The 98.6℉ Myth: Why Everything You Think You Know About Body Temperature Is a Lie

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When you were kid, you probably knew that to score a magical sick day home from school, you needed to have a fever. When the thermometer came out of your mouth, it had to read higher than 98.6℉—the long-accepted "normal" human body temperature. (If you wanted to really seal the deal, you may have hoped to hit 100℉.) Since then, you may have used a temperature above 98.6℉ as a metric to work from home (or call out sick entirely).

But here's the thing: The average body temperature isn't actually 98.6℉—a fact that we've known for more than 25 years. The myth originated in the 19th century with a single doctor, and despite evidence to the contrary, it's persisted ever since.

THE GIANT—AND FAULTY—ARMPIT THERMOMETER

In 1851, Carl Wunderlich, the director of the hospital at Leipzig University, began going from room to room with a comically large thermometer in tow. He wanted to understand how body temperature is affected by different diseases, so in each room, he would hold the foot-long device in patients' armpits for a full 20 minutes, waiting for a temperature to register. Once it did, he'd note the temperature on the patient's chart (Wunderlich is thought to be the first physician to do so). He and his staff did this for years, repeatedly taking the temperatures of some 25,000 patients and logging them on their charts, until he had millions of readings. In 1868, he finally published this data in Das Verhalten der Eigenwarme in Krankheiten (On the Temperature in Diseases: A Manual of Medical Thermometry). He concluded that the average human body temperature was 98.6℉, underscoring the idea that fever is a symptom of illness, not a cause.

No one questioned Wunderlich's methods, or his average, for about 140 years. Then, in the early 1990s, internist Philip Mackowiak—a professor of medicine at the University of Maryland, a medical historian, and, apparently, a clinical thermometer junkie—saw one of the physician's instruments at the Mutter Museum in Philadelphia. He told the Freakonomics podcast that he'd always had doubts about the 98.6℉ standard. "I am by nature a skeptic," he said. "And it occurred to me very early in my career that this idea that 98.6 was normal, and then if you didn't have a temperature of 98.6, you were somehow abnormal, just didn't sit right."

Getting his hands on Wunderlich's thermometer—which the museum let him borrow—only deepened his doubts. The huge thermometer was unwieldy and non-registering, meaning, Mackowiak explained, "that it has to be read while it's in place." Not only that, but Wunderlich had used the device to measure temperatures in the armpit, which is less reliable than temperatures taken in the mouth or rectum. The instrument itself also wasn't terribly precise: It measured up to 2 degrees Centigrade higher than both ancient and modern instruments.

In 1992, Mackowiak decided to test Wunderlich's average. Using normal-sized oral thermometers and a group of volunteers, he determined that the average human body temperature actually hovers around 98.2℉. Mackowiak found that body temperature tends to vary over the course of the day, with its lowest point around 6 a.m. and its highest in the early evening. Body temperature can also fluctuate monthly (with the menstrual cycle) and over a lifetime (declining decade by decade with age), and may even be differentially linked to sex and race assignments. He concluded that normal body temperature is so unique to each person that it's almost like a fingerprint and, given that wide variation, not actually a very reliable indicator of illness.

As a result of his study, Mackowiak proposed raising the threshold for fever to 98.9℉ for temperatures taken in the morning (and 99.9℉ at other times). While it's a relatively minor change in terms of actual degrees, this fever threshold is actually lower than the CDC's, which is a temperature of 100.4℉ or higher.

There are potential real-life consequences in this gap, for everyone from students (who'd have to attend school with what would be considered a low-grade fever by Wunderlich's 98.6℉ standard) to employers and daycares (who use temperature to set attendance policies). What's more, anyone who is actually sick but ignores a low-grade fever—one that meets Mackowiak's threshold but still falls under the CDC's—could pose a risk to people with compromised immune systems trying to avoid unnecessary exposure to illness in public places.

THE BALANCING POINT

There's a reason the average trends near 98℉ instead of 92℉ or 106℉. As endotherms, mammals expend a great deal of energy maintaining body temperature when compared with cold-blooded creatures. To find and conserve a just-right body temperature, central nervous system sensors gather data (too warm? too cold? just right, Goldilocks?) and send that information to the pebble-sized hypothalamus near the base of the brain. There, the data is converted into action: releasing sweat and widening the blood vessels if too warm; raising metabolism, constricting the blood vessels, and inducing shivering if too cold.

