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."

12 Facts About Fibromyalgia

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To people living with fibromyalgia, the symptoms are all too real. Muscle tenderness, full-body pain, and brain fog make it hard to function—and getting a restful night’s sleep isn’t much easier. To the frustration of patients, other aspects of the chronic condition—such as what causes it, how to diagnose it, and how to treat it—are more of a mystery. But after decades of rampant misconceptions, we know more facts about fibromyalgia than ever before.

1. SYMPTOMS FEEL DIFFERENT FOR EVERYONE.

Symptoms of fibromyalgia can vary widely. The defining characteristic of the condition is widespread pain, or pain felt throughout the entire body, but how often this pain occurs and how intensely it’s felt is different in each patient. Some people may feel pain reminiscent of a sunburn, a pins-and-needle sensation, sharp stabbing, or some combination of the above. Beyond pain, the condition can come with fatigue, disrupted sleep, depression and anxiety, and trouble focusing (known as “fibro fog").

2. IT AFFECTS MOSTLY WOMEN.

Most fibromyalgia patients are female, making it more prevalent in women than breast cancer. Not only are women more likely to have fibromyalgia than men, but they report experiencing the symptoms more acutely as well. Researchers still aren’t sure why the condition has a disproportionate impact on women, but they speculate that because the diagnosis is most common during a woman's fertile years, it may have something to do with estrogen levels. Some experts also suspect that the condition may be under-diagnosed in men because it’s often labeled a woman’s problem.

3. IT’S RARE.

Though it has gained visibility in recent years, your chances of experiencing fibromyalgia are still slim. According to the Centers for Disease Control and Prevention, it affects roughly 4 million adults in the U.S., or 2 percent of the population. Fibromyalgia’s similarity to other mysterious conditions also means it is likely overdiagnosed, so that number may be even lower.

4. MOST PEOPLE GET IT IN MIDDLE AGE.

People who have fibromyalgia tend to develop it well into adulthood. The condition is most common in 30- to 50-year-olds, but people of all ages—including children and seniors—can have it. Fibromyalgia in patients 10 and younger, also called juvenile fibromyalgia, often goes unrecognized.

5. IT’S HARD TO DIAGNOSE.

There’s no one medical test that you can take to confirm you have fibromyalgia. Instead, doctors diagnosis patients who exhibit the condition’s most common symptoms—widespread pain, fatigue, trouble sleeping, and muscle tenderness in certain points on the body—by process of elimination. Polymyalgia rheumatica and hypothyroidism (or an underactive thyroid gland) provoke similar symptoms, and both show up in blood tests. Doctors will usually tests for these conditions and others before diagnosing a person with fibromyalgia.

6. THE NAME IS RELATIVELY NEW.

People have suffered from fibromyalgia for centuries, but it received its official name only a few decades ago. In 1976, the word fibromyalgia was coined to describe the condition, with fibro coming from fibrous tissue, myo from the Greek word for muscle, and algia from the Greek word for pain. The name replaced fibrositis, which was used when doctors incorrectly believed that fibromyalgia was caused by inflammation (which -itis is used to denote).

7. IT MAY BE ASSOCIATED WITH PTSD.

Health experts have long known that post-traumatic stress disorder can manifest in physical symptoms—now they suspect the disorder is sometimes connected to fibromyalgia. According to a study published in the European Journal of Pain in 2017, 49 percent of 154 female fibromyalgia patients had experienced at least one traumatic event in childhood, and 26 percent had been diagnosed with PTSD. Researchers also saw a correlation between trauma and the intensity of the condition, with subjects with PTSD experiencing more and worse fibromyalgia pain than those without it.

8. IT’S NOT “ALL IN YOUR HEAD.”

As is the case with many invisible illnesses, fibromyalgia patients are often told their symptoms are purely psychological. But findings from a 2013 study suggested what many sufferers already knew: Their pain is more than just a product of mental distress or an overactive imagination. The small study, published in the journal Pain Medicine, found extra sensory nerve fibers around certain blood vessel structures in the hands of 18 of 24 female fibromyalgia patients compared to 14 of 23 controls. The study proposed that the nerve endings—once thought to merely regulate blood flow—may also be able to perceive pain, an idea that could help dispel a harmful myth surrounding the condition.

9. IT’S CONNECTED TO ARTHRITIS, CHRONIC FATIGUE SYNDROME, AND IBS.

For many patients, fibromyalgia isn’t the only chronic condition they suffer from. Fibromyalgia has been linked to chronic fatigue syndrome, irritable bowel syndrome, sleep apnea, migraines, rheumatoid arthritis, and other medical problems. In some cases, as with chronic fatigue syndrome, the two conditions have such similar symptoms that their diagnostic criteria overlaps. Others conditions like irritable bowel syndrome are related to fibromyalgia—not confused with it.

