"An immigrant has fully naturalised in Finland when he thinks +25 C is hot outside but +65 C is cold in a sauna."
There are about 1.6 million saunas in Finland, most people going to sauna 1-3 times a week. (They are also found in Russia (banya), Estonia and Sweden (bastu) in lesser numbers, where the concept is slightly different in each country.) It's not a luxury or anything special - Finns grow up with it. In practical use, it's used in the same role as a hot shower. Sauna is not "popular", but ubiquitous - almost every house has one. Finns dislike arbitrary social customs and unwritten rules, so there aren't any for saunas, either. Despite this, I'd like to point out few things:
- Sauna has nothing to do with sex. Nudity (partial or complete) is natural there due to the heat. Swimwear does feel uncomfortable. If you have a problem with nudity, you should think if you're grown up and comfortable with yourself or still in some Freudian stage of development.
- Sauna is not a form of exercise either, and physical exertion is not recommended. Quite the opposite: many gyms and other fitness centers have a sauna for relaxing after the exercise.
- It is not a "sauna" if it's a just a warm drying cabinet. We're talking about 60-70 C at minimum, the ability to get löyly (vaporizing water on the kiuas i.e. stove), wooden paneling and a drain inlet.
- If your host asks you to go to sauna, go for it. It's a part of the expected hospitality to offer the guest a sauna bath. It feels like you'd be a child if you fear it. We'll understand, but hey, it's you who's losing the experience!
- Behave like in a holy place - no excessive talking, scuffling, etc. Scuffling is dangerous, because you can fall on the hot kiuas. Saunas are not "officially" holy, but they are places set apart for repose and quiescense.
- Even in a sauna, if something feels bad, it is. Don't burn your ears.
Most of the descriptions imply that the sweating would be the point, but this is not true. In a sauna, you forget about the sweating. So, why would sauna feel good? Normally, 5-10 % of blood flow goes through the skin. In sauna, this percentage increases dramatically to 50-70 %. The skin is initially heated up to 40 C, which makes the cutaneous capillary vessels dilate. A rapid flush of warm blood goes through the whole body. The blood flow gets 2-3 times stronger than normal, with no exercise. So the reason why sauna feels good is that the muscles can enjoy the blood flow of heavy exercise, while relaxed. This speeds up metabolism, especially lactic acid metabolism in the muscles.
Sauna feels good, as long as you don't overdo or underdo it. It's not the temperature of the air that tells how hot it feels, because the skin doesn't sense the temperature of surroundings, but its own temperature. Hot air causes a flow of thermal energy into the skin, but doesn't scald it, because the still air can't raise the temperature of the skin to the blistering 45 C. It is NOT like in a desert, where the radiation of the Sun fries you and makes you feel sick.
The optimal temperature of air is 70-80 C. It feels like hot shower, but it is more refreshing and is felt throughout the whole body. 60 C causes a heavy, lazy feeling, like in a hot bath, unless you throw a lot of water to the kiuas. This makes the sauna too humid. Over 80 C means that the air has to be quite dry, so that löyly doesn't burn. Be also careful not to scald your ears, feet or hands, if you like sauna hot. Large areas of skin have a lot of water to heat, so they are quite insensitive to the heat. Surprisingly, the face is also insensitive. The heat isn't felt on the nose or on the forehead, unlike in a desert.
One thing is important to understand about sauna: sauna is a wet and humid place. The air is so humid that most of the "sweat" is really condensated humidity. Condensation, particularly in a hot sauna, is a process that gives off heat much like evaporation consumes it. The Finnish term löyly refers to this heat you feel on your skin, as a wet surface - as your wet skin - has a tendency to adsorb even more water from the air.
Because of the humidity, textiles will get wet or moist. You simply can't wear any clothes, except swimming trunks. Electronic equipment such as mobile phones will get wet and corrode or short circuit to death. Eye glasses feel uncomfortable and you can't see through them. Swimming suits are not recommended, because sweat accumulates on them, so they're not hygienic. In many swimming halls only patients with cancer are allowed to wear a swimming suit in sauna.
Sauna and psychoactive drugs or large amounts of alcohol don't mix. A beer or two is safe and refreshing. But if you're drunk, then you don't feel it when you've been too long there. Thirst may disappear, leading to dehydration. Salmiakki Koskenkorva will make that worse. And you'd stumble on the kiuas. There are guard rails in home saunas, but summer cottage saunas usually have little or no protection.
The word sauna is pronounced /sauna/ where /a/ is a back vowel (and the corresponding English spelling would be "sownuh"). The word is a genuine Finnish word, as it is Proto-Finnish (giving modern Finnish, Estonian, Sami). A similar word is found in Sami, meaning "a pit dug in snow to warm oneself in". This reflects the history of sauna. Proto-Finnish people would dig a shallow hole in the ground or in the snow, pile something atop it, and go there to warm oneself in cold weather. They would discover that if you light a fire there, the smoke would be intolerable. But, if you piled up a kiuas from stones, lit the fire under it, and let the stones absorb the heat, you could get a nice, comfortable room when the fire was put out. They would also discover that throwing snow on the kiuas would give off löyly - humidity, and heat of condensation. This is essentially, technologically and concept-wise the Finnish sauna, and incredibly, this kind of sauna (savusauna, the smoke sauna) is still in modern use by some enthusiasts, although in an architecturally improved form.
Newer technologies are the use of wooden building with a wooden floor, which gives a cleaner environment and a higher temperature. Modern Finnish saunas achieve a higher temperature by using a stove (modern kiuas) to warm the stones continuously. This isn't ancient, but an early 20th invention. There are wood-burning as well as electric kiuases. The cleanliness is helped by modern developments like running warm water and a separate showering room. The sauna is traditionally the most important part of the house; in the olden days, you would first build the sauna, and live there until the house itself was finished. Most houses today don't have a different building for a sauna; summer cottages, however, do.
Foreigners' attempts to build a sauna are usually futile and look half amusing, half saddening. Rarely they do enough research on the subject. The major reason for failure is cultural. The fact is, that in Finland, sauna is an independent cultural artifact. The sauna is not a part of some other cultural manifestation; it's not exercise, it's not sex, it's not health and fitness, it's not ascetism, it's not an ancient religious tradition. Finnish sauna is Finnish sauna as itself; trying to forcibly insert it into some foreign subculture doesn't look pretty.
The technology itself is a bit nontrivial, too. I heard about Swedish "bastu", that is, their "sauna". There was a fitted carpet in the floor. Next a phone, maybe? I have been in German swimming hall sauna. The temperature was set at 90 C and no löyly was allowed. But who'd want to be in such a dry and hot place? Löyly was made by squeezing water from a sponge to a kiuas. A Danish hotel sauna was otherwise OK, but the designers had placed the kiuas right next to the door. There were no guard rails. This is dangerous! The wet clinker floor might be slippery and make you trip over right on the kiuas. The resistors of the kiuas were glowing white-hot, because the element was too small in proportion to the volume of the room. The floor was plane. (The floor has to be slightly tilted, so that the water drains away to the right direction.)
Finnish Sauna Society has a lot of info in English in their web site. http://www.sauna.fi/
MD Lasse Viinikka. Sauna and Health. Available in the aforementioned URL.