According to a study by Aviv Bergman and Arturo Casadevall in the journal mBio, the precise balancing point for ideal body temperature is the sweet spot where the metabolic cost for all this thermoregulation balances with the evolutionary advantage of warding off fungal disease. (While warm-blooded animals are prone to bacterial or viral infections, they rarely experience fungal infections because most fungi can't withstand temperatures above 86℉. Cold-blooded animals, on the other hand, are prone to all three.) For Bergman and Casadevall, this benefit even explains what tipped Darwin's scales in favor of mammals, allowing them to edge out other vertebrates for dominance after the Cretaceous-Tertiary mass extinction wiped out the dinosaurs.

Of course, rules call for exceptions, and the one place where human body temperature demonstrates sustained elevation is outer space. Astronauts on prolonged missions clock significantly higher average body temperatures than they do when terrestrial—even up to 104℉. This so-called "space fever" is probably a product of some combination of radiation exposure, psychological stress, and immune response to weightlessness. Researchers believe this phenomenon could yield crucial information about thermoregulation—and may even offer insight into how humans might adapt to climate change.

WHY THE MYTH PERSISTS

It's been 26 years since Mackowiak's study, yet the newer data has not taken hold among medical professionals or the public. What gives?

Mackowiak tells Mental Floss that he finds it a bit mystifying that the myth persists, especially since many people, when pressed, know that the so-called "average" temperature varies. Part of the problem may be psychological: We cling to beliefs despite evidence to the contrary—a phenomenon called belief perseverance [PDF]. It's a significant force upholding a surprising number of medical myths. The idea humans should drink eight glasses of water a day? Not science. Sugar causes hyperactive behavior? Nope. Reading in dim light harms eyesight? Not really.

Unlearning persistent myths—especially ones loaded with the weight of medical authority—is difficult. "Deep down, under it all," Mackowiak says, "people want simple answers for things."

Watch: Woman Plays Violin During Brain Surgery to Help Doctors Avoid Damaging Her Fine Motor Skills

Violinist Dagmar Turner played George Gershwin's "Summertime" and other selections during her brain surgery performance.
Violinist Dagmar Turner played George Gershwin's "Summertime" and other selections during her brain surgery performance.
Furtseff/iStock via Getty Images

When 53-year-old Dagmar Turner told neurosurgeons she was right-handed, they said that was no problem—the brain surgery they were planning to remove a tumor only ran the risk of affecting fine motor skills in her left hand. To Turner, a lifelong violinist and member of the Isle of Wight Symphony Orchestra, that was still very much a problem.

Turner told ITV News she suggested playing the violin during the procedure so the surgeons at King’s College Hospital in London could ensure they weren’t damaging coordination in either hand. They agreed.

According to NBC News, the violinist played George Gershwin's “Summertime” and selections by Gustav Mahler and Julio Iglesias while surgeons extracted the tumor from the right frontal lobe of her brain. BBC News reports that she’s been living with the growth since 2013, and doctors decided it was time to operate in November 2019.

You can watch her play in the video below. (The top of Turner’s head is completely obscured by plastic and other surgical materials, so there’s nothing graphic in the clip. Having said that, anyone who’s especially squeamish about the inside of an operating room should proceed with caution.)

Professor Keyoumars Ashkan, a neurosurgeon who helped plan the procedure, said in a statement from the hospital that the mid-surgery performance was a first for him.

“We perform around 400 resections (tumor removals) each year, which often involves rousing patients to carry out language tests, but this was the first time I’ve had a patient play an instrument,” he said. “We managed to remove over 90 percent of the tumor, including all the areas suspicious of aggressive activity, while retaining full function in her left hand.”

[h/t BBC News]

15 Historic Diseases that Competed with Bubonic Plague

Jan Josef Horemans, Interior with a surgeon and his apprentice attending to a patient (1722), Wellcome Collection // CC BY-NC 4.0
Jan Josef Horemans, Interior with a surgeon and his apprentice attending to a patient (1722), Wellcome Collection // CC BY-NC 4.0

In 1665, about a quarter of all Londoners died of the Great Plague—but bubonic plague was not the only deadly disease circulating in the city. A published register, called London’s Dreadful Visitation, or, A Collection of All the Bills of Mortality, recorded the causes of death and the number of victims in London between December 20, 1664 and December 19, 1665. The systematic, parish-by-parish tally reveals the rapid spread of plague throughout the capital: a total of one victim, recorded in the first week, increased to 7165 during the week of September 12-19, 1665.

