What's the average human body temperature—and is it cooling down? - National Geographic UK

For 150 years, 37 degrees Celsius, or 98.6 degrees Fahrenheit, was thought to be the average body temperature for a healthy human being. But that number is wrong.

But for at least the past two decades, researchers have known that the average body temperature is actually colder, about 37.5 degrees Celsius (°C) – 97.8 degrees Fahrenheit (°F) – and that anywhere between 35.7 °C (96.3°F) and 37.3°C (99.3 degrees °F) is within a normal range for the human body. Yet 37°C, or 98.6°F, has endured as the magic number among concerned parents and doctors alike, displayed on everything from pharmacy thermometers to medical centres' webpages.

"Doctors are no different from anybody else," says Julie Parsonnet, an infectious disease doctor at Stanford University. "We've been raised with that as the normal value since we were little."

Every person is different, and many factors can affect body temperature, including age, body type, activity, diet, disease, time of day, and how the temperature is measured. Temperature is commonly measured inside the ear, under the tongue, in the armpit, rectally, or on the forehead. There is even an ingestible thermometer pill, and each of these different types of thermometers come up with slightly different average temperatures.

By studying the temperatures of healthy people and what factors can push the body outside a safe range, researchers can develop a better understanding of the workings of the body as a whole. To function properly, the human body needs to stay within a narrow temperature range spanning about 1.6 degrees Celsius, or 3 degrees Fahrenheit. Outside that range, neurones slow down and muscles and organs work less efficiently. Even the proteins in cells could be affected. So the body works hard to stay at a safe temperature by doing things like sweating when it is hot or constricting blood vessels in cold weather.

"That's all done by signalling from the [brain's] hypothalamus that tells you: Your blood is the wrong temperature," Parsonnet says.

Body temperature can also change in response to illness. A fever happens when the body turns up its temperature by a few degrees above normal, which is thought to kill some types of microbes and help the immune system work faster.

Because body temperature is so commonly measured and provides an important tool for studying health, researchers say that it is past time to reevaluate the internal heat of the human body.

Cooling off?

Parsonnet thinks 98.6 might not have always been wrong, but that it changed. People are cooling down, she suggests.

Initially interested in studying why the average American citizen has gotten heavier over time, Parsonnet began looking at the ways in which body temperature could be related to metabolism. "I've been thinking for years about trying to find a cohort that could show us what the [average body] temperature was decades ago," she says.

That cohort came from the Civil War Veterans Series, a data set of health information about Union Army veterans that Nobel Prize-winning economist Robert Fogel began compiling in 1978. The report contains information about the occupations, diseases, and disabilities of the veterans, including the temperature statistics that Parsonnet had been looking for.

In 2019 Parsonnet and her colleagues at Stanford combined the Civil War data with a health survey from the 1970s and modern data from Stanford University Medical Centre for a full data set of almost 200,000 thermometer measurements over the last 160 years. After analysing the data, they found that the average American's body temperature has decreased by about one degree Fahrenheit since the Industrial Revolution.

It's a change that tracks with Parsonnet's theory that human metabolism might be decreasing over time, along with other changes to the human body caused by better access to food and healthcare in industrialised countries.

People have "gotten taller, they've gotten fatter, they've gotten colder, and they live longer," Parsonnet says. "All those four things kind of go together."

A faulty thermometer?

Philip Mackowiak, professor emeritus at the University of Maryland School of Medicine, has a different theory for why 37°C (98.6°F) doesn't match modern temperature measurements. He thinks the number was never correct to begin with.

The standard temperature of 37°C (98.6°F) can be traced back to a book published in 1870 by the German physician Carl Rinhold August Wunderlich. "He was in a clinic in Germany, and according to his book … accumulated a million temperatures," Mackowiak says. "When he averaged out those temperatures, he came up with 37 degrees centigrade or 98.6."

Mackowiak says there are a number of reasons the original estimate of average temperature might have been off. For example, it's not clear exactly how Wunderlich came up with his average, and Mackowiak says the total data set was so large that Wunderlich likely only averaged a small number of his measurements.

