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."

Can You Guess How These Diseases Were Treated in the 1700s?

17 Bizarre Natural Remedies From the 1700s

In the late 1740s, John Wesley—a British evangelist and the co-founder of Methodism—published Primitive Physick, or, An Easy and Natural Method of Curing Most Diseases. The tome gave regular people ways to cure themselves with natural remedies, using items they could find in their own homes.

When in doubt, Welsey thought that drinking cold water or taking cold baths could cure most illnesses (including breast cancer); some of his suggestions, like using chamomile tea to soothe an upset stomach, have survived today. Other natural remedies he whipped up, though, are decidedly strange. Here are a few of them.

1. To Cure An Ague

Wesley describes an ague as “an intermitting fever, each fit of which is preceded by a cold shivering and goes off in a sweat.” There are many natural remedies for curing it, but all must be preceded by taking a “gentle vomit,” which, if taken two hours before the fit, Wesley says will generally prevent it, and may even cure the ague. If the vomiting fails, however, Wesley suggests wearing a bag of groundsel, a weed, “on the pit of the stomach, renewing it two hours before the fit.” The weed should be shredded small, and the side of the bag facing the skin should have holes in it.

Should this not work, Wesley suggests a remedy that requires a stronger stomach: “Make six middling pills of cobwebs, take one a little before the cold fit: Two a little before the next fit: The other three, if Need be, a little before the third fit. I never knew this fail.”

2. To Cure a Canine Appetite

Wesley turns to a Dr. Scomberg for the cure to this condition, which is defined by Wesley as “an insatiable desire of eating”: If there’s no vomiting, canine appetite “is often cured by a small Bit of Bread dipt in Wine, and applied to the Nostrils."

3. To Cure Asthma

Tar water, sea water, nettle juice, and quicksilver are all acceptable cures for what Wesley calls "moist Asthma" (which is characterized by “a difficulty of breathing … the patient spits much”). But a method that “seldom fails,” Wesley says, is living “a fortnight on boiled carrots only.”

Dry and convulsive asthma, meanwhile, can be treated with toad, dried and powdered. “Make it into small pills,” Wesley writes, “and take one every hour until the convulsions fade.”

4. To Prevent or Cure Nose Bleeds

Drinking whey and eating raisins every day, Wesley says, can help prevent nose bleeds. Other methods for preventing or curing the phenomenon include “hold[ing] a red hot poker under the nose” and “steep[ing] a linnen rag in sharp vinegar, burn[ing] it, and blow[ing] it up the nose with a Quill.”

5. To Cure a “Cold in the Head”

Getting rid of this common ailment is easy, according to Wesley: Just “pare very thin the yellow rind of an orange," he writes. "Roll it up inside out, and thrust a roll inside each nostril.”

6. To Cure “An Habitual Colick”

Today's doctors define colic as a condition suffered by "a healthy, well-fed infant who cries for more than three hours per day, for more than three days per week, for more than three weeks." But adults can get it, too; it's characterized by severe stomach pains and spasms (which, we now know, can be an indication of other conditions, like Crohn's disease and irritable bowel syndrome). To cure it, Wesley suggests this odd remedy: “Wear a thin soft Flannel on the part.”

6. To Cure “White Specks in the Eye”

While it's unclear exactly what "white specks in the eye" actually is—eye floaters, maybe—Wesley suggests that, when “going to bed, put a little ear-wax on the Speck.—This has cured many.”

7. To Cure the Falling Sickness

Those who suffer from this illness “fall to the ground, either quite stiff, or convulsed all over, utterly senseless, gnashing his teeth, and foaming at the mouth.” To cure the condition, Wesley recommends “an entire milk diet for three months: It rarely fails.” During fits, though, “blow up the nose a little powder’d ginger.”

8. To Cure Gout

“Regard not them who say the gout ought not to be cured. They mean, it cannot,” Wesley writes. (They, here, might be referring to regular practitioners of medicine.) “I know it cannot by their regular prescriptions. But I have known it cured in many cases, without any ill effect following.” Gout in the foot or hand can be cured by “apply[ing] a raw lean beef-steak. Change it once in 12 hours, ‘till cured.”

