Now average body temperature is 97.9 degrees

Over the past few decades, evidence has been mounting that the average human body temperature is not really 98.6 degrees Fahrenheit. Instead, most people’s baseline is a little bit cooler.

The standard of 98.6 was established over 150 years ago by the German physician Dr. Carl Wunderlich, who reportedly took over a million measurements from 25,000 people. Temperatures ranged from 97.2 to 99.5, and the average was 98.6. Dr. Wunderlich also established 100.4 degrees as “probably febrile.”

However, a study published in September that evaluated the temperatures of more than 126,000 people between 2008 and 2017 found that the average is closer to 97.9 degrees. Other modern-day studies have reported similar numbers.

Experts who study body temperature have differing opinions about why we appear to have gotten cooler over time, and whether that matters when it comes to evaluating fevers and diagnosing infections.

Experts think humans really have gotten cooler over the past 150 years. Our temperatures may have declined because “we are so lucky to be healthier than we used to be,” said Dr. Julie Parsonnet, a professor of medicine and of epidemiology and population health at Stanford Medicine, who led the September study on body temperature.

Regardless of the reason for the shift, the experts interviewed for this article agreed that 98.6 degrees should no longer be considered the universal human standard. But instead of shifting the average temperature down a degree or so, it should be given as a range, said Dr. Waleed Javaid, a professor of medicine at the Icahn School of Medicine at Mount Sinai, who published a 2019 review paper on body temperature.

A range would account for the natural variability in temperature that occurs across gender and age — women tend to run slightly warmer than men, and older adults run cooler than younger people. Additionally, everyone’s body temperature fluctuates throughout the day — it is typically lowest in the morning and highest in the late afternoon. “Like there’s a range for heart rate, there’s a range for blood pressure,” temperature also has a range, Dr. Javaid said.