fringe claim or an exposé, it is ordinary industrial chemistry that the people making polyester would describe the same way.
What the research actually covers

This is the part that most writing on this subject gets wrong, so it is worth being slow about.

There is a large, well-replicated body of research on microplastics from textiles. Almost all of it is about laundering. Washing machines, agitation, detergent, water. That is where the fiber shedding numbers everybody quotes come from.

Taking a shedding figure derived from a washing machine and applying it to a hot dry room is not a valid move. It is, however, what a great many wellness articles do, usually without mentioning that the underlying study involved a washing machine at all.

There is research that gets closer. A paper in Science of the Total Environment looks at thermally driven polymer degradation in synthetic fabrics during high-temperature disinfection, which is nearer to the question than laundering is, though still not a sauna.

There is also published work from groups at Plymouth and Geneva, in a toxicology journal around 2020, on antimony release from polyester textiles into artificial sweat solutions. Of everything we found, that is the closest analogue to the actual scenario: a synthetic fabric, soaked in sweat, against skin.

The honest synthesis

So, heat, plus heavy sweating, plus a synthetic fabric held against skin, repeated weekly for years. That specific combination has not been studied properly by anyone.

We are not claiming harm. We are pointing at an absence of evidence running in both directions, and noting that when a material is new to a use, the burden of demonstrating it is fine sits with the person selling it rather than with the person wearing it.

That is a defensible position. "Polyester hats are poisoning you" is not, and you should be suspicious of anyone who tells you otherwise, including anyone who sells the natural alternative.

What would actually settle this

We should be specific about what the missing study looks like, because "more research is needed" is what people say when they want to sound careful without committing to anything.

You would want a polyester textile held at sauna temperatures, around 190°F, in high humidity, in contact with a sweat-equivalent solution, for an hour at a time, repeated over a realistic product lifetime. Then you measure what comes off it: particle counts by size, and dissolved metals, antimony in particular.

That is not an exotic or expensive experiment. The equipment exists in any decent materials lab. It has not been done because nobody has had a commercial reason to fund it, and the people with the strongest commercial reason to fund it are the people selling synthetic sauna textiles.

Until someone runs it, anybody telling you the answer with confidence is guessing, in one direction or the other.

The other synthetics in the room

Fairness demands pointing out that a hat is not the only synthetic in most saunas, and probably not the most significant one.

Polyester and poly-blend towels are extremely common. Many commercial benches are finished with a sealant or a coating. Gym saunas often have vinyl or composite trim. Some prefab cabins use adhesives in the panel construction that are doing something at 190°F, though nobody can tell you precisely what.

We are not going to pretend our products solve any of that. If the question genuinely concerns you, the towel against your skin for twenty minutes is at least as worth thinking about as the hat on your head, and it is a cheaper thing to change.

"Natural" is not an argument

One more thing, because this page would be dishonest without it.

Wool is not safe because it is natural. Plenty of natural things are terrible for you, and the word "natural" on a label is marketing rather than information. Wool happens to have a long record in this specific use, and that record is the argument. The naturalness is incidental.

Wool also has real downsides that a synthetic doesn't. Some people react to it against bare skin. Untreated wool attracts moths, which is why most wool on the market is treated with something, and why ours being untreated means you have to store it sensibly. It takes longer to dry. It costs several times more.

If you are allergic to wool, a synthetic sauna hat is a completely reasonable thing to own, and we would rather you wear one of those than cook your scalp on principle.

Why this shaped what we made

Wool has been worn in hot wet rooms for something like a century and a half of continuous practice in the banya tradition, and much longer than that as felt generally. The failure modes are known and boring. It wears out. 

A synthetic substitute has been in the same use for a couple of decades, in a setting nobody has instrumented, with a residual metal catalyst in the fiber and an open question about what heat and sweat do at the surface.

Given a choice between a material with a long boring record and a material with a short unstudied one, in an application where the thing is pressed against your head while you sweat, we went with the boring one. That is the whole reasoning. It is a variance argument, not a toxicity argument.

The one synthetic component in our hat is the sewing thread, and we are working on changing that, which is only possible at order numbers larger than we can commit to right now. Mentioning it in a piece about synthetics is the only way the rest of this page is worth anything.