What Should You Wear in a Sauna? Sauna-Safe Swimwear Explained

What's really in your swimsuit, what happens to it in the heat, and what to wear instead

Quick answer: What should you wear in a sauna? Traditionally nothing at all, sitting on a cotton or linen towel. Where swimwear is required, as it is in many UK public saunas, choose loose, breathable swimwear made mostly from certified organic natural fibres, with no metal parts and no rubbery prints.

I love a sauna. The heat that sinks right into your bones. The quiet. That lovely heavy feeling afterwards when your whole body has finally let go.

And I'm clearly not alone. The British Sauna Society counted around 45 public saunas in the UK in early 2023. By early 2025 there were 147. Today the society maps more than 600 publicly accessible saunas across the country [1].

UK public saunas counted by the British Sauna Society [1]

But a few years ago I started asking a question I couldn't shake. Why is almost all swimwear made from plastic?

Once you ask that, a second question follows pretty quickly. What happens to that plastic when you sit in a room heated to somewhere between 70 and 100°C, sweating, for 15 minutes?

It's not about fear, it's about awareness.

We celebrated the launch of Earthcove in a London sauna, surrounded by friends talking about exactly this. So let's look at what we know, and what we don't.

What is most swimwear made from?

Pick up almost any swimsuit and check the label. You'll usually find polyester or nylon (also called polyamide) blended with Lycra (also called elastane or spandex).

A typical swimwear fabric is around 80% polyester or nylon and 20% Lycra. All three are synthetic fibres made from fossil fuels. In other words, plastic.

They're used because they stretch, hold their shape and dry fast. But synthetic fabrics are often treated with chemical finishes to get there. That's where things get more concerning.

What chemicals can be found in plastic swimwear?

Not every swimsuit contains every chemical below. But these three come up again and again in independent testing of synthetic clothing.

What is PFAS, and why is it in swimwear?

PFAS (per and polyfluoroalkyl substances) are a family of more than 10,000 synthetic chemicals. They're known as "forever chemicals" because they barely break down in the environment or in our bodies.

In clothing, PFAS are used to make fabric repel water, stains and grease. In swimwear, that means water resistance and quick drying.

And they do turn up. The Environmental Working Group found PFAS in kids' swimwear from Carter's and Disney [2]. A global study by IPEN and partner organisations tested 72 clothing items, including swimsuits, and found PFAS in 64% of them [3][4].

PFAS were found in 64% of clothing items tested [3]

Some PFAS build up in the body over time. The US EPA links exposure to certain PFAS with effects on the immune system, the liver and birth weight [5]. Children are more vulnerable because their bodies are smaller and still developing.

What are phthalates, and why are they in swimwear?

Phthalates are chemicals used to make hard plastics soft and flexible, mostly PVC.

They aren't usually what makes polyester or nylon stretchy. In clothing, they mainly turn up in plastisol prints (those thick, rubbery logos and patterns), PVC coatings and some printing inks. Greenpeace testing has found phthalates in the printed areas of everyday clothing at up to 11% by weight [6].

Phthalates are hormone disruptors. That's why several are restricted in children's toys and childcare items in the EU and US.

What is BPA, and why is it in swimwear?

BPA (bisphenol A) is a chemical used to make plastics clear and hard. Most of us know it from the push to get it out of baby bottles.

Fewer people know it turns up in clothing. The Center for Environmental Health (CEH) tested sports bras, leggings and athletic shirts made from polyester and Lycra, the same fibre family used in most swimwear. Some could expose wearers to up to

 California's safe limit for BPA [7]. CEH has not found BPA in socks made mainly from cotton [8].

BPA mimics oestrogen. The plastics industry maintains it's safe at the low levels most people are exposed to. Many scientists disagree, and research is ongoing.

What happens to plastic swimwear in a sauna?

Here's where I want to be careful, because this is where a lot of sauna marketing gets carried away.

Nobody has yet run a study measuring what happens to swimwear chemicals on people in a sauna. What we do have are lab studies using donated human skin and 3D skin models. They're useful. But they aren't people.

• Using human skin samples in the lab, researchers found that raising skin temperature from 25°C to 39°C increased absorption of every substance tested [9].
• A University of Birmingham study found 15 of 17 PFAS tested were absorbed through a 3D model of human skin [10].
• A 2024 study in Environment International found flame retardants carried by microplastics were absorbed more by a sweaty skin model than a dry one [11].

