A new study has found evidence that the average blood temperature of people in the U.S. has lowered over the past century and a half—probably because we’re in better health.
The authors of the new study, published in the journal eLife, looked at the medical records of Americans, including Civil War veterans, from three periods of time: 1860–1940, 1971–1975, and 2007–2017. They found that people’s average body temperature, after accounting for things like age, height, and weight, has steadily declined over time, by roughly 0.05 Fahrenheit every decade since the 1860s.
The study isn’t the first to show that the golden number of a healthy blood temperature—98.6 degrees Fahrenheit—is outdated. That number came from a single study in the 1800s, conducted by a single doctor, that eventually became accepted as a universal truth. But we know now that our body temperature goes up and down all the time even when we’re healthy, depending on factors like the time of day or our age (the older we are, the colder we tend to be). And more recent studies have found that even the average body temperature of people today is below 98.6.
“Our temperature’s not what people think it is,” said study author Julie Parsonnet, professor of medicine and of health research and policy at Stanford University, in a release by the university. “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”
According to the authors, this is the first study to try and figure out whether this disparity in body temperature between the past and present is largely due to our changing environment or just because the thermometers back then were less accurate.
To rule out the possibility of a measurement error, they specifically looked at a group of people in the Civil War era who were tracked for decades, presumably using the same basic thermometer technology. Because they too had a slight downtick in their body temperature as the years went along, the authors argue, it’s likely that our bodies really have been cooling off over time.
Biologically, the authors say, there are plenty of plausible reasons why Americans are cooler today. One theory is that since we experience fewer infections, thanks to vaccination and sanitation, our bodies have less constant inflammation. We also have reliable heating and cooling in our buildings and homes, lessening the need for the body to work so hard to maintain homeostasis.
“Physiologically, we’re just different from what we were in the past,” Parsonnet said. “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.”
Parsonnet and other researchers have also argued that since body temperature so easily changes over the day and from person to person, it’s not really worth creating a new “healthy” number for people to obsess over. If you’re feeling hot, with a temperature above 100.4, then you likely have a fever. But otherwise, you’re probably fine.