IRA FLATOW: This is Science Friday. I’m Ira Flatow. When I say 98.6 degrees Fahrenheit, you know what I’m talking about, right? That’s the normal human body temperature. That standard was said over 150 years ago by the German physician Carl Reinhold August Wunderlich. That number may no longer be the average. And the human body temperature might be lowering. Producer Alexa Lim has the story.

ALEXA LIM: During the pandemic, you may have been taking your temperature a bit more frequently. Just a quick check-in to see if you’re running at the quote unquote “average temperature.” Some people might even say they run a little hot or a little cold. But researchers have found that the average human body temperature has actually decreased from that 98.6 degrees Fahrenheit mark. So how does human body temperature work? And what can it tell us about overall human health? My next guest is here to talk about that. Dr. Julie Parsonnet is a Professor of Medicine in infectious diseases and epidemiology and population health at Stanford University. Welcome.

JULIE PARSONNET: Thank you very much for having me.

ALEXA LIM: Let’s just talk about what is body temperature? Your temperature is an indicator of what’s happening inside your body. So what factors determine our temperature?

JULIE PARSONNET: So your temperature is an indicator of your metabolic rate, how much heat your body is burning to allow the body’s functions to actually happen– to have your heartbeat, and to have your brain work, and to have your breathing, and your kidneys, and all these other things actually function. Your metabolic rate goes up with some things– when you exercise, when you go outside and run. When you eat, your metabolic rate will go up, because you take some energy to digest food. But most of what your body temperature comes from is from the processes of just keeping your body working to make sure all your proteins and everything else functions. You have to maintain a certain temperature. So you’re burning calories to be able to do that.

ALEXA LIM: So what’s the thermostat in your body? What’s kind of regulating that?

JULIE PARSONNET: So there are a lot of things that are regulated, but largely it’s from your endocrine system, which is the system in the body that kind of monitors that all your systems are really going right.

ALEXA LIM: I want to play a question from our VoxPop app. This question is from Caitlin from New York City.

CAITLIN: If the human body temperature is 98.6 degrees, why are we so uncomfortable when the temperature outside is near 99 degrees, as well?

JULIE PARSONNET: Just because outside it’s 99 degrees, doesn’t mean that our human body feels comfortable at that temperature. We actually feel comfortable at what’s called the “thermal neutral zone,” which is around the area between about 65 degrees and 75 degrees. And that area is when our body doesn’t have to do much to maintain our temperature, when we don’t have to do that extra work.

When you get above that, we have to do extra work to keep our temperature low. We’re fighting to keep our temperature from getting too hot. When the temperature is lower than 65 degrees, we have to work harder to keep it at that normal level. And I also point out that when we’re in 99 degrees, we’re typically not outside naked.

[CHUCKLES]

ALEXA LIM: That’s key.

JULIE PARSONNET: We’re wearing clothes. And there are other things that are keeping us even warmer. So we do have to prevent ourselves from overheating when we get at those high temperatures.

ALEXA LIM: So I want to get to this idea of this 98.6 degrees, and that maybe it’s decreasing. I mean, where did we even get this 98.6 degrees as a standard in first place?

JULIE PARSONNET: There was a guy named Carl Wunderlich, back in the middle of the 19th century, who was very interested in body temperature. And he took millions of temperatures from tens of thousands of people over time. And he looked at what those temperatures meant. He looked at what time of day they were higher, when they were higher than women than men, what they looked like with people with various diseases, what was abnormal, what was normal. And it was an enormous piece of work. And we think of having computers to look at $2 million pieces of data, but he sat down there with his pencil, and he looked at all these data. And he worked very hard to do it right. And he’s the one who came up with this normal of 98.6 degrees that’s been carried on since the 19th century.

ALEXA LIM: Wow, so it’s been like 150 years of just going on this one data set, basically.

JULIE PARSONNET: Yes. But in the last 30 years-40 years, there have been– at least 30 years– there have been a number of people who’ve said that number is wrong. Dr. Phil Makowiak, who’s at University of Maryland, did some really nice work back in the 1990s, showing that our body temperature was lower than that. His conclusion was that Wunderlich’s thermometers were wrong. And he actually went back and found one of Wunderlich’s thermometers and tested it, and said, “You see, it’s not good. It’s not a good thermometer.” And that’s the reason.

Wunderlich was very careful about calibrating his thermometers. And he worked very hard to get them to be accurate at the time. And you can do that, because the temperature of boiling water and the temperature freezing are always the same. They haven’t changed over time. So you can calibrate your thermometers pretty well. And I think just the thermometer that Professor Makowiak used was probably just not calibrated anymore.

ALEXA LIM: And so you did your own study that looked at body temperature in the United States. And you started with data during the Civil War. What did you find out?

JULIE PARSONNET: So we had three groups that we looked at. We had a cohort of veterans of the Civil War. Some of them might have been born as early as the beginning of the 19th century and had been followed after the Civil War, until about the 1920s for pension purposes. And then there was a second cohort that was a group that the NHANES, which is a national survey that the NIH runs, that looks at temperatures in a segment of the United States– healthy group of people the United States. And then we looked at temperatures at my institution, Stanford, and compared those. So the temperatures from NHANES were the 1970s. And then ours currently.

What we found is that temperatures have been declining pretty consistently since the middle of the 19th century– since the Industrial Revolution essentially– at about 0.03 degrees centigrade per decade. So the average temperature is now about 97.4 degrees, somewhere in that range, rather than the 98.6 degrees that we see now.

