Body temperature in humans is closely regulated to keep it nearly independent of environmental temperature, and it has been used for health diagnosis since ancient times (this was one of the first Human thermal comfort 4 applications of physical thermometry, in 17th century). It was soon discovered that the deeper inside the body, the lesser disturbances from the environment, so that the most reliable body core measurements are taken with more or less invasive means (hypodermic needle, inside the oesophagus, inside the rectus, below the tongue, under the axilla, or at the tympanum), although non-invasive measurements are taking aver, with infrared detection at the tympanum, the temporal artery, at the front, and so on. Body core temperature is around 37 ºC, but during long-distance running competitions (say >10 km), and shortly afterwards, people may reach core-temperature may rise up to 38.5 ºC or even 39.5 ºC
