Some facts about Finnish Sauna:

The word sauna is Finnish word. It has been an essential part of Finnish way of life for at least a 2000 years.

To day we have in Finland 2,3 million households 1,3 building for housing and about over 2 million saunas! 

Originally sauna was a wooden house with a big stove that consisted of piles of natural stones. In Finnish nature granite bedrock is very visible and present all over the country, so it was not a big deal to collect a heap of stones. The stove had a fireplace in the bottom. The fireplace didn’t have any chimney, so all the smoke filled the room. AT the end of the room there was a small hatch, which was open during the burning of the wood, so the smoke went out through the hatch. When the sauna was warmed up, the hatch was closed. The whole interior was gradually covered with soot. 

The construction changed during 17 century. A chimney was added to the sauna. Before the saunas were sensitive to fire, but the situation changed  after the chimney.

The current sauna got its form about 100 years ago. The insulation of the sauna has increased so the heat doesn’t escape the sauna. Saunas are warmed around 80 - 100 C ( 176 - 212 F ). Steam has a very essential role in sauna. When people go to sauna, they start to throw water on the stove. It will evaporate immediately and the steam will fill the sauna. The steam feels hotter on skin, than the dry air even as its temperature can be even a bit lower the the air’s. People will stay in sauna in average 15 - 30 min, but there are a lot of people that will stay in sauna over an hour. It is common also to have some short breaks and then return to the sauna again. A lot of saunas have been built by the waters. In that case people take a swim during their sauna session. 

In Finland all  people go to the sauna naked and at least families ( parents and children ) go  to the sauna together. We have also had public saunas throughout history. They were mixt sex saunas until early 19 hundreds and since the are mainly sepated male nad female saunas. If there is only one sauna, then women and men gos there in turns. 

We have also mixed saunas all naked but there are quite a few of them. 

In central Europe in German speaking areas the is also a strong sauna culture, that differs a bit from Finnish, but is basically the same. They are mainly public and there are mixed sexses naked.

This is the reason why nudity is not an issue in Scandinavia or central Europe, because people are used to seeing people of all ages from their childhood. From my perspective, this is a very sound and healthy attitude to nudity and  I have difficulties to understand the Anglo-American attitudes towards nudity and for instance nipples. I am astonished how people can think that violence is less harmful for children than seeing the natural human body! 

The health effects of saunas are very good. Staying in a sauna will lower one's blood pressure and the effect will last at least sometime afterwards. It has no negative effects.

There are a couple of links to information on sauna.

https://sauna.fi/en/

https://en.wikipedia.org/wiki/Saun