IRA FLATOW: This is Science Friday. I’m Ira Flatow. You know, whenever the weather turns hot, the conversation turns to sweat. You hate sweat, right? Your clothes stick. Your head is dripping. Your deodorant as well. Well, let’s not go there.

On the other hand, lots of people seek out sweat. Whether it’s hot yoga or a steam bath, there’s nothing like a good [NON-ENGLISH] as we used to say. So is it good, or isn’t it?

My next guest is here to suggest you celebrate that sweat. No matter how profuse, don’t be salty. The chemistry is cool, even. It’s our evolutionary superpower as human beings. And if we didn’t have it, she adds in a new book, we might be left doing some even less savory things to keep cool. Yes, we’ll talk about that.

Here with me now is Sarah Everts, science journalist, author of The Joy of Sweat– Strange Science of Perspiration. Welcome, Sarah.

SARAH EVERTS: Thank you for having me.

IRA FLATOW: Let’s talk about the joy of sweat for a moment, because there are people who do seek it out. They go into a steam bath. They like hot yoga. It feels good to sweat.

SARAH EVERTS: Yeah. And in fact, when you sweat profusely, you release a happy hormone– the same sorts of things that give you the runner’s high. And so I think there is an emotional catharsis that we have when we sweat.

And most cultures, at one point or another, have some sort of sweating ceremony. From the sweat lodges of the Indigenous peoples of the Americas, or the jjimjilbangs in Korea, or the banya in Russia, or the saunas in Finland. And so we all seek out some sort of sweaty catharsis at some point or another.

IRA FLATOW: Let’s get into what sweat really is, because I’ve had, for many years, a misconception that sweat is just water and salt. But it’s actually very closely related to our blood. Where does it come from? What happens to it before it appears on our skin? Why does it get there? Give us a little bit of the ABCs.

SARAH EVERTS: Sweat is actually sourced from the watery parts of blood– blood plasma. So the red blood cells and the platelets and the immune cells have been filtered out. And that liquidy part is what keeps your body on the inside wet, so we are salty oceans inside.

And when your body gets overheated, and you get the temperature directive to start to sweat, your sweat glands source that perspiration from this fluid that is percolated out of blood. It’s called interstitial fluid.

And so pretty much anything that’s small and is circulating around in your blood system can emerge out your sweat pores. I had my sweat analyzed by a forensic scientist, actually, who took an analysis of even just a fingerprint of mine. So fingerprints are just sweat prints, right?

And she could tell that I had had a morning coffee, because there was caffeine that had emerged out in my sweat pores. If I had, for example, added a little shot of whiskey to my coffee or a little something more illegal, all of that also emerges out in your sweat because it is circulating in your blood. As well as glucose, urea, proteins. All sorts of interesting things come out in sweat.

IRA FLATOW: Do you think, someday, we might be able to use sweat as a fingerprint? Because maybe you have a unique sweat profile or something like that.

SARAH EVERTS: Well, I do know that forensic scientists are certainly interested in sweat fingerprints. So normally, when you think of forensic scientists looking at fingerprints, they’re looking at the whorls and swirls. They’re looking at how it physically looks, and they’re comparing an image of a fingerprint to that of a database.

Well, chemists are now actually analyzing the chemistry of fingerprints, and they’re able to find out all sorts of information. And in fact, that scientist who analyzed my fingerprint– she works with law enforcement trying to develop– this is a technique. And she, for example, analyzed a single fingerprint lifted from a window sill where a stalker had tried to break into a house and found that he had been consuming alcohol and, actually, cocaine.

And so I do think that there will be forensic analysis of fingerprints coming up, but I also think a lot of people are really into personal measurement, and that can also give us super interesting information.

So say you have a little Band-Aid-like sweat patch analyzing what’s coming out of your skin or a smartwatch add on. And you get a little push alert, because your sweat patch has noticed that your blood alcohol level is probably higher, because there’s alcohol in your sweat. So it tells you, maybe don’t drive home after the bar. Take a cab.

Or you can imagine coaches on the sideline keeping tabs on the sweating of their players. Say, in a really important match, a player starts getting stressed and starts releasing stress hormones or signs of fatigue. That might ping the coach to hey, let’s switch out that player for somebody new.