None of these tested swimwear. None of them tell us what happens in a real sauna. But they point the same way. In the lab, warm, damp skin let more through.

The lab studies used heat well below a real sauna

A sauna is hot and humid. You're sitting still, often for 15 minutes or more, with your swimsuit pressed against your skin. For me, that's reason enough to choose natural fibres where I can.

What about the air in the sauna?

Synthetic fabrics release volatile compounds into the air, and heat speeds this up. One lab study of synthetic mattress textiles found emissions rose significantly between 25°C and 50°C [12].

A sauna is hotter than that, and it's a small, shared room. Research here is still early. But it's one more reason to think about what we wear into the heat.

Why do people love saunas?

Let's balance this out, because saunas are wonderful.

The word sauna comes from Finland, where there are around 3 million saunas for 5.5 million people. Finnish sauna culture is recognised by UNESCO as part of the world's intangible cultural heritage [13]. For Finns, it's about cleansing the body and the mind.

People sauna to relax, to warm up in winter, to unwind after exercise and to spend quiet time with friends. Here in the UK, community saunas have become a whole new kind of social space. A place to talk, or to say nothing at all.

What is the best thing to wear in a sauna?

Saunas were made for bare skin. In the Finnish tradition you go in naked and sit on a cotton or linen towel. It's still the most natural way to sauna.

But many public saunas in the UK, the US, France and elsewhere require swimwear. So what's the next best thing?

What makes swimwear sauna-safe?

• Natural fibres first. Look for swimwear made mostly from organic cotton rather than polyester or nylon.
• Independent certification. A label that says "eco" or "natural" isn't enough. GOTS (the Global Organic Textile Standard) covers the whole supply chain, from organic fibre to finished garment, and sets strict rules on the chemicals used along the way.
• No rubbery prints or plastic coatings. These are where phthalates tend to hide.
• No metal. Clasps, underwires and rings heat up fast and can burn your skin.
• A comfortable, breathable fit. Tight synthetics hold heat and sweat against your skin.

Your sauna swimwear checklist. Save this one for your next sauna trip

What are the best tips for your next sauna?

• Sit on a cotton towel. It's better for hygiene and puts a natural layer between you and the bench.
  
  
• Shower first. Rinse pool chlorine off your skin and your swimsuit before you go in.
  
  
• Go bare where you can. If you have a private sauna, do it the Finnish way.
  
  
• Drink plenty of water. Before and after, every time.
  
  
• Start with one swap. Replace the swimsuit you wear most. You don't need to replace everything at once.

Sauna-safe swimwear from Earthcove

This is exactly why we created Earthcove.

Earthcove is the world's first organic cotton swimwear range. Every piece is GOTS certified and UPF 50+. It's made from 98% organic cotton, with 2% Lycra for stretch and fit. We're open about that 2% because we'd always rather tell you the full story than make a claim we can't stand behind. So no, it isn't plastic free. It's a lot less plastic against your skin.

Typical swimwear fabric compared with Earthcove

Earthcove swimwear is GOTS certified, which bans PFAS, phthalates, formaldehyde and harmful azo dyes. Our prints use water based, PVC free inks.

Our Sauna Safe collection brings together the grown up styles I reach for in the heat. Two favourites:

• The Womens Support Swimming Costume, designed with maximum support for fuller busts.
• The Womens Scoop Neck Swimming Costume, a soft, classic scoop neck that's also mastectomy friendly, with discreet pockets for a prosthesis.

Every small change adds up

You don't need to overhaul your entire life overnight. Maybe it's one swimsuit. Maybe it's a cotton towel for the sauna bench. Maybe it's going bare next time you're somewhere that allows it.

Every small change adds up. And remember, it's not about fear, it's about awareness.

Enjoy the heat. And the quiet.

Emma x

Sauna swimwear: your questions answered

What should I wear in a sauna?

Traditionally, nothing at all, sitting on a cotton towel. Where swimwear is required, choose loose, breathable swimwear made mostly from certified organic natural fibres, with no metal parts or rubbery prints.

Do I have to wear swimwear in a UK sauna?