10. IT'S PROBABLY NOT GENETIC—BUT IT CAN CLUSTER IN THE FAMILIES.

If you're closely related to someone with fibromyalgia, you're more likely to have it yourself. Studies have shown that the diagnosis tends to cluster in families. At first this seems to suggest that the condition is genetic, but scientists have yet to identify a specific gene that's directly responsible for fibromyalgia. The more likely explanation for the trend is that members of the same family experience the same environmental stressors that can trigger the symptoms, or they share genes that are indirectly related to the issue.

11. ANTIDEPRESSANTS CAN HELP ...

Since we don't know what causes fibromyalgia, it's hard to treat. But patients are often prescribed antidepressants to ease their symptoms. These medications have been shown to alleviate some of the most debilitating hallmarks of the condition, such as general pain and restless nights. Doctors who support antidepressants as a fibromyalgia treatment are quick to note that that doesn’t make the condition a mental disorder. While these drugs can lift the depressed moods that sometimes come with fibromyalgia, they also function as painkillers.

12. ... AND SO CAN EXERCISE.

One of the most common pieces of advice fibromyalgia patients get from doctors is to exercise. Hitting the gym may seem impossible for people in too much pain to get off the couch, but physical activity—even in small doses—can actually alleviate pain over time. It also works as treatment for other fibromyalgia symptoms like depression and fatigue.

Could Leonardo da Vinci's Artistic Genius Be Due to an Eye Condition?

Young John the Baptist, Leonardo da Vinci (1513-16, Louvre, Paris).
Young John the Baptist, Leonardo da Vinci (1513-16, Louvre, Paris).
Christopher Tyler, JAMA Ophthalmology (2018)

Leonardo da Vinci was indisputably a genius, but his singular artistic vision may have been the result of seeing the world differently in more ways than one. A new paper argues that he had strabismus, a vision disorder where the eyes are misaligned and don’t look toward the same place at the same time. This disorder, visual neuroscientist Christopher Tyler argues, may have helped the artist render three-dimensional images on flat canvas with an extra level of skill.

Tyler is a professor at City, University of London who has written a number of studies on optics and art. In this study, published in JAMA Ophthalmology, he examined six different artworks from the period when Leonardo was working, including Young John the Baptist, Vitruvian Man, and a self-portrait by the artist. He also analyzed pieces by other artists that are thought to have used Leonardo as a model, like Andrea del Verrocchio’s Young Warrior sculpture. Leonardo served as the lead assistant in the latter artist’s studio, and likely served as the model for several of his works. Leonardo was also a friend of Benedetto da Maiano, and possibly served as a model for his 1480 sculpture of John the Baptist. Tyler also looked at the recently auctioned Salvator Mundi, a painting that not all experts believe can be attributed to Leonardo. (However, at least one scientific team that examined the painting says it’s legit.)

With strabismus, a person’s eyes appear to point in different directions. Based on the eyes in Leonardo’s own portraits of himself and other artworks modeled after him, it seems likely that he had intermittent strabismus. When he relaxed his eyes, one of his eyes drifted outward, though he was likely able to align his eyes when he focused. The gaze in the portraits and sculptures seems to be misaligned, with the left eye consistently drifting outward at around the same angle.

'Vitruvian Man' with the subject's pupils highlighted
Vitruvian Man, Leonardo da Vinci (~1490, Accademia, Venice)
Christopher Tyler, JAMA Ophthalmology (2018)

“The weight of converging evidence suggests that [Leonardo] had intermittent exotropia—where an eye turns outwards—with a resulting ability to switch to monocular vision, using just one eye,” Tyler explained in a press release. “The condition is rather convenient for a painter, since viewing the world with one eye allows direct comparison with the flat image being drawn or painted.” This would have given him an assist in depicting depth accurately.

Leonardo isn’t the first famous artist whose vision researchers have wondered about. Some have speculated that Degas’s increasingly coarse pastel work in his later years may have been attributed to his degenerating eyes, as the rough edges would have appeared smoother to him because of his blurred vision. Others have suggested that Van Gogh’s “yellow period” and the vibrant colors of Starry Night may have been influenced by yellowing vision caused by his use of digitalis, a medicine he took for epilepsy.

We can never truly know whether a long-dead artist’s work was the result of visual issues or simply a unique artistic vision, but looking at their art through the lens of medicine provides a new way of understanding their process.

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