There’s all sorts of applications like that that are less dystopian than the forensic applications, too.

IRA FLATOW: We don’t just have one kind of sweat, either. There’s regular sweat and then that funky armpit stuff that we get starting with puberty. Tell us about the differences between those two.

SARAH EVERTS: Yeah. So eccrine sweat, the stuff that we’ve been talking about, that’s responsible for cooling us down. But there is another. And those are the apocrine glands, and those are found anywhere where hair grows at puberty. That kind of sweat isn’t watery at all. It’s actually more waxy.

And when bacteria living in your armpits eat that sweat, they metabolize it into the very stinky odors that start emerging out our armpits at puberty. So it’s kind of like a good news, bad news situation, right? Most sweat when it emerges from our pores is not smelly, and the thing that’s responsible is the bacteria in your armpit.

But on the downside, it’s actually effectively bacterial poop that’s making you stinky. So I’ll leave you to decide whether you find this heartening or not.

IRA FLATOW: So it’s not just your armpit, then, that may be stinky. It may be anywhere where the sweat collects and bacteria can get to it.

SARAH EVERTS: Exactly.

IRA FLATOW: Yeah. You open your book with a story– and you have to tell the story– of a woman who sweated red, and how it baffled medical professionals.

SARAH EVERTS: Yeah. How alarming is that? So it certainly baffled medical professionals, and it stressed her out, but it also super excited the medical professionals. Because can you imagine how often would you get to analyze red sweat?

So she was a nurse, and she started noticing that around the collars of her white uniform and in the armpits, there were red sweat patches. And she’d have to soak her work clothes for hours to get it out.

So when they analyzed her body, they found that she was a super healthy 20-something nurse. Could not figure out what was wrong. And at a follow up appointment was finally the time where they cracked the case, because she shows up, and her fingers have that kind of reddish brown color that people who roll their own cigarettes sometimes get– that kind of stain.

And they knew that she was not a smoker, and so they’re like, what? What is on your fingers? And she’s like, oh, it was my favorite chips. It’s a spicy corn tomato chip. And effectively, she had been eating upwards of 45 bags of chips a week.

IRA FLATOW: Wait, wait. 45 bags a week?

SARAH EVERTS: Yeah, of spicy tomato corn chips. Yeah. And because anything that you consume can end up in your blood system, and your sweat is sourced from the watery parts of blood, some of that red-colored dye had emerged out her pores.

And so when they put her on an elimination diet, her sweating red cleared up, and she just went back to the normal complaints we have about sweat. Dank odor and patches, but not colorful ones.

IRA FLATOW: That’s interesting. You mentioned urea. How is sweat different from urine, if they’re both derived from our blood?

SARAH EVERTS: Right. So this gets to, probably, my biggest pet peeve, which is when people talk about going for a good sweat as a detox strategy. This is total hogwash.

So effectively, because anything in your blood can emerge out in your sweat, lots of good stuff comes out, like glucose and hormones, as well as bad stuff. But if you were to detox by sweating profusely, you would literally have to get rid of all the water in your blood out your sweat pores. That would completely dehydrate you, and you would dry up and die.

Instead, your kidney filters your blood for that nasty stuff floating around your bloodstream. Filters it out and then dispatches it out in urine. And so sometimes there’s urea in your blood, and that gets siphoned off by the kidneys and dispatched out in pee. As well as all the other bad stuff. That’s why we evolved the kidney. Sweat is entirely– at least that salty stuff– just for cooling down.

IRA FLATOW: Speaking of unusual sweat, let me go to a clip we have from Brant from Brooklyn. He has a question on the SciFri VoxPop app.

BRANT: I don’t just sweat in the summer. I sweat year round. I do have sweaty armpits, but they don’t bother me as much as my excessively sweating hands, because I have to use my hands for things. I have had Botox injections to help with the sweating. They do work, but they’re expensive. They’re painful. And they only last for about five months, and then the sweating comes right back.

IRA FLATOW: And he wants to know if there’s anything more effective or inexpensive on the horizon.

