Time to redefine normal body temperature?

Normal body temperature is 98.6˚ F, right?

That's certainly what we're all taught, and it's the right answer on a test. I know it seems crazy, but 98.6˚ F may not, in fact, represent the best estimate of normal body temperature. Not only that, but normal body temperature may be falling over time, according to data samples reaching back almost 160 years.

Where did 98.6 degrees come from?

In the mid-1800s a German physician, Carl Wunderlich, measured axillary (armpit) temperatures from about 25,000 people and found that the average was 98.6˚ F (37˚ C). And so, we've believed that ever since.

But more modern studies have called this time-honored truth into question, and have found that normal body temperature

• Varies throughout the day, tending to rise later in the day.
• Varies among individuals. Women tend to have higher body temperature than men, and younger people tend to have higher temperatures than older folks. Height and weight also may affect body temperature, according to a 2023 study.
• May be less than 98.6˚ F. Average oral temperature was 97.5˚ F in an analysis of 20 studies published between 1935 and 1999. Similarly, the 2023 study and an earlier study of more than 35,000 people pegged average body temperature at 97.9 F.
• Seems to have fallen over the years. Normal body temperature has been drifting down over the last two centuries, according to a 2020 study.

Are humans getting cooler?

Yes. That 2020 study analyzed temperature recordings from three periods of time over 157 years:

• 1860–1940: A mix of armpit and oral temperatures of nearly 24,000 veterans of the Civil War were measured.
• 1971–1975: Oral temperatures of more than 15,000 people from a large population study (the National Health and Nutrition Examination Survey) were analyzed.
• 2007–2017: Oral temperatures of more than 150,000 people in another large research project (the Stanford Translational Research Integrated Database Environment) were reviewed.

During the nearly 160 years covered by the analysis, the average oral temperature gradually fell by more than one degree. As a result, the new normal seems closer to 97.5˚ F.

Why would average body temperature be falling?

Two key possibilities are:

• Lower metabolic rate: One of the biggest determinants of body temperature is your metabolic rate. Like a car engine that's idling, your body expends energy just keeping things going, and that generates heat. A lower metabolic rate in modern times could be due to higher body mass (some studies link this with lower metabolic rate), or better medical treatments, preventive measures, and overall health.
• Lower rates of infection and inflammation: In Wunderlich's day, tuberculosis, syphilis, chronic gum disease, and other inflammatory conditions that can raise body temperature were common, and treatments were limited.

What about changes in how body temperature is measured?

The method of temperature measurement varied in these different studies. But the repeated findings that normal body temperature seems to be falling suggest that this is unlikely to have affected the results. And average body temperature dropped even during periods of time when methods of measurement did not change.

Why body temperature — and changes over time — matter

Body temperature is vital to health; that's why it's among the "vital signs" — along with blood pressure, heart rate, and breathing rate — routinely checked by your doctor. These measures are critical when evaluating someone who may be sick, because significant abnormalities can indicate major, even life-threatening, illness.

Thousands of chemical reactions occurring simultaneously and continuously in the body require a rather narrow range of temperature. As a result, our bodies do not tolerate wide fluctuations in temperature very well. In fact, severe hypothermia (low body temperature) or hyperthermia (high body temperature) may cause permanent organ damage or death. That's why the body has such an elaborate thermoregulation system that keeps the its temperature close to ideal most of the time.

Does the definition of fever also need to change?

Fever is usually defined as any temperature above 100˚ F. The most common cause of fever is any infection in the body, but there are other causes, including heat stroke or a drug reaction. Although you can be sick with a normal temperature, body temperature is clearly an important and useful indicator of health.

Metabolic rate, infection, and inflammation in the body all influence human health and longevity. So, a falling average body temperature over the last century and a half could reflect important changes and warrant additional research. However, the new studies did not assess how fever is defined –– or whether it needs reassessment.

The bottom line

While news that the normal body temperature may be drifting down over time is intriguing, it is not cause for alarm. We'll need to rely on additional research to tell us how important these findings may be. In the meantime, it's probably time to abandon the assumption that 98.6˚ is a normal temperature. Something closer to 97.5˚ may be more accurate.