Heat, infrared and the body's response
Core body temperature
Core body temperature is the temperature of the deep tissues and organs, as opposed to the skin and limbs. It is normally held close to 37 °C, varying about half a degree across the day. External heating raises it; the body then works to bring it back.
Also called core temperature, deep body temperature
The body is not one temperature. Physiologists distinguish the core, held within a narrow range, from the shell — skin, hands, feet — which is allowed to swing widely and is used as a radiator. Core temperature follows a daily rhythm of roughly 0.5 °C, lowest in the early morning hours and highest in the late afternoon.
Measurement. Rectal, oesophageal and ingestible-capsule measurements estimate core temperature directly. Oral, axillary and tympanic readings are convenient approximations and read lower or noisier. Sauna research that reports core temperature almost always uses rectal measurement.
What heat does to it. In a heated sauna, heat gain exceeds heat loss and core temperature drifts upward. The body responds by dilating skin blood vessels and sweating. If evaporation is blocked by humidity, the sweating response cannot do its job and the core rises more readily than the sauna’s temperature alone would suggest.
Do not confuse the sauna’s temperature with body temperature. We publish first sessions at 30 to 37 °C, with only healthy people going above 37 °C, and note that ozone begins breaking down above about 40 °C. Those are settings for the air inside the sauna, not measurements of the person in it. A 37 °C setting does not mean a 37 °C client, and it does not mean the client’s core is unchanged.
Why it matters here
The number on the display describes the air in the sauna, not the person in it. A 37 °C setting does not mean a 37 °C client, and the load the body is carrying does not show up on the display.