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Archaeologists have found Neolithic structures in Scotland’s Orkney Islands from around 4000 BCE with spaces for fire and bathing, and similar structures in Greenland and Newfoundland. The word sauna may trace to a Proto-Germanic root shared with the English word stack, meaning a pile of stones. The earliest Finnish saunas were pits dug into earthen slopes and used as winter dwellings, heated by rocks that stored a fire’s energy. Modern saunas often reach 100 degrees Celsius, survivable only because the air is dry, while steam rooms near 100 percent humidity are kept around 50 degrees Celsius. In 15th-century Korea under Sejong the Great, Buddhist monks ran domed stone sweat rooms as medical facilities for the sick, elderly and poor.

The episode explains heat shock proteins, which refold damaged proteins, and Finnish studies linking sauna use four to seven times a week to lower blood pressure and lower risk of sudden cardiac death. Alcohol is named as the leading cause of sauna deaths. The 1936 Berlin Olympics, where Finland built a sauna in the athletes’ village, and World War II front-line saunas spread the practice into Germany, Austria and Switzerland, while Russian banyas, Native American sweat lodges and Baltic folklore developed their own traditions.

• When water hits the rocks, the burst of löyly condenses on the bather’s cooler skin, releasing latent heat, so much of the sudden wetness is not sweat.
• Bryan Johnson has published data showing an 85 percent drop in microplastics in his blood and semen during intense daily sauna use.
• The German Aufguss is a silent, scheduled ritual led by a sauna master who whirls scented steam with a towel.
• Estonia’s smoke sauna tradition is protected by UNESCO, and a Finnish company invented the electric sauna stove in 1938.
• Finnish folklore’s sauna tonttu, an elf under the benches, punishes bathers who argue, sleep or make noise.