Average body temperature among Americans has dropped about 0.02 °C (0.05 F) every decade since the early 1800s and 0.03 °C every decade since 1960 

March 8, 2024
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Mackowiak PA, Wasserman SS, Levine MM. A Critical Appraisal of 98.6°F, the Upper Limit of the Normal Body Temperature, and Other Legacies of Carl Reinhold August Wunderlich. JAMA. 1992;268(12):1578–1580. doi:10.1001/jama.1992.03490120092034 Conclusions.  —Thirty-seven degrees centigrade (98.6°F) should be abandoned as a concept relevant to clinical thermometry; 37.2°C (98.9°F) in the early morning and 37.7°C (99.9°F) overall should be regarded as the upper limit of the normal oral temperature range in healthy adults aged 40 years or younger, and several of Wunderlich’s other cherished dictums should be revised.(JAMA. 1992;268:1578-1580)

Katz, Brigit. Human Body Temperature Is Getting Cooler, Study Finds –
Our average normal temperature may no longer be 98.6 degrees Fahrenheit Smithsonian Magazine January 16, 2020. Accessed March 8, 2024 Quote:“In previous studies people who found lower temperatures [in more recent times] thought the temperatures taken in the 19th century were just wrong,” study co-author Julie Parsonnet, a professor of medicine at Stanford’s School of Medicine, tells Alice Park of Time magazine. “I don’t think they were wrong; I think the temperature has gone down.” (I think she’s right about the temperature going down but I disagree on most of the whys…almost all of them.)

Protsiv, Myroslava et al. “Decreasing human body temperature in the United States since the industrial revolution.” eLife vol. 9 e49555. 7 Jan. 2020, doi:10.7554/eLife.49555 Abstract: In the US, the normal, oral temperature of adults is, on average, lower than the canonical 37°C established in the 19th century. We postulated that body temperature has decreased over time. Using measurements from three cohorts—the Union Army Veterans of the Civil War (N = 23,710; measurement years 1860–1940), the National Health and Nutrition Examination Survey I (N = 15,301; 1971–1975), and the Stanford Translational Research Integrated Database Environment (N = 150,280; 2007–2017)—we determined that mean body temperature in men and women, after adjusting for age, height, weight and, in some models date and time of day, has decreased monotonically by 0.03°C per birth decade. A similar decline within the Union Army cohort as between cohorts, makes measurement error an unlikely explanation. This substantive and continuing shift in body temperature—a marker for metabolic rate—provides a framework for understanding changes in human health and longevity over 157 years.

Basal body temperature

Main article: Basal body temperature

Basal body temperature is the lowest temperature attained by the body during rest (usually during sleep). It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature. In women, temperature differs at various points in the menstrual cycle, and this can be used in the long term to track ovulation both to aid conception or avoid pregnancy. This process is called fertility awareness.

Core temperature

Core temperature, also called core body temperature, is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Significant core temperature elevation (hyperthermia) or depression (hypothermia) over more than a brief period of time is incompatible with human life.

Hypothermia

Main article: Hypothermia

In hypothermia, body temperature drops below that required for normal metabolism and bodily functions. In humans, this is usually due to excessive exposure to cold air or water, but it can be deliberately induced as a medical treatment. Symptoms usually appear when the body’s core temperature drops by 1–2 °C (1.8–3.6 °F) below normal temperature.

Hypothermia Pathophysiology

Heat is primarily generated in muscle tissue, including the heart, and in the liver, while it is lost through the skin (90%) and lungs (10%). Heat production may be increased two- to four-fold through muscle contractions (i.e. exercise and shivering). The rate of heat loss is determined, as with any object, by convection, conduction, and radiation. The rates of these can be affected by body mass index, body surface area to volume ratios, clothing and other environmental conditions.

  • Nuckton TJ, Claman DM, Goldreich D, Wendt FC, Nuckton JG (October 2000). “Hypothermia and afterdrop following open water swimming: the Alcatraz/San Francisco Swim Study”. American Journal of Emergency Medicine. 18 (6): 703–7. doi:10.1053/ajem.2000.16313. PMID 11043627.
  • Hanania NA, Zimmerman JL (1999). “Accidental hypothermia”. Critical Care Clinics. 15 (2): 235–49. doi:10.1016/s0749-0704(05)70052-x. PMID 10331126.
  • https://en.wikipedia.org/wiki/Hypothermia

Many changes to physiology occur as body temperatures decrease. These occur in the cardiovascular system leading to the Osborn J wave and other dysrhythmias, decreased central nervous system electrical activity, cold diuresis, and non-cardiogenic pulmonary edema.

