Whenever a person thinks they have a fever, the first step is to take their temperature and see if they’re far off from what’s considered healthy — 98.6 degrees Fahrenheit. That’s been the “normal” temperature for over a century, based on research from 1871 of millions of readings in the U.S. and Europe. But a new study found that body temperatures have actually decreased, and that number is likely outdated.
Researchers at Stanford University wrote in a paper published in the journal eLife that human body temperature has decreased since that early research, and doctors likely need to update what is considered normal, TIME reported.
Using information from three databases, the researchers analyzed over 677,000 temperature readings from almost 190,000 people that was collected between 1862 and 2017.
Over that span, temperatures dropped on average by 0.03 degrees Celsius in men and 0.29 degrees Celsius in women. That would change the normal body temperatures to 98.5 degrees Fahrenheit in men and 98.1 in women.
Julie Parsonnet, a professor of medicine at Stanford University School of Medicine who led the study, told TIME that human bodies have changed in other ways, so it makes sense that their average temperature would too.
“We have grown in height on average, which changes our temperature, and we have gotten heavier, which also changes our body temperature,” she said. “[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. It’s not beyond the imagination that our body temperatures would change as a result.”
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Parsonnet added that the new medicines and vaccines that have stopped many of the infectious diseases more common in the 19th century are also a major factor.
“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 said. “Plus, we conquered general inflammation with non-steroidal anti-inflammatory drugs and statins, all of which enable us to live almost inflammation-free.”
While this research points to a need for a new average temperature, Parsonnet said that doctors should instead think about a creating personalized average temperatures based on a person’s height, weight, age, time of day and the outside temperature.
“We can estimate better what a normal temperature is for an individual person than we currently do,” she said. “Now we use one number, but that value may not make sense, since a normal temperature may be different for different people.”