What Exactly is 'Normal' Human Body Temperature—and Has It Really Decreased?

That's what a new study published in the journal eLife says, but not all experts agree.

It's a number you've heard your entire life, either at the doctor's office, or from your mother: 98.6°F—the standard normal human body temperature.

But that number is more than just a reading on a thermometer. While it varies from person to person, your body temperature is essential to your health, and can signal if something is just slightly off, or if something is majorly wrong.

Okay, first: What is normal—and abnormal—when it comes to the human body temperature?

According to the US National Library of Medicine, a division of the National Institutes of Health (NIH), human body temperature can vary by age, activity, and time of day, but it's generally accepted as 98.6°F (or 37°C). But that's a very specific number, so it's important to think of normal human body temperature as more of a range: 97°F (36.1°C) to 99°F (37.2°C) is the best estimate, per the NIH.

That number—98.6°F—has been the standard for nearly 150 years (German physician Carl Reinhold August Wunderlich first determined it in 1871, after conducting readings in millions of patients).

Human body temperature becomes abnormal when it dips below or rises above that range of readings. According to the NIH, a fever is any temperature over 99.5°F in adults—and it's not necessarily a diagnosis or disease, but rather a symptom: It's a sign that your body is trying to fight off an illness or infection. (Just FYI: most bacteria and viruses thrive in your normal body temperature, but when that temperature increases, it's harder for them to survive. Fevers also activate your immune system, to help your body fight off illness.)

Conversely, human body temperature can dip too low—readings below 95°F are considered hypothermia, according to the Centers for Disease Control and Prevention (CDC). It is caused by prolonged exposure to extremely cold temperatures, which forces your body to lose heat faster that it's produced, eventually using up your body's stored energy. Eventually, the brain will be impacted by low body temperature, and the individual might have trouble thinking clearly or moving well—often, someone who is experiencing hypothermia won’t even know that it’s happening and won’t be able to react.

What about the new study that says 98.6°F isn't the normal human body temperature anymore?

So, according to new research published in the journal eLife, what we consider “normal” body temperature may have dropped in the last century and a half by nearly one entire degree. For the new study, published January 7, 2019, researchers from Stanford University analyzed over 650,000 readings taken from over 190,000 people spanning over two centuries, concluding that the “norm” has dropped to 97.9°F.

“Our temperature’s not what people think it is,” Julie Parsonnet, MD, the study’s senior author and a professor of medicine and of health research and policy at Stanford University, said in a statement. “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”

Using three sets of data taken over the years, researchers observed that average body temperature has actually decreased over time—on average dropping between 0.03°C and 0.29°C per decade for men and women, respectively.

Dr. Parsonnet believes this could be due to various factors. “We have grown in height on average, which changes our temperature, and we have gotten heavier, which also changes our body temperature. [Today,] we have better nutrition, better medical care, and better public health. We have air conditioning and heating, so we live more comfortable lives at a consistent 68°F to 72°F in our homes, so it’s not a struggle to keep the body warm,” she explains. “It’s not beyond the imagination that our body temperatures would change as a result.”

Then, there is the fact that many of the infectious diseases which would cause a spike in temperature that were present 150 years ago, are now treatable. “We have gotten rid of many of the inflammatory conditions that people had—tuberculosis, syphilis, periodontal disease, wounds that didn’t heal, dysentery, diarrhea—with antibiotics and vaccines,” she continued. “Plus, we conquered general inflammation with non-steroidal anti-inflammatory drugs and statins, all of which enable us to live almost inflammation-free.”

However, some medical experts are skeptical about the findings of the study. “While intriguing, there are so many methodological issues with the study, it's hard to know how valid these results are,” Bethesda, MD internist Matthew Mintz, MD, tells Health.

Additionally, he points out that even if body temperature has dropped, it is unclear if it even means anything. “The body temperature is fairly constant because it is at this temperature where the body's chemical processes function the best,” he points out. “Even if you are in the hot sun or freezing cold, the body maintains its temperature for optimal functioning."

Philip Mackowiak, MD, emeritus professor of medicine at the University of Maryland School of Medicine, also has some hesitation about the new study. In an interview with Scientific American, Dr. Mackowiack said that scientific data dating as far back as the Civil War is "inherently suspect," adding that different variables may not have been controlled for, like whether or not the soldiers were sick, where the thermometer was inserted, and what type of instrument was used.

But, while experts may not entirely agree on whether normal human body temperature is actually declining, they do apparently agree on one thing: The temperature that is currently considered a fever is still a fever and should not be changed. "Temperature can be helpful in determining whether or not you’re ill and, based on its level, how ill you might be,” Dr. Mackowiak told Scientific American.

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