  • Marx J (2010). Rosen’s emergency medicine: concepts and clinical practice 7th edition. Philadelphia, PA: Mosby/Elsevier. pp. 1869–1870. ISBN 978-0-323-05472-0.
  • https://en.wikipedia.org/wiki/Hypothermia

Research has shown that glomerular filtration rates (GFR) decrease as a result of hypothermia. In essence, hypothermia increases preglomerular vasoconstriction, thus decreasing both renal blood flow (RBF) and GFR.

  • Broman M, Källskog O (1995). “The effects of hypothermia on renal function and haemodynamics in the rat”. Acta Physiologica Scandinavica. 153 (2): 179–184. doi:10.1111/j.1748-1716.1995.tb09849.x. PMID 7778458.
  • Broman M, Källskog O, Kopp UC, Wolgast M (1998). “Influence of the sympathetic nervous system on renal function during hypothermia”. Acta Physiologica Scandinavica. 163 (3): 241–249. doi:10.1046/j.1365-201x.1998.00356.x. PMID 9715736.
  • https://en.wikipedia.org/wiki/Hypothermia

Diagnosis

Atrial fibrillation and Osborn J waves in a person with hypothermia. Note what could be mistaken for ST elevation.

Accurate determination of core temperature often requires a special low temperature thermometer, as most clinical thermometers do not measure accurately below 34.4 °C (93.9 °F). A low temperature thermometer can be placed in the rectum, esophagus or bladder. Esophageal measurements are the most accurate and are recommended once a person is intubated. Other methods of measurement such as in the mouth, under the arm, or using an infrared ear thermometer are often not accurate.

  • Brown DJ, Brugger H, Boyd J, Paal P (November 2012). “Accidental hypothermia”. The New England Journal of Medicine. 367 (20): 1930–8. doi:10.1056/NEJMra1114208. PMID 23150960. S2CID 205116341.
  • McCullough L, Arora S (December 2004). “Diagnosis and treatment of hypothermia”. American Family Physician. 70 (12): 2325–32. PMID 15617296.
  • https://en.wikipedia.org/wiki/Hypothermia

As a hypothermic person’s heart rate may be very slow, prolonged feeling for a pulse could be required before detecting. In 2005, the American Heart Association recommended at least 30–45 seconds to verify the absence of a pulse before initiating CPR. Others recommend a 60-second check.

The classical ECG finding of hypothermia is the Osborn J wave. Also, ventricular fibrillation frequently occurs below 28 °C (82 °F) and asystole below 20 °C (68 °F). The Osborn J may look very similar to those of an acute ST elevation myocardial infarction. Thrombolysis as a reaction to the presence of Osborn J waves is not indicated, as it would only worsen the underlying coagulopathy caused by hypothermia.

  • Hanania NA, Zimmerman JL (1999). “Accidental hypothermia”. Critical Care Clinics. 15 (2): 235–49. doi:10.1016/s0749-0704(05)70052-x. PMID 10331126.
  • Marx J (2010). Rosen’s Emergency Medicine: Concepts and Clinical Practice (7th ed.). Philadelphia: Mosby/Elsevier. p. 1862. ISBN 978-0-323-05472-0. p.1869
  • https://en.wikipedia.org/wiki/Hypothermia

Nazi human experimentation

Nazi human experimentation was a series of medical experiments on prisoners by Nazi Germany in its concentration camps mainly between 1942 and 1945. There were 15,754 documented victims, of various nationalities and age groups, although the true number is believed to be more extensive. Many survived, with only a quarter of documented victims killed. Survivors generally experienced severe permanent injuries.

At Auschwitz and other camps, under the direction of Eduard Wirths, selected inmates were subjected to various experiments that were designed to help German military personnel in combat situations, develop new weapons, aid in the recovery of military personnel who had been injured, and to advance Nazi racial ideology and eugenics, including the twin experiments of Josef Mengele. Aribert Heim conducted similar medical experiments at Mauthausen.

After the war, these crimes were tried at what became known as the Doctors’ Trial, and revulsion at the abuses perpetrated led to the development of the Nuremberg Code of medical ethics. The Nazi physicians in the Doctors’ Trial argued that military necessity justified their experiments and compared their victims to collateral damage from Allied bombings.

Experiments

The table of contents of a document from the Subsequent Nuremberg trials prosecution includes titles of the sections that document medical experiments revolving around food, seawater, epidemic jaundice, sulfanilamide, blood coagulation and phlegmon. According to the indictments at the subsequent Nuremberg Trials, these experiments included the following:

Freezing experiments

A cold water immersion experiment at Dachau concentration camp presided over by Ernst Holzlöhner (left) and Sigmund Rascher (right). The subject is wearing an experimental Luftwaffe garment.

In 1941, the Luftwaffe conducted experiments with the intent of discovering means to prevent and treat hypothermia. There were 360 to 400 experiments and 280 to 300 victims, indicating that some victims suffered more than one experiment.

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