A sauna (, ), or sudatory, is a small room or building designed as a place to experience dry or wet heat sessions, or an establishment with one or more of these facilities. The steam and high heat make the bathers perspire. A thermometer in a sauna is typically used to measure temperature; a hygrometer can be used to measure levels of humidity or steam. Infrared therapy is often referred to as a type of sauna, but according to the Finnish sauna organisations, infrared is not a sauna. History The oldest known saunas in Finland were made from pits dug in a slope in the ground and primarily used as dwellings in winter. The sauna featured a fireplace where stones were heated to a high temperature. Water was thrown on the hot stones to produce steam and to give a sensation of increased heat. This would raise the apparent temperature so high that people could take off their clothes. The first Finnish saunas were always of a type now called savusauna; "smoke sauna". These differed from present-day saunas in that they were operated by heating a pile of rocks called a kiuas by burning large amounts of wood for about 6 to 8 hours and then letting out the smoke before enjoying the löyly, a Finnish term meaning, collectively, both the steam and the heat of a sauna. A properly heated "savusauna" yields heat for up to 12 hours. As a result of the Industrial Revolution, the sauna evolved to use a wood-burning metal stove with rocks on top, kiuas, with a chimney. Air temperatures averaged around but sometimes exceeded in a traditional Finnish sauna. As the Finns migrated to other areas of the globe, they brought their sauna designs and traditions with them. This led to a further evolution of the sauna, including the electric sauna stove, which was introduced in 1938 by Metos Ltd in Vaasa. Although sauna culture is more or less related to Finnish and Estonian culture, the evolution of the sauna took place around the same time in Finland and other Baltic countries; all have valued the sauna, its customs and traditions until the present day. The sauna became very popular especially in Scandinavia and the German-speaking regions of Europe after the Second World War. German soldiers had experienced Finnish saunas during their fight against the Soviet Union on the Soviet-Finnish front of WWII, where