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."

10 Facts About Hepatitis

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Even if you've been vaccinated against it, you may have a lot of unanswered questions about hepatitis. The condition, which is characterized by inflamed liver tissue, can be caused by a variety of factors, including viruses, an overactive immune system, and alcohol abuse. Hepatitis symptoms also vary widely, from a flu-like feeling that clears up in a few weeks to liver failure. Here are some facts worth knowing about every type of hepatitis—including the most common types, hepatitis B and hepatitis C.

1. There are five types of viral hepatitis.

Every case of hepatitis is characterized by inflammation of the liver tissue. When looking at viral hepatitis specifically, the treatments, modes of transmission, and duration of symptoms vary from according to which virus strain is causing it. Hepatitis A is an acute illness that often goes away on its own over time. It spreads primarily via the oral-fecal route, usually when someone ingests food or water contaminated with the hepatitis A virus. The second type, hepatitis B, can be either acute or chronic, and it spreads through bodily fluids like blood and semen. Hepatitis C mainly spreads through blood and is most likely to develop into a chronic condition.

The fourth and fifth types of viral hepatitis are hepatitis D and E, though they aren’t talked about much in the U.S. Like hepatitis A, hepatitis E is mostly spread through oral-fecal contamination. Hepatitis D can only be contracted if the patient has already had hepatitis B. Both types are less common in the U.S. compared to countries that lack access to clean drinking water.

2. Non-viral hepatitis can be caused by alcohol and other factors.

Catching a virus isn’t the only way to contract hepatitis. Even if you’re up-to-date on your shots and practice good hygiene, you can get it from exposure to toxic chemicals, taking prescriptions or over-the-counter-drugs, or abusing alcohol. All of these conditions are known as toxic hepatitis. There’s also autoimmune hepatitis, which occurs when the body’s immune system attacks the liver and treats it like a hostile invader. Doctors aren’t entirely sure why this happens, but it’s more common in people with a history of infections or other immune diseases.

3. Chronic hepatitis may not show any symptoms.

Chronic hepatitis is diagnosed when the condition lasts longer than six months. Sometimes it develops following a bout of acute hepatitis, but more often it’s asymptomatic. Vague signs of this form of hepatitis may include malaise, fatigue, and nonspecific upper abdominal discomfort. It’s under-diagnosed, but if patients suspect they have hepatitis symptoms, they can get a liver function test, a viral serologic test, or other blood work done to confirm it’s there.

4. Yellow eyes and skin are common symptoms of acute hepatitis.

Unlike chronic hepatitis, acute hepatitis quickly presents clear signs. These include pale stool, dark urine, fatigue, loss of appetite, and flu-like symptoms. One of the tell-tale symptoms of hepatitis is jaundice, which is characterized by yellowish skin or eyes. This occurs when bilirubin, an orange-colored waste material produced by the normal breakdown of red blood cells, builds up in the blood because the liver isn’t functioning properly.

5. Some types of hepatitis can be prevented with vaccines.

Hepatitis types A and B can both be protected against with vaccines. The hepatitis A vaccine is administered in two doses six to 18 months apart and the hepatitis vaccine is doled out in three shots over six months. Cases of hepatitis B in the U.S. have dropped by as much as 73 percent since the vaccine was first introduced in the 1980s and hepatitis A cases have declined by 95 percent in the same time period.

6. There's no vaccine for Hepatitis C—but doctors are working on it.

Hepatitis C is the most common form of viral hepatitis, but there's still no vaccine for it. Scientists have identified at least six genetically distinct types of the virus, and about 50 different subtypes. This makes it difficult to develop a one-size-fits-all vaccine for hepatitis C, but medical experts have been working on one since the disease was first detected 25 years ago.

7. Some types of hepatitis can be cured.

There’s no specific therapy for hepatitis A once you contract it, but treating it is simple: With plenty of bed rest and hydration, the symptoms should clear up on their own within a few weeks or months. Hepatitis B, on the other hand, has a cure. Pegylated interferon-alphaA, a weekly shot administered over six months, eradicates hepatitis B in 25 percent of people. When it doesn’t work, patients can take oral medications, like amivudine and adefovir, that suppress symptoms. People with hepatitis C can take a combination of pegylated interferon and ribavirin tablets to recover from the condition, but this treatment doesn’t always work and can cause harsh side effects that are hard for some patients to tolerate.

In people with non-viral hepatitis, avoiding the cause—whether it’s drugs, alcohol, or toxic chemicals in their environment—is the first and most important step toward protecting their liver. Patients with autoimmune hepatitis may need to take drugs like Prednisone that lower their immune activity. If chronic hepatitis has gone untreated for a long time and the liver is severely damaged, a liver transplant may be the only option.

8. Long-term effects of hepatitis can be deadly.

If left untreated for too long, chronic hepatitis can have severe health effects. Even when symptoms aren’t immediately apparent, hepatitis takes its toll on the liver. One of the more dire outcomes of this condition is cirrhosis, a deadly liver disease that occurs when scar tissue starts to overtake healthy tissue inside the liver. This stops the liver from functioning properly and can lead to gallstones, swelling of the legs and feet, increased blood pressure, chronic bruising and bleeding, and poisoning of the brain. Liver cancer is another potential long-term side effect of chronic hepatitis.

