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 ang high heat makes the bathers perspire. A thermometer in a sauna is typically used to measure heat, while 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 organizations, infrared is not a sauna.

The oldest known sauna's 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.

Under many circumstances, temperatures approaching and exceeding 100 degree's Celsius (212 degree's Farenheit) would be completely intolerable and possibly fatal if exposed to long periods of time. Sauna's overcome this problem by controlling the humidity.

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