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Average Adult Body Temperature Is 97.9, Study Suggests

While 98.6 degrees Fahrenheit is often cited as the standard for normal body temperature, adults' average body temperature may be closer to 97.9 degrees Fahrenheit, according to a study published Sept. 5 in JAMA Internal Medicine.

Researchers analyzed data on 396,195 outpatient encounters at Stanford (Calif.) Health Care that included temperature measurements from April 28, 2008, through June 4, 2017. Patients that had diagnoses or medications associated with extremely high or low body temperatures were excluded from the analysis.

Normal body temperatures ranged from 97.3 to 98.2 degrees Fahrenheit, and the overall average was 97.9 degrees Fahrenheit.

Researchers also found normal body temperature varies from person to person based on physical characteristics and can fluctuate throughout the day. Statistical modeling revealed temperatures decreased with age and height but increased with weight. Time of day had the most influence on body temperature — patients' temperatures were lowest in the morning and highest around 4 p.M. 

"Most people, including many doctors, still think that everyone's normal temperature is 98.6 F. In fact what's normal depends on the person and the situation, and it's rarely as high as 98.6 F," study author Julie Parsonnet, MD, the George DeForest Barnett Professor in Medicine and professor of epidemiology and population health at Stanford Medicine, said in a news release.

Normal Body Temperature Found To Vary Between People

You might think you know what a normal body temperature is, but there is no such thing.

Analyzing the age-old belief that 98.6 Fahrenheit is normal human temperature, scientists at Stanford Medicine found that your temperature is personal.

It also depends on age, sex, height and weight, and changes throughout the day.

"Most people, including many doctors, still think that everyone's normal temperature is 98.6 F. In fact, what's normal depends on the person and the situation, and it's rarely as high as 98.6 F," said senior study author, Dr. Julie Parsonnet, a professor of medicine.

The normal temperature of a tall, underweight 80-year-old man in the morning could be a degree lower than the afternoon temperature of a 20-year-old woman who is obese, she explained in a Stanford news release.

Past research at Stanford found that Americans' average body temperature has dropped from 98.6 F by about 0.05 F every decade since the 19th century. This is likely due to better health and living conditions that reduce inflammation.

Today, a so-called normal body temperature is nearer to 97.9 F, the researchers noted.

The idea of 98.6 F comes from a German study published in the 1860s. But even then, researchers noted that men and the elderly had lower temperatures than women and young adults. Temperatures in that study were also higher in the afternoon.

"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," said lead author Catherine Ley, a senior research scientist. "We've used an average value to create a false dichotomy of what's normal and what's not."

To provide updated research on this, Stanford scientists analyzed more than 618,000 oral temperature measurements from adult outpatients seen at Stanford Health Care from 2008 to 2017.

The research tracked time of day, along with each patient's age, sex, weight, height, medications and health conditions.

The investigators also applied a machine learning algorithm to identify diagnoses and medications that were disproportionately associated with extremely high or extremely low temperatures, to keep the data from being distorted by illness.

About one-third of patients were excluded from the analysis for having these diagnoses. These included infectious diseases linked to high temperature and type 2 diabetes linked to low temperature, which was a new finding.

The upshot: Adults have normal temperatures ranging from 97.3 F to 98.2 F, with an overall average of 97.9 F.

Men tended to have lower temperatures than women. Temperatures decreased with age and with height, and increased with weight.

Time of day exerted the biggest influence with temperatures coolest in the early morning and warmest around 4 p.M.

About one-quarter of temperature variability from person to person could be credited to age, sex, height, weight and time of day.

That means other factors not studied account for the remainder. This might include clothing, physical activity, menstrual cycle, measurement error, weather, and drinking a hot or cold beverage.

These individualized benchmarks could make body temperature a more accurate and useful vital sign, Parsonnet said, recalling her elderly mother-in-law's experience.

In her case, she was not diagnosed with a serious heart infection for weeks because her temperature never reached a conventional fever, usually defined as higher than 100.0 F or 100.4 F.

Future studies could look at personalized definitions of fever and whether having a consistently higher or lower normal temperature affects life expectancy, Parsonnet said.

"There's a lot of temperature data in the world, so there's a lot of opportunity to actually learn something about it," she said.

The study was published Sept. 5 in JAMA Internal Medicine.

More information: Catherine Ley et al, Defining Usual Oral Temperature Ranges in Outpatients Using an Unsupervised Learning Algorithm, JAMA Internal Medicine (2023). DOI: 10.1001/jamainternmed.2023.4291

The U.S. National Library of Medicine has more on fever.

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Best Weather Model Shows Warmer-than-normal For A Long Time

Many of you are asking when it's going to turn cold. Let's try to figure that out.

Firstly, remember that I don't say if it's going to be cold or warm. I look at if it's going to be colder than normal or warmer than normal. Obviously at this time of year we are losing the heating power of the sun. Warmer than normal temperatures at Halloween will be in the 60s with a shot at 70 degrees. That's why I don't tell you if it will be hot or cold. I let you decide if 70 degrees late in October is warm.

I guess I just let the cat out of the bag on the projection for temperatures through October.

The European Model, which is usually the best model for extended forecasts, has an output that goes out 45 days in the future. That long of a projection brings us to November 5.

Here is the upper-level wind flow forecast for the next 45 days. If you see red it means the atmosphere is expanded and taller due to warmer-than-normal conditions.

Forecast of wind flow and upper-air conditions from September 25 ro November 5

The main take-away here is the atmosphere over most all of the U.S. Is expected to be expanded due to warm air aloft. You don't see much blue on this map.

But who cares about what's going on in the upper atmosphere, right? What goes on aloft eventually transfers down to the surface. If it's warmer than normal aloft it will also be warmer than normal at the ground.

The next forecast is the surface temperature anomaly for the next 45 days out to November 5.

Surface temperature anomaly forecast from September 25 to November 5.

There are a couple of quick things to go over here. The output is in degrees Celsius. To turn the temperature anomaly forecast into something meaningful double the Celsius output. This means we start October a few degrees warmer than normal, warm to 10 degrees above normal a week into October and then ease back to a few degrees warmer than normal to end the month.

Also notice anomalously cold air is nowhere in sight as we head toward November.

Winter will eventually get here, but right now fall looks very warm.

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