But quite a few Londoners met their fates in other ways. Here’s a look into the antiquated diseases that managed to kill those that Yersinia pestis couldn’t catch.

1. Winde

Winde is listed throughout the Bills as a constant cause of death. According to the Oxford English Dictionary, winde referred to paroxysms of severe gastrointestinal pain, which could have been symptoms of numerous diseases.

2. Purples

Purples described purple blotches on the skin caused by broken blood vessels, indicative of an underlying illness, such as scurvy or a circulation disorder. It could also mean the most severe stage of smallpox.

3. Livergrown

People who died of livergrown suffered from an enlarged (or failing) liver. Doctors could diagnose it through the combination of other symptoms, like jaundice and abdominal pain. It was commonly a result of alcoholism, but could be caused by a number of disorders.

4. Chrisomes

Infant mortality was extremely high before the advent of modern medicine. The Bills distinguished abortive (miscarried), stillborn, infant, and chrisom deaths—the latter term specified infants who died within the first month of life, around the time they were baptized with special white cloths (which were called chrisomes).

5. Rising of the Lights

18th century illustration of lungs and heart
Jacques-Fabien Gautier d'Agoty, The Lungs and the Heart (1754), Wellcome Collection // CC BY-NC 4.0

Physicians and scholars have debated the origin of the term rising of the lights. According to the OED, the condition indicated any kind of illness characterized by a hoarse cough, difficulty breathing, or a choking sensation. Croup, asthma, pneumonia, and emphysema were all culprits.

6. Timpany

The condition of having serious swelling or bloating in the digestive tract, which produces a hollow sound when tapped, is still called tympany today. The sort that would have proven fatal to humans could have been caused by kidney disease, intestinal infections, or cancerous tumors.

7. Tissick

The term tissick, a corruption of phthisis, originated in ancient Greek and persisted through Latin, French, and English for thousands of years, only to end up an obsolete word referring to a “wasting disease of the lungs,” according to the Online Etymology Dictionary. In the 17th century, that could indicate the wheezing and coughing associated with asthma, bronchitis, or possibly tuberculosis.

8. Meagrome or Megrim

We recognize this obscurely spelled ailment as migraine. During the years of the Great Plague, any internal head trauma, from an aneurysm to a brain tumor, would be filed under megrim.

9. Imposthume

Imposthume was a swelling, cyst, or abscess, usually filled with pus or other putrescence. At the same time that it was being recorded as a cause of death, imposthume took on a metaphorical meaning and referred to an egotistical or corrupt person “swollen” with pride.

10. Head Mould Shot

In newborns, the bony plates of the skull are not fused together, which makes it easier to fit through the birth canal. Head mould shot described a condition where the cranial bones were so compressed by delivery that they overlapped (or overshot) each other and caused fatal pressure on the brain. Today, the condition, now known as craniosynostosis, is treatable with surgery.

11. Quinsie

18th century illustration of a woman getting her throat examined in a pharmacy

Quinsie, which evolved from a Latin word meaning “choke,” is still occasionally used in modern England. It describes a complication of tonsillitis in which an abscess grows between the tonsil and the throat. Unless the abscess was removed, a patient could suffocate from the blockage.

12. Surfeit

A surfeit means an excess of something. In the Bills of Mortality, it’s hard to identify the substance in question. Sometimes, as in the case of King Henry I and his lampreys, it can refer to overeating a food that becomes poisonous if taken in large enough quantities.

13. French Pox

When people across Europe came down with syphilis beginning in the 1490s, they blamed the French. (Perhaps they should have blamed Christopher Columbus and the Spanish, whom historians believe brought the bacterial infection back from the New World.) Rightly or wrongly, French pox is what the Bills of Mortality lists for deaths by advanced syphilis, whose symptoms included rash, blindness, organ failure, and tissue necrosis.

14. Bloody Flux

Dysentery, a.k.a. bloody flux, was common among densely crowded Londoners without clean drinking water. People contracted dysentery from food or water contaminated with one of several pathogens, and its main symptom was bloody diarrhea (the aforementioned flux) and severe dehydration.

15. Plannet

Plannet is likely a shorthand for “planet-struck.” Many medical practitioners believed the planets influenced health and sanity. A person who was planet-stricken had been suddenly maligned by the forces of particular planets. They would likely present symptoms also associated with aneurysms, strokes, and heart attacks.

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