Turns out 37C isn’t normal body temperature after all

In case parents needed to hear another thing we’ve been getting wrong all along, researchers from Stanford University published a study recently that suggests we might need to rethink what we’ve been told are normal and fever temperatures. The study analyzed databases from the mid 1800s to 2017, and found that humans’ normal body temperatures are now running almost 0.5 degrees Celsius cooler, compared to the 1800s.

This means that normal human body temperature isn’t the 37C or 98.6F that most of us have been taught. Instead, ‘normal’ seems closer to 36.5C or 97.7F. What’s perhaps most surprising is that this isn’t even news. A number of studies in recent decades have shown that normal isn’t 37C.

We spoke to Jonathan Hausmann, a rheumatologist at Boston Children’s Hospital and Beth Israel Deaconess Medical Center, about what this means for parents.

Why are our normal body temperatures going down?

I'm not sure normal body temperature is going down. The ‘normal’ body temperature of 98.6F comes from research done in the mid-1800s by a German physician, Dr. Carl Reinhold August Wunderlich. It’s been called into question whether his thermometer was properly calibrated, so it’s possible the measurement was wrong to begin with.

But this most recent study is really interesting. It includes many more datasets, outside of the Wunderlich study, and shows declining temperatures over the years. There could be many reasons for this. It’s possible that people in the earlier datasets had a higher incidence of chronic infections, thus raising the average body temperature. We’re also using more medications today, including anti-inflammatories, that may be lowering temperatures in those who take them.

In my opinion, it’s more likely there are environmental causes or measurement differences that explain the differences found in temperature over the years, rather than the fact we are somehow "evolving" to have lower temperatures.

With so many studies in recent years debunking that 37C (98.6F) number, why is it still widely used?

It’s one of those ideas that’s failed to die. Even within the medical community, most physicians would tell you that that 98.6F is normal and 100.4F means fever.  Perhaps this is because in Celsius, 37 degrees (normal) and 38 degrees (fever) are convenient, round numbers. But when you look at the evidence behind it, there isn’t any, except for that study by Wunderlich over a century ago.

You did your own study on normal body temperature. What did it conclude about what is normal and what is feverish?

In our 2018 study, we recruited over 300 people to record 5,000 temperatures and enter their data in a smartphone app. We found average temperatures around 97.7F and we defined a fever as 99.5F. We also looked at the circadian rhythm to temperature relationship, which has been described in other studies, and found that temperatures are lowest at around 4 a.m and highest at 4 p.m. These results are supported by other trials done under controlled conditions.

How should this change how we measure our kids’ temperatures?

For parents, it’s helpful to know that if the temperature is 97F, that’s probably within normal limits, and when it’s 100F, especially if it’s early in the morning, that may be a true fever, and that may represent a mild infection.

It’s important to be aware of what time of day it is when you’re taking the temperature because normal body temperature naturally varies. So a temperature of 99 degrees at 4 in the morning may be a fever, whereas that same temperature at 4 in the afternoon is within normal limits.

Does this mean we should be treating kids’ fevers at lower thresholds?

I don’t think this information should change how parents manage illnesses. Parents hear advice that they should treat fevers above 102F, for example, but I haven't seen any evidence that says you must treat a fever above 102F.

In fact, some studies that have shown that if you give patients with the flu medications that reduce fevers, they have symptoms for longer.

Higher temperatures above 105F are concerning because they are more likely to indicate more serious infections. It’s important to remember it’s our own immune system that is creating fever, and it likely serves the purpose of fighting infections.

Read more: Everything you need to know about treating a child's fever Is there such thing as a teething fever?