Curing the gout in any limb can be accomplished by beginning this ritual at six in the evening: “Undress and wrap yourself up in Blankets. — Then put your Legs up to the Knees in Water, as hot as you can bear it. As it cools, let hot Water be poured in, so as to keep you in a strong Sweat till ten. Then go into a Bed well warm'd and sweat till Morning. — I have known this to cure an inveterate Gout.”

9. To Cure Jaundice

Wesley suggests curing jaundice—which turns the skin and whites of the eyes yellow (thanks to too much bilirubin in the blood, we now know)—by wearing "leaves of Celandine upon and under the feet." Other possible cures include taking a small pill of Castile soap in the morning for eight to 10 days, or "as much lies on a shilling of calcin’d egg-shells, three mornings fasting; and walk till you sweat.”

10. To Cure “The Iliac Passion”

This decidedly unpleasant condition—which Wesley defines as a “violent kind of Colic ... the Excrements are thrown up by the mouth in vomiting,” eww—has a few cures, including “apply[ing] warm Flannel soaked in Spirits of Wine.” Most delightful, however, is the cure recommended by a Dr. Sydenham: “Hold a live Puppy constantly on the Belly.”

11. To Cure “the Palpitation or Beating of the Heart”

Among the remedies for this ailment are the mundane “drink a Pint of cold Water,” the stinky-but-probably-not-effective “apply outwardly a Rag dipt In vinegar,” and the very exciting “be electrified” (which is suggested for a few other illnesses as well).

12. To Cure Pleurisy

This illness is characterized by “a Fever attended with a violent pain in the Side, and a Pulse remarkably hard.” (It's caused, we now know, when the double membrane that surrounds the lungs inside the chest cavity becomes inflamed.) Wesley’s first suggested remedy involves applying “to the Side Onions roasted in the Embers, mixt with Cream." Next up is filling the core of an apple with frankincense “stop[ping] it close with the Piece you cut out and roast[ing] it in Ashes. Mash and eat it.” Sounds delicious!

13. To cure Quinsy

“A quinsy,” Wesley explains, “is a Fever attended with Difficulty of Swallowing, and often Breathing.” (Today, the condition is called peritonsillar abscess and it's known to be a complication of tonsillitis.) He suggests applying “a large White-bread Toast, half an Inch thick, dipt in Brandy, to the crown of the Head till it dries.”

14. To Cure “A Windy Rupture”

Wesley doesn't say what, exactly, this condition is, though a Google search brings up the term hernia ventosa, which another medical book of the same time defines as a "false hernia ... where the wind is pent up by the coats of the Testes, inflating and blowing up the inguen," or the groin area. Wesley prescribes the following method to cure it: “Warm Cow-dung well. Spread it thick on Leather, [throwing] some cummin seeds on it, and apply it hot. When cold, put on a new one.” This, he says, “commonly cures a Child (keeping his Bed) in two Days.”

15. To Cure a "Tooth-ach"

Wesley suggests being electrified through the tooth. If that’s too extreme for you, try “rub[bing] the Cheek a Quarter of an Hour ... Or, put[ting] a Clove of Garlick into the Ear.”

16. To Stop Vomiting

Induced vomiting was an important part of Wesley's medical theories (remember the "gentle vomit" that could stop the ague?). But if a patient was vomiting and it wasn't a part of the prescribed method for curing him, Wesley advised "after every Vomiting, drink a pint of warm water; or, apply a large onion slit, to the Pit of the Stomach."

17. To Heal a Cut

Wesley suggests holding the cut closed "with your thumb for a quarter of an hour" (what we might call applying pressure these days), then dipping a rag in cold water and wrapping the cut in it. Another method: "Bind on toasted cheese," Wesley writes. "This will cure a deep cut." Pounded grass, applied fresh every 12 hours, will also do the trick.

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