People are “warm-blooded,” meaning that the human body has ways of maintaining a fairly constant internal body temperature, despite changes in the temperature of the surroundings. One advantage of such temperature regulation is that the muscles are always ready to work. When the body is in cooler air or water, extra heat is produced by shivering. Cooling is accomplished by evaporation of sweat.
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The classic study of human body temperature was published by Carl Wunderlich¹ in 1868. In a survey of 25,000 adults, recording temperatures to the nearest degree centigrade, he arrived at an average of 37° centigrade. Later authors converted this figure to degrees Fahrenheit, but added extra, unjustified precision, giving the traditional 98.6°F.
A modern study, carried out in 1991 at the University of Maryland² with much better instrumentation, found a mean temp of 98.2 degrees Fahrenheit. Average temperatures of individuals varied by as much as 4.8°F, and a healthy individual’s temperature might vary during the course of a day by as much as 1.09°F. The lowest temperatures were at 6 am, and the highest between 4 and 6 pm. Women, on the average, were slightly warmer than men (by 0.3°F, 0.2°C). Other studies have found the highest temperatures between 6 and 10 pm, and the lowest between 2 and 4 am.
1. C. Wunderlich.
Das Verhalten der Eigenwarme in Krankheit.
Leipzig: Otto Wigard, 1868.
Phillip A. Mackowiak, Steven S. Wasserman and Myron M. Levine.
A critical appraisal of 98.6°F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich.
JAMA, volume 268, no. 12, pages 1578-1580 (Sept 23/30, 1992).
Horvath, H. Menduke, G. M. Piersol.
Oral and rectal temperatures of man.
JAMA, volume 144, pages 1562-1565 (1950).
Body temperatures of 114°F (45.6°C) and above are “incompatible with life.”
In children, temperatures of 106°F and above are often accompanied by convulsions. At 108°F, brain damage is common.
Heat exhaustion may occur at body temperatures in the range 37°C- 40°C, sometimes, ominously, accompanied by confusion and other mental symptoms. Lacking steps to cool (and hydrate) the patient, heat exhaustion may progress to:
Heat stroke, a life-threatening condition, may occur at a body temperature as low as 104°F (40°C), but temperatures as high as 113°F (44.4°C) have been recorded. Heat stroke may be caused by hot weather, exertion or both. Children, the elderly, and those in poor health, especially the obese, are most susceptible. A body temperature of 106°F (41.1°C, measured rectally) is a medical emergency requiring immediate treatment. See the advice from the Mayo Clinic. As first aid, ice packs can placed around the neck, in the armpits and over the groin, and the patient can be sprayed or doused with cold water and mechanically fanned. The most effective treatment is immersion in cold water. In emergency rooms, heat stroke victims are sometimes given intravenous transfusions of chilled saline.
A series of pages by the CDC, especially good on the types of people most liable to be affected, and with practical advice.
H. B. Simon.
JAMA, volume 236, page 2419 (1976).
Abderrezak Bouchama and James P. Knochel.
New England Journal of Medicine, vol 346, pages 1978-1988 (20 June 2002).
A review article.
C. H. Wyndham.
Heatstroke and hyperthermia in marathon runners.
Annuals of the New York Academy of Science, volume 301, page 128 (1977).
In case you are a runner.
Emma A. Nye, Lindsey E. Eberman, Kenneth E. Games and Colin Carriker.
Comparison of whole-body cooling techniques for athletes and military personnel.
International Journal of Exercise Science, vol. 10, no. 2, pages 294-300. (1 March 2017).
“Our results support multiple organizations that deem CWI as the only acceptable treatment, when compared to the Polar Life Pod® and ice sheets.”
Hypothermia begins at temperatures below 95°F (35°C). Humans lose consciousness at about 91°F (32.8°C).
Severe hypothermia begins when body temperature falls below 86°F (30°C). Old people and infants are particularly susceptible: hypothermia has occurred even at room temperatures as high as 60°F. Check the links.
Mark S. Blumberg.
Body Heat. Temperature and Life on Earth.
Cambridge (MA): Harvard University Press, 2002.
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Last revised: 24 July 2017.