"Statistics were not in common use then, much less computers," Macowiack says. "So how he could have processed a million data points and come up with the results that he did, it's just impossible to imagine."

Wunderlich's thermometers, which measured temperature from the armpit and had to be read while held in place for 15-20 minutes, might also have been a source of error. One of those original mercury thermometers is kept in Philadelphia's Mütter Museum, and Mackowiak borrowed the instrument for inspection.

"We did some careful studies to check its range," Macowaick says, "and [it] was calibrated a degree and a half higher than either modern or contemporary thermometers."

The long-standing inaccuracy has bothered Mackowiak for decades. In a 1992 article for the Journal of the American Medical Association, he and his colleagues wrote: "We believe that 37°C (98.6°F) should be abandoned as a concept having any particular significance for the normal body temperature."

When asked about the study suggesting average U.S. body temperature is decreasing, Macowiak says he has doubts. He thinks the data set in the paper was missing key factors, such as what types of thermometers were used and what time of day the temperatures were recorded.

"I have no way to know for certain one way or another," Macowiak says. "But my intuition is that no, it has not been decreasing over time."

Temperatures of the Tsimane

Michael Gurven, an anthropologist at the University of California, Santa Barbara, was also unsure at first about the decreasing temperatures reported in the Stanford study. "We were skeptical," he says. "So we reanalysed their data."

When Gurven and his team crunched the numbers, they found the same cooling trend. "We don't have a really great understanding about exactly why, but it does seem real that there's been a decline," he says.

Gurven was interested in the body temperature research because of his own work with the Tsimane people, subsistence farmers and hunters who live in Bolivia, relatively isolated from the outside world.

"Some of the villages are an hour's walk from the nearest town," Gurven says. "Other villages are several days away in a dugout canoe."

After reading the paper on decreasing body temperatures in the U.S., Gurven was curious to look at the average Tsimane body temperature. If changing lifestyles in industrialised countries are the reason that body temperatures have been decreasing, then people living without modern conveniences would in theory have higher temperatures.

To his surprise, Gurven and his colleagues found Tsimane body temperatures have also decreased over time. "We were seeing a similar level of decline, but over like one-tenth of the time period," he says.

Since Gurven started working with the Tsimane in 2002, he has found their average temperature has decreased from about 37°C (98.6°F) to 36.6°C (97.7°F).

The research provides another independent data set that suggests people are cooling down. But if that is true, there's no obvious reason why. "Everyone and their dog had a different idea," Gurven says.

Some of the possible causes include air conditioning, diet, chronic illness, immune system activity, dental disease, parasites, sleep patterns, and anti-inflammatory drugs.

For the Tsimane, it's possible to rule out some factors, like fast food and thermostats, but lots of little things have changed for the Tsimane over the past two decades that might have contributed to the change. They have better access to healthcare, leading to lower rates of disease that can affect body temperature, as well as more consumer goods like blankets, which could help people stay at a more comfortable temperature year-round.

All of these changes could add up to an overall decrease in average temperature, Gurven says.

Changing picture

So how does all this research relate to everyday health? If your temperature is 37 degrees Celsius (98.6 degrees Fahrenheit) should you be concerned?

"98.6 is a normal temperature, but not the normal temperature," Mackowiak says. "There is no the normal temperature."

If you must have a number, it may be useful to know the temperature that Mackowiak calls an "actionable fever," which is a temperature so high that a person should receive medical treatment even if they have no other symptoms. He says for hospitals, that number is generally 38.3 degrees Celsius – 101 degrees Fahrenheit – or above.

But temperature is not always a clear-cut measure of sickness. Even with a body temperature in the normal range, other symptoms can indicate something is wrong.

Body temperature "is just one indication of illness," Parsonnet says. "We like it because it's a number and everybody likes numbers … But the fact is, if you feel sick, you're sick, regardless of what your temperature is."