SARAH EVERTS: So what he’s describing, hyperhidrosis, is a pretty serious sweating condition. And people who have it– some can’t even hold a cell phone or a pencil because it slips out of their hands.

And I am really saddened that there has not been more research on this. Botox is one solution, but it’s only a temporary one, and it’s expensive. Some people try to take drugs to control their sweating, but there’s often a lot of side effects. Quite honestly, I wish that there were more strategies available, and I wish that more researchers dug into hyperhidrosis.

IRA FLATOW: One would think, with all the people who have this, that the drug companies would be salivating– and maybe it’s the wrong analogy– to try to find a drug for it.

SARAH EVERTS: Another bodily fluid.

IRA FLATOW: You know?

SARAH EVERTS: Yeah.

IRA FLATOW: Yeah. Let’s go into other kinds of disordered sweating. Tell us about any other ones.

SARAH EVERTS: Well, there are some individuals who don’t actually sweat at all. They have a genetic condition that interrupts the development of sweat glands in utero. And actually, that is really debilitating because whether you find sweat annoying or not, it is essential for keeping you alive.

Because, effectively, you are sweating a tiny bit at all times, making micro-adjustments to your body temperature. Because as that sweat is dispatched onto your skin, the evaporation of the water pulls away the heat from the surface of your skin. Meanwhile, your blood is rushing by.

So have you ever noticed when light-skinned people get really hot, they turn red? That’s because their vasculature system has pushed up veins as close to the surface of the skin as possible, so that the cooling evaporation of sweat can cool the blood rushing by. And so then that blood can go back into the interior and cool you down.

And so people who don’t have sweat glands at all– they have to spritz themselves with water constantly. It’s very uncomfortable to live in even a slightly warm climate, because their body can’t make those micro-adjustments to body temperature.

IRA FLATOW: So it must be dangerous.

SARAH EVERTS: Oh, yeah. Yeah, it’s life threatening. I mean, as much as it’s kind of annoying to be drippy on a hot day, that’s your body just trying to do its thing to keep you alive. Heat stroke is a terrible way to die.

IRA FLATOW: I knew before I read this book that people are some of the only animals that sweat. But you really want us to see sweating as what makes us special– our evolutionary superpower, even. What makes it so super for us?

SARAH EVERTS: Right. Well, it makes it so super because we can exercise and run and, effectively, cool down at the same time. So if you think about our evolutionary history, most of our prey sprints way faster than us.

But because we have this huge naked surface area of skin– most other animals are covered in fur. We’re a naked ape. We have this enormous surface area for cooling down. So our prey would sprint away way faster than us, and we would start running after them. And eventually, they would have to stop and cool down, so they didn’t overheat. And we could catch up, forcing them to sprint again. And catch up and sprint again, until they were so exhausted, or that they were easy to kill, or they died of overheating.

And so the modern incarnation of this is marathons, of course. We can run great distances and cool down while on the move. And if you just think about dogs, for example, the way a dog cools down is by panting. And it’s sticking out its tongue, and it’s also evaporating water, but it’s evaporating water from saliva. And it’s evaporating it off the only naked surface area it has, which is a tiny little tongue in comparison to their whole body.

And if you think about that, we have such a larger surface area off of which we can cool down, and this allows us to live in really hot climates. It’s allowed humans to populate a good chunk of the world, for better or for worse.

IRA FLATOW: I also notice that some of the options animals have for keeping cool are, how shall I put it, pretty gross.

SARAH EVERTS: Alarming at best is how I do it. Yeah.

IRA FLATOW: I mean like, peeing on their feet. Pooping even, sometimes.

SARAH EVERTS: Yeah, so this is the thing, right? So evaporation of water off the surface of your body. This is the most efficient way to cool down. And so if not sweat, then another bodily fluid. And so dogs use saliva, which is arguably gross, but not as gross as urine or poop.

So for example, vultures will poop on their own legs– it’s quite a liquidy poop– to evaporate the heat off themselves. Seals urinate on themselves. Honeybees vomit on themselves to get water onto the surface of their bodies to evaporate away the heat.