Usually, yes. Most UK gyms, spas and community saunas ask you to wear swimwear. A few offer textile free or naked sessions, and in a private sauna it's your choice. Always check the venue's rules before you go.

Is it safe to wear a polyester swimsuit in a sauna?

Polyester won't melt at normal sauna temperatures, but it isn't my first choice. Synthetic swimwear can carry chemical finishes such as PFAS. Lab skin models suggest warm, sweaty skin absorbs more of some chemicals, although we don't yet have studies in people.

Can you wear a cotton swimsuit in a sauna?

Yes. Cotton is breathable, copes well with heat and doesn't need the synthetic finishes common in plastic swimwear. Look for GOTS certified organic cotton for independent assurance on the chemicals used.

What is the best fabric for sauna swimwear?

Natural fibres from plants, ideally certified organic cotton. GOTS certification covers the fibre, the processing and the finished garment, including strict rules on harmful chemicals.

Can I wear my swimsuit in the sauna after swimming in a chlorinated pool?

You can, but rinse first. Shower and rinse your swimsuit in fresh water before you go in. Many saunas ask you to shower anyway. Chlorine left in the fabric wears down any swimsuit faster over time, cotton included.

Will a cotton swimsuit shrink or lose its shape in a sauna?

Sitting in a sauna isn't the same as a hot wash or a hot tumble dry, which is where cotton shrinkage usually happens. Earthcove swimwear has 2% Lycra so it holds its shape when wet. To keep it at its best, rinse it in fresh water after every swim or sauna, machine wash cold, and line dry or tumble dry on low.

How hot is a sauna, and how long should I stay in?

A traditional Finnish style sauna usually runs between 70 and 100°C. Infrared saunas are cooler, roughly 45 to 65°C. Most guidance suggests beginners start with five to 10 minutes and build up to 15 or 20, with cool down breaks between rounds. Listen to your body, drink water and check with your GP first if you're pregnant or have a health condition [14].

Sources

1. 1. British Sauna Society figures, reported in European Spa Magazine (2025) UK wild sauna boom is the subject of a new book and survey, europeanspamagazine.com, and Health Club Management (2026) Public Sauna-Bathing Charter launches ahead of International Sauna Congress as sector expands, healthclubmanagement.co.uk

   2. Environmental Working Group (2022) New baby textile product tests show concerning levels of toxic forever chemicals. ewg.org

   3. IPEN and Arnika. PFAS forever chemicals found in outerwear and clothing sold globally. ipen.org

   4. CHEM Trust. New study finds banned forever chemicals being used in children's clothing worldwide. chemtrust.org/pfas-childrens-clothing

   5. US Environmental Protection Agency. Our current understanding of the human health and environmental risks of PFAS. epa.gov/pfas

   6. Greenpeace Research Laboratories (2014) A Little Story About the Monsters in Your Closet, Technical Report. greenpeace.org

   7. Center for Environmental Health (2023) New testing shows high levels of BPA in sports bras and athletic shirts. ceh.org

   8. Center for Environmental Health. Frequently asked questions: BPA in socks. ceh.org

   9. Kilo, S. et al. (2020) Impact of physiologically relevant temperatures on dermal absorption of active substances: an ex vivo study in human skin. Toxicology in Vitro 68. doi:10.1016/j.tiv.2020.104954

   10. Ragnarsdóttir, O., Abdallah, M.A. and Harrad, S. (2024) Dermal bioavailability of perfluoroalkyl substances using in vitro 3D human skin equivalent models. Environment International. doi:10.1016/j.envint.2024.108772. University of Birmingham summary at birmingham.ac.uk

   11. Abafe, O.A. et al. (2024) Assessment of human dermal absorption of flame retardant additives in polyethylene and polypropylene microplastics using 3D human skin equivalent models. Environment International 186: 108635. doi:10.1016/j.envint.2024.108635

   12. Kim, K.H. et al. (2015) Emissions of DMF, formamide and hazardous VOCs from mattress textiles at elevated temperature. Ecotoxicology and Environmental Safety 114: 350 to 356. doi:10.1016/j.ecoenv.2014.07.008

   13. UNESCO (2020) Sauna culture in Finland. ich.unesco.org

   14. Healthline (2024) How long should you stay in a sauna? healthline.com (secondary source)