Finland has about 3.3 million saunas for 5.5 million people, so most Finns grow up with one. Here are the things people outside Finland most often get wrong, and what is actually true.
They are different rooms. A Finnish sauna runs at about 80–100 °C with low humidity, around 10–20%, and you add moisture yourself with löyly. A steam room sits around 50 °C at 100% humidity. In a sauna the air stays above the dew point, so you do not see fog.
Throwing water is the point. The steam, löyly, is what Finns go to the sauna for. Gym "no water" signs are about liability and maintenance; most heaters with stones are built for it. How to throw water safely
Heater makers suggest about 65–80 °C for a home sauna. Finland's own "World Sauna Championships" in Heinola, where contestants endured 110 °C with half a litre of water thrown every 30 seconds, ended after a finalist died in 2010. A good sauna is about soft, even heat, not endurance.
Sweat is mostly water and salts. The liver and kidneys handle detoxification, and researchers have found that pollutants stored in body fat are unlikely to leave through sweat. Finns go to the sauna to relax and get clean, not to detox.
A session can take off around half a kilo of sweat, but that is water, and it comes back as soon as you drink. Drink water after the sauna; that weight is meant to return.
For most healthy people the opposite looks true: long Finnish studies link regular sauna use with fewer fatal heart events, though they show an association rather than proof. People with unstable chest pain, a recent heart attack or severe aortic stenosis should avoid the sauna, and alcohol is the real danger. What the research shows
Nudity in the sauna is ordinary and not sexual. Families and friends bathe together, public saunas are often separate for men and women, and mixed public saunas usually mean swimsuits. A traditional saying goes that in the sauna one behaves as in church.
Cooling off is part of the ritual, but how you do it is up to you: a cool shower, a swim, a lake, or snow in winter. Plenty of Finns just sit on the porch.
There is no target. Most rounds last about 10–15 minutes, and heater manuals advise against more than 30 minutes at a time. Leave when you have had enough.
Infrared panels heat your body directly and run at around 45–60 °C, without stones or löyly. UNESCO's description of Finnish sauna culture lists infrared among the kinds of sauna, but the experience is different from a Finnish sauna with a stove.