ALEXA LIM: That sounds like a pretty big decrease in what seemingly is a short amount of time.

JULIE PARSONNET: It is a pretty fast decrease. To me, it seems like that. But there’s this recent paper that’s been published by a group studying Native Indigenous population in Bolivia, where they found a very similar decline over even a much more rapid period– so just over a couple of decades. So what we’re basically seeing is that human beings can change with modernization and industrialization. And we don’t really know what the factors are that are leading to that. We’re not absolutely certain why that’s happening. But as we are getting less likely to get infectious diseases, and as we’re having better diets, and as we are able to control our climate better, and sort of control our environment around us a little bit better, we’re finding that our temperature doesn’t have to be as high as we once thought it had to be.

ALEXA LIM: So does that mean that, since we’re controlling infections better, that our body doesn’t have to work as hard as at controlling them? Or what does that mean exactly?

JULIE PARSONNET: Yeah. So I mentioned when we talked about where our body is comfortable. We are comfortable at a certain outside external temperature. And our bodies are probably also more comfortable at places where we’re not fighting infection. There are certain things that turn us on and say, OK, we have to work harder. Our bodies have to work harder to keep our body going in a normal way.

And there are threats– like infectious diseases, all those vaccine-preventable diseases, tuberculosis, all those other things– that make us work a lot harder to keep healthy. And then, again, there are things like being in cold weather, having no heat in your home, that may also make you work a lot harder. And all those things we’ve been able to manage pretty well over the last 100 years. We’ve gotten rid of a lot of infectious diseases, a lot of chronic infectious diseases, particularly like tuberculosis.

And we live in environments that are much easier for us to maintain our weight and our metabolism. I think it is a sign of being healthier. I don’t know that we know that for sure. But I do think it is a sign of being healthier. I think we have, again, over the same 150 year period of time, we’ve gotten taller, fatter, our life expectancies have gotten longer, and we’ve gotten colder. And I think all those four things are intertwined. We are actually seeing a decrease in life expectancy in many parts of the world, including the United States. And I’ll be curious to see whether we see reverses in this temperature changes, and what’s going to happen with that over time.

ALEXA LIM: And like I mentioned, people might be taking their body temperatures more often these days. Is this actually an effective way to monitor our own individual health, or at least set a benchmark for ourselves?

JULIE PARSONNET: I think it’s a really interesting question and there are a lot of companies that are very interested– a lot of groups that are developing body monitors that you can wear all the time. They’ll look at your curves of your temperature and see what is your normal, your personal normal. We know, for instance, that temperatures are lower in the morning than they are in the evening by a fair amount. And they’re higher in women who are premenopausal, and they’re higher during parts of the menstrual cycle. And we all know that, because we use that for work on fertility.

So each person is going to have differences based on who they are– by their height, their weight, women, men, time of day. So taking your temperature at any point of the day without knowing how all those factors are coming together may make that number really hard to interpret. So it is pretty important to kind of get a sense of who this person is, and what is a normal in that particular person to be able to at least look at small variations and have an understanding of what they mean.

And I’ll also point out that there’s a lot of errors in thermometers. And some of the errors in thermometers can be much higher than the variation in a person. So if you take two different thermometers, and you use them, or you’ve been wearing a hat, or you just had a cup of coffee, or whatever, your temperature may vary a fair amount that has nothing to do with your body’s physiology. These small changes in temperature that people are looking at– or whether it’s 99.1 degrees or not– those are pretty inaccurate.

We’re still very good when you– 104 degrees– nobody’s going to say, OK, that’s not a temperature. When you’re temperature 104 degrees, 102 degrees, even 101 degrees, everybody’s going to say, well, that’s not normal. That’s off normal. But these little small variations that people are finding in their daily temperatures are kind of hard to interpret, at least right now.

ALEXA LIM: What’s the most interesting thing you ran across in this Civil War data study?

JULIE PARSONNET: For me? Well, so there are a couple of things I found out that were really interesting to me, which is that platypuses have really low temperatures.

ALEXA LIM: [CHUCKLES]

JULIE PARSONNET: That was very interesting to me on hippos. I kind of thought, wow, that’s kind of cool that they have such low temperatures. But it made me think that we can actually function– proteins and mammalian proteins can function at much lower levels than I would normally have thought they could.

But with respect to the Civil War data, what I thought was most interesting is, first of all, that we have those data. And Professor Fogel, who is at the University of Chicago, an economist, won the Nobel Prize actually, in part, for putting together all these data from these veterans of the Civil War, which I thought was really interesting. And those are all available online. So anybody who’s interested in history can kind of go back and see who these people were back in the Civil War. Because you can find out what their illnesses were, and what jobs they did. And which, to me, as somebody who loves history, it’s just fantastic to have these data available.

And the other thing that was striking to me is the different names of all the medical conditions back there. It was really hard to figure out what they were talking about with all these “flux” and all sorts of things that we don’t use in medicine anymore, and to look back and see how much we’ve advanced, and how we understand disease.

ALEXA LIM: Thanks for joining us, Dr. Parsonnet.

JULIE PARSONNET: It’s a pleasure. Thank you so much.

ALEXA LIM: Dr. Julie Parsonnet it is a Professor of Medicine and Infectious Diseases and Epidemiology and Population Health at Stanford University. For Science Friday, I’m Alexa Lim.

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