9. Baby boomers are more likely than other age groups to have hepatitis C.

Baby boomers, a.k.a. people born between 1945 and 1965, are five times more likely to have hepatitis C than the rest of the population [PDF]. Transmission of hepatitis C reached its peak in the 1960s through the 1980s, before regular screenings for the virus became common, which is when most Boomers living with the disease today likely contracted it. Health experts recommend that everyone in this age group be tested for hepatitis C even if they don’t exhibit symptoms.

10. Viral hepatitis kills more people than malaria.

There are more than 325 million people around the world living with viral hepatitis today—that’s roughly equivalent to 4 percent of Earth's population. Every year, the disease leads to 1.34 million fatalities, which makes it deadlier than HIV, tuberculosis, and malaria. While the death rates associated with those diseases are on the decline, deaths caused by viral hepatitis increased 22 percent between 2000 and 2015. In 2017, Charles Gore, then president of the World Hepatitis Alliance, said the spike can be blamed on a lack of funding and prioritization of hepatitis compared to other global health threats. Lack of awareness is also a problem: Just 5 percent of people with viral hepatitis realize they have it.

8 Scientific Benefits of Napping

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Even on the best of days, life can be exhausting. If you find your energy flagging in the middle of the day, you might like to know that 34 percent of Americans nap. Napping is a healthy way to restore the deficits of sleep deprivation. Whether you bow down to the ritual of a mid-afternoon siesta or never stop to snooze, you may think twice about the power of napping after reading about these eight benefits—just in time for National Nappy Day.

1. Napping can boost your immune system.

Sleep deprivation—particularly repeated, chronic lack of sleep—takes a toll on your neuroendocrine and immune functions by increasing inflammatory molecules known as cytokines, as well as stress hormones like cortisol and norepinephrine. A 2015 study in the Journal of Clinical Endocrinology & Metabolism took 11 healthy young men and restricted them to a night of only two hours of sleep. Blood and urine tests measured higher cytokines and levels of norepinephrine in both groups after sleep deprivation. The following day, one group was given two half-hour naps, while the control group did not have any naps. Blood and urine samples of those who napped showed that their cytokines and norepinephrine levels had returned to normal, as though they had never lost a night of sleep.

2. A nap can improve night alertness.

For people who work at night, or through the night, several studies have shown that naps from between 30 minutes and four hours long that are taken in advance of the shift—what's known as a "prophylactic nap"—improve performance and alertness. These naps can also improve nighttime driving alertness on the way home from the shift. However, most of these studies also include the administration of caffeine, which likely contributed. Yet a 1995 study in Sleep, which compared naps and caffeine, found that "naps, in general, provided longer and less graded changes in performance, mood and alertness than did caffeine, which displayed peak effectiveness and loss of effect within about six hours."

3. Naps + caffeine are a one-two punch against sleepiness. Just ask a surgeon!

Surgeons must often perform continuous surgery for hours longer than the average person would ever have to persist at a task. A 1994 study in the journal Ergonomics found that naps were indeed effective at keeping surgeons who had to remain awake for 24 hours alert, but only when caffeine was administered, too. Neither naps or caffeine alone were sufficient.

4. Frequent naps can improve daytime alertness.

Daytime napping also appears to improve mental alertness and performance, according to a number of laboratory studies. However, researchers found that shorter naps were more effective than longer ones. The most effective time of them all was 10 minutes, which produced the best outcomes in all sleep measures including "subjective sleepiness, fatigue, vigor, and cognitive performance." A 30-minute nap could produce the same effects but brought about "a period of impaired alertness."

5. Naps can help you learn new skills.

If you want to get better at learning a new skill, you might want to take more frequent naps. A 2006 study in Biological Physiology broke participants into two groups: those who napped frequently and those who napped sporadically. Each group was given a nap before a reading task. Habitual nappers—people who reported napping frequently—did better on the reading and retention task. Researchers determined that the brains of habitual nappers consolidated motor learning better, which is part of the process of learning a new skill.

6. Napping can improve your physical stamina.

It turns out that napping is not only just good for mental processes, but has a positive impact on physical stamina and performance as well. A 2007 study in the Journal of Sports Sciences put 10 healthy men through a series of sprints before and after a 30-minute, post-lunch nap. Sprint times improved after the naps, suggesting to the researchers that a post-lunch nap "improves alertness and aspects of mental and physical performance following partial sleep loss." They suggest that napping may be an important part of the regimens of athletes who are undergoing restricted sleep during training or competition.

7. Want to improve your memory? Take a nap!

One of the many functions of regular nighttime sleep is to consolidate memory. A 2010 study in Neurobiology of Learning and Memory set out to see whether daytime naps also improve memory processes, particularly associative memory (the ability to make connections between unrelated objects). Thirty-one healthy participants were given a learning task at 12 p.m. to memorize two sets of face-object photograph pairs. The objects in each pair occurred in both sets but were paired with different faces. Participants were broken into two groups: those who had a 90-minute daytime nap or those who did not. At 4:30 p.m., participants who napped showed notably better retention of associative memory.

8. A 90-minute nap is as good as a full night's sleep for perceptual learning.

Previous research demonstrated that people perform better on a visual texture-distinguishing task after a night of sleep than they do immediately after learning it. A 2003 study in Nature Neuroscience found that people performed just as well on the test after a 60- to 90-minute nap as they did after a full night of slumber.

"What's amazing is that in a 90-minute nap, you can get the same [learning] benefits as an eight-hour sleep period," lead author Sarah Mednick said in an interview with the American Psychological Association. "The nap is having an additive benefit on top of a good night of sleep."

This article originally ran in 2017.

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