Credit, BBC/Serenity Strull
In the 1860s, German physician Carl Wunderlich faced a fundamental question: was fever a disease in itself or just a symptom of an underlying illness?
At a time when thermometers were not routinely used, he embarked on an ambitious mission. Over 15 years, he collected the axillary temperatures of 25,000 patients — of all ages and both sexes — totaling more than a million measurements.
From this vast data set, Wunderlich calculated an average of 37°C (98.6°F) for healthy adults. This value has become the reference for monitoring normal human body temperature. According to him, if the temperature exceeded 38 °C (100.4 °F), it would indicate the presence of a fever.
Although other researchers had already reached the same conclusion, Wunderlich's data set was so extensive that its value came to enjoy "privileged status" as a universal diagnostic tool, says Philip Mackowiak, a medical historian and infectious disease expert at the University of Maryland.
More recently, however, different research has shown that the average body temperature considered normal has changed over the last century and a half.
Could it be that the original benchmark was incorrect? Or is a rapidly changing world altering our biology?
Cooling down
Wunderlich "established much of what we know to be true" about body temperature variation, says Julie Parsonnet, an infectious disease physician at Stanford University.
From his dataset—which comprised mostly working-class patients at the Leipzig hospital, where he served as medical director—Wunderlich discovered that normal temperature varies. He observed that women have higher temperatures than men, that the temperature decreases with advancing age and that it also increases throughout the day.
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However, it was found that, today, the average temperature of individuals is lower than that of their peers 150 years ago; in the case of American men, the temperature is about 0.59 °C lower than that recorded at the beginning of the 19th century.
A decade ago, Parsonnet's team analyzed three large data sets, comprising multiple temperature measurements of nearly 190,000 people in the United States taken between 1862 and 2017. "The new normal [for adults] appears to be closer to 97.5°F (36.39°C)," she says.
Several studies, done before and since, corroborate Parsonnet's findings.
This downward trend is not just restricted to people living in developed countries. The average body temperature of the Tsimané — a semi-nomadic indigenous community that inhabits the Amazon rainforest — was around 37 °C in the early 2000s, according to biological anthropologist Adrian Jaeggi, from the University of Zurich, Switzerland. However, in 2018, after two decades of rapid socioeconomic transformations, this average fell to 36.5 °C.
Why have average body temperatures decreased?
There are several possible explanations for the lower averages we see today. Parsonnet, for example, believes that this is a reflection of a generally healthier population. In Wunderlich's time, life expectancy was 38 years; today, it is almost double that.
"People weren't leading the kind of healthy lives we do now," she says. "They often suffered from a variety of health problems, such as tuberculosis, poor dental conditions, dysentery, rheumatic fever and osteomyelitis."
"When the immune system tries to defend us against all these threats that put our lives at risk, it consumes calories. And calories are, in practice, a form of heat," explains Parsonnet.
Likewise, among the Tsimane, vaccines and improved basic sanitation have brought health gains, says Jaeggi. Using bathrooms and shoes has also become more common today than it was 20 years ago, he notes, which reduces the incidence of hookworm (parasitic worm) infections contracted by walking barefoot on contaminated soil.
The air temperature indoors is also easier to control these days. Air conditioning is present in at least 88% of homes in the United States, while a similar proportion of homes in the United Kingdom have central heating.
"We spend most of our lives in an ideal climate — what we call the zone of thermal neutrality — where we don't need to make any effort to maintain our body temperature," says Parsonnet. "And, if we don't need to make effort, we don't waste energy and, consequently, we don't produce heat."
Could it be that Wunderlich was mistaken?
Mackowiak, however, believes that the drop in average body temperature may have an entirely different cause.
When he had the opportunity to personally examine one of Wunderlich's glass thermometers, he noticed that the instrument was calibrated about 2°C higher than modern thermometers. This means that any measurement made with these devices would present a value 2°C higher than the real one.
At that time, measuring temperature was a laborious task, says the medical historian.
The mercury-in-glass thermometers, about 30 centimeters long, needed to be calibrated in advance based on fixed points, such as boiling water and melting ice. Patients then needed to remain still for 15 to 20 minutes, as "it took a while for the temperature to register on these thermometers," he explains.
In other words, the value obtained by Wunderlich may have been incorrect from the beginning. Furthermore, he took measurements in the armpit, and not in the mouth — as is more common today —, which makes it difficult to compare his data with current data.
Parsonett, however, does not believe that improper calibration is the cause of Wunderlich's overestimation. When analyzing, with his team, body temperature data collected between 1862 and 1930 from veterans who served in the Army during the American Civil War, they found that individuals born earlier, in fact, had higher average temperatures than those born later.
"As we compare people born in different years who had their temperature measured at the same time, and people born in the same year who were measured at different times, it is possible to isolate the effect of the instrument from the age and year of birth variables," she points out.
Parsonett's team got additional confirmation by examining two other, more recent data sets in which they observed a similar drop. This data comprised temperatures recorded between 1971 and 2017 "using thermometers that were presumably subject to equivalent calibration standards."
A new temperature guide
Does all this mean it's time to rethink the importance attached to 37°C (98.6°F)?
Some have already done so, including two reference sources widely used by doctors: Harrison's Principles of Internal Medicine and the online database UpToDate.
However, while having a baseline for your own body temperature can be a useful guide, most experts caution against placing too much emphasis on any single value.
"It's important not to take these types of limits too seriously, as they don't represent rigid divisions — no one would argue that 36.99°C is in any way radically different from 37.01°C," says Jaeggi.
Still, when your temperature drops too much or rises too much — typically below 35 °C (96 °F) or above 40 °C (104 °F) in adults — this is a sign that something more serious may be happening in your body.
*All content in this report is provided for general informational purposes only, and should not be considered a substitute for medical advice from a healthcare professional.
Read the original version of this report (in English) on the BBC Future website.
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