Stanford University researchers recently debunked the common knowledge that 98.6 degrees is the average body temperature. 

The notion dates back to the mid-19th century, established by German physician Carl Reinhold August Wunderlich in 1851. A recent British study found the average temperature of 25,000 patients to be 97.9 degrees. 

Figuring out the true cause of temperature change is very challenging, said Dr. Julie Parsonnet, senior author of the study and professor of medicine and of health research and policy, Stanford University. For the study, which was published in the journal eLife, Parsonnet and her colleagues analyzed three sets of temperatures from three distinct historical periods: between 1862 and 1930, 1971 to 1975 and 2007 to 2017. 

On average males born in the 2000s have a body temp 1.06 degrees lower than that of men born in the early 1800s. For women that number is on average 0.58 degrees lower. That’s a decrease in body temperature of 0.05 degrees every decade. 

One of the things researchers looked at is whether thermometers are just more precise now. In the mid-1800s the science of thermometers was pretty new; today’s are much more accurate, Parsonnet said. By examining each data set researchers ruled error out. Analyzed separately, each set showed a pattern of declining temperatures over time. 

What exactly is causing the decrease in body temp is unknown. 

“I could list thousands of correlations but none of those prove causation,” Parsonnet said. 

One reason our temperatures may be decreasing is because we’re healthier. 

“In the 19th century when the 98.6 value was established, many people lived with chronic diseases. Life expectancy was less than [age] 40. A significant percentage of the population would have had diseases that are rare today in their chronic forms like tuberculosis, syphilis and rheumatic heart disease. They also would have had recurrent bouts of diarrhea, skin infections, chronic non-healing wounds and all those vaccine-preventable diseases that we don’t see” in modern times, Parsonnet said. 

Additionally, because they had little heating and no air conditioning, people’s bodies had to consume more calories to stay at a good temperature. 

“So it should be surprising that their temperature, which is a measure of how hard the body has to work to keep the body functions going, might be revved up,” Parsonnet said. The main significance of the finding is that body temperature is changing over time, she said. “We all have heard about 98.6 since our mothers told us, ‘You’re not sick. Your temperature is 98.5. You need to go to school.’ So everyone is saying in their heads, ‘I told you I was sick, mom!,’” Parsonnet said. “But I do want to say that the significance of our finding is not that temperature is lower than 98.6. People paying attention have known that 98.6 is too high for decades.” 

Credit Dr. Philip Mackowiak at the University of Maryland, who brought the issue to light in 1997 with a study published in JAMA, Parsonnet said. 

It’s better to think about temperature as fluctuating. “Personally, I think people are way too fixated on body temperature as a marker of illness,” Parsonnet said. “Yes, it can be quite useful when it is at the extremes. A temperature of 103 or 104 is definitely a cause for alarm and should result in a medical assessment.” 

A little movement is OK. “If you feel sick and your temperature is only 98.6 or even 97.8, it doesn’t make you not sick. It’s the big picture, not the number that matters,” she said. Remember that people can transmit infections even when they are feeling well, either because they don’t have symptoms or they have a mild illness. 

“We should be rethinking the rules about staying home from school or work. Shouldn’t we care more about how people feel and whether they are coughing or sneezing, than that specific number?” she said.