For decades, 98.6 degrees Fahrenheit has been the widely accepted “normal” average temperature for the human body.
But new research adds to the growing body of evidence that humans actually run a bit cooler. In addition, the study suggests there’s no such thing as a “normal” body temperature, because readings vary greatly depending on a variety of factors, from the person’s age to the time of day.
Researchers described these and other findings last month in JAMA Internal Medicine.
Scientists analyzed the oral temperature measurements of more than 126,000 adults seen at Stanford Health Care between 2008 and 2017. They also compiled information about each patient, such as their height, weight, sex and age, and made note of the time of day each temperature was taken.
They found the average human body temperature to be around 97.9 degrees, nearly a degree lower than 98.6. But are humans actually getting cooler? Maybe, but maybe not.
The belief in the 98.6 degree average dates back to 1868, when German physician Carl Wunderlich took more than a million temperature measurements from 25,000 people.
But a lot has changed in the world of scientific and medical research over the last 150 years, so holding up today’s studies against Wunderlich’s may not necessarily be an apples-to-apples comparison.
“Statistics were not in common use then, much less computers,” Philip Mackowiak, a physician at the University of Maryland, told National Geographic’s Brian Gutierrez last year. “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.”
Where on the body Wunderlich took the patients’ temperatures may also have something to do with the difference. For instance, oral temperatures tend to be lower than rectal temperatures.
In addition, 98.6 degrees was simply the number that stuck in the public consciousness when, in reality, Wunderlich reported a range of temperatures. He also noted that measurements varied depending on the time of day, as well as patients’ age and sex.
“Instead of thinking about a distribution in temperatures, which is what the initial study showed, we’ve taken a mean of 98.6 F and used it as a cutoff value,” says Catherine Ley, an epidemiologist at Stanford University who co-authored the new study, in a statement. “We’ve used an average value to create a false dichotomy of what’s normal and what’s not.”
It is also possible that average body temperatures have dropped in recent decades because, by and large, humans are getting healthier. Thanks to advances in medicine and dentistry, patients are dealing with less inflammation than they likely were 150 years ago, reports the New York Times’ Dana G. Smith.
The new study also finds variations in temperature depending on patient characteristics and time of day. For example, women tend to run hotter than men. Older people tend to have lower temperatures than younger people. And, among all demographics, temperatures tend to be lower in the morning and higher in the afternoon. (If you’re curious about what your temperature might be throughout the day, the team also built an online personalized temperature range calculator using their findings.)
An elevated temperature, or fever, suggests the body might be fending on some sort of infection—and that means the immune system is doing its job. Fevers are “actually not necessarily something we have to eliminate at all times," says Angela Mattke, a pediatrician at the Mayo Clinic, to CBS News’ Aliza Chasan.
But, since there’s so much variability in body temperature, a high reading for one person could be normal for someone else. Taken together, these and other recent findings suggest doctors should rethink the temperature benchmarks they use to determine when a patient is healthy and when something might be wrong. Individualized temperature evaluations and personalized fever thresholds might be more useful than comparing everyone to the average, the researchers say. For example, a doctor might consider how a patient’s current temperature compares to their historical readings.
“Most people, including many doctors, still think that everyone’s normal temperature is 98.6,” says study co-author Julie Parsonnet, also an epidemiologist at Stanford, in a statement. “In fact what’s normal depends on the person and the situation.”