The average human body temperature has dropped, study says

The temperature 98.6 degrees Fahrenheit may be seared into your brain as the average temperature of the human body, but a recent study shows that that’s no longer the case.

Researchers at the Stanford University School of Medicine found the average human body temperature has dropped since the 1800s. That’s when the standard temperature of 98.6 degrees Fahrenheit was established by German physician Carl Reinhold August Wunderlich.

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Julie Parsonnet, MD, professor of medicine and of health research and policy, said in an eLife podcast that the physician "took about a million temperatures on 25,000 people and on the little pieces of paper, wrote them down and did all these calculations figuring out what is the right temperature for the time of day and for women and for men ... But things have changed since the 19th century."

Wunderlich published the number in an 1868 book. Another recent study found that on average, British patients’ temperatures were 97.9 F.

“Our temperature’s not what people think it is,” said Parsonnet in a press release. “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”

In a study published in the open-access journal eLife, Parsonnet and her colleagues analyzed temperatures from Civil War veterans in the Union Army. Military service, medical and pension records data from 1862 and 1930 were taken into account for the soldiers, some of whom were born in the early 1800s. Then, researchers compared the veteran's temperatures to two other groups.

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One was a set from the U.S. National Health and Nutrition Examination Survey I, which includes data from 1971 to 1975. The other was clinical data from the Stanford Translational Research Integrated Database Environment. It includes data from adult patients who sought treatment from Stanford Health Care between 2007 and 2017. Researchers analyzed 677,423 temperature measurements and developed a linear model that gradually interpolated temperature.

“We found that the temperatures were declining over time, and we couldn’t explain it by just the thermometers that were used or the way they took their temperatures,” Parsonnet said in the podcast.

Parsonnet added that since the 19th century, people’s homes and hygiene has improved. They’ve also gotten taller, bigger and colder. People are also much healthier, she said, noting those who lived in the 1800s “just had a really harder life. They had a lot more infections —things like tuberculosis and syphilis and rheumatic heart disease — and they lived under harsher conditions ... Their lives were just harsh enough that they had to rev up their bodies all the time to keep themselves safe, to keep themselves healthy.”

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Researchers found that on average, the body temperature of men born in the early to mid-1990s is 1.06 F lower than average body temperatures of men born in the early 1800s. Similar results followed for women. Researchers discovered that the average body temperature of women born in the early to mid-1990s is 0.58 F lower than women born in the 1890s. With these calculations, body temperature decreased an average of 0.05 F every decade.

“Physiologically, we’re just different from what we were in the past,” Parsonnet said in a press release. “The environment that we’re living in has changed, including the temperature in our homes, our contact with microorganisms and the food that we have access to. All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re actually changing physiologically.”