And so when you know what could have been, when you know what evolution might have bequeathed us, you know, sweat is a lot less gross than all of those other things. I mean, imagine a subway in the dead of summer, where people are peeing, puking, licking themselves so that they can cool down. In contrast, sweating is so much less gross.

IRA FLATOW: We have to take a short break, but when we come back, there’s more. Yes, we’re going to keep on sweating with author Sarah Everts, author of The Joy of Sweat.

This is Science Friday. I’m Ira Flatow. We’re talking about sweat– the chemistry, the physiology, and even the forensics of it– with my guest Sarah Everts, author of the book The Joy of Sweat– The Strange Science of Perspiration. And boy, are we finding out just how strange some of this is.

I want to bring in a question from Lynetta in California. She sent this in via the SciFri VoxPop app, and it’s a question I have, too.

LYNETTA: I recently learned that there are differences between tears depending on why they’re produced. I’m wondering if the same is true with sweat. Is the sweat that the body produces because of stress the same as the sweat that’s produced because of heat?

IRA FLATOW: Thank you for that question, Lynetta, because I have the same question about nervous sweat. Why do we sweat when we’re nervous at all? What does that have to do with cooling off the body, and are there two kinds of sweat?

SARAH EVERTS: Yes, I love this question. So we can sweat because our body gets hot, right? As soon as our temperature rises, and all of our two to five million sweat glands open up.

But another way to open up the floodgates is stress hormones, like adrenaline. And so if you’re panicked, you can also start the sweating. We don’t know exactly why that is evolutionarily.

But you can imagine that most of the time when you’re fearful or at least in our history, you had to run away really quick or climb a tree or do something like that. And so it’s possible that our body is effectively assuming that we’re going to need to cool down pronto.

But what’s really interesting about fearful sweat is that there might be a unique odor that we produce when we are stinky. So researchers have followed up on this kind of weird idea that we might produce an anxious odor, and they gave people t-shirts to wear and put them in front of a television screen. And they watched either a nature documentary, or they watched a really scary movie, and got the subjects to sweat.

And then, they took away these odor samples and gave it to a panel of sniffers. And what’s really interesting is that these complete strangers could distinguish just normal BO from the body odor produced during a moment of anxiety.

And so we do sniff out information about others around us. And, yeah, chemists are hard at work trying to pluck that molecule out. But they haven’t been successful yet, but they’re certainly working on it.

IRA FLATOW: I know you also investigated up close another mystery of sweat, and that is we can be attracted to other people’s sweat smells. Tell us what you learned about sweat and love.

SARAH EVERTS: OK. So I went to Moscow to go to a sweat dating event, where people sniff body odor as a way to find love and romance. And the idea is that whether or not you find somebody attractive or likeable, or the hobbies match, at some point, you’re going to smell the body odor of the person you are with. And it’s going to be a make-or-break moment.

And so, why not cut to the chase– or eliminate the chase– and do your filtering for potential dates by body odor? And certainly, humans have a body odor print. We know this because dogs can track a specific human based on a sample of their t-shirts, right?

And we do smell one another. In fact, parents can identify the body odor of their newborns just within hours of birth. Siblings can identify a long-lost brother or sister after two years of being apart. So we do recognize the body odor of others.

And in fact, there’s been all sorts of tantalizing research that suggests that how our partners smell is involved in whether or not we’re attracted to them. So the famous t-shirt study by Claus Wedekind is when women were given the t-shirts of men– and by the way, all this research is very heteronormative with cisgendered, straight couples, and I wish it weren’t so. I wish that they would evaluate a greater diversity of human sexuality.

But when women were given these stinky t-shirts of men to smell, they found the men with the most complementary immune systems to be the most attractive. And by complementary I don’t mean same. I mean, different enough that any progeny that they would have together would have a very strong immune system.

And if you think about it, it makes sense. For most of human history, our major foes have been microbial, right? We’ve died from plagues and pathogens. And so it behooves us to try and find a mate that will create children that can survive these pathogens.

IRA FLATOW: Let’s talk about all the tricks we use to sweat less or reduce the smell of our sweat– antiperspirants and deodorants. Have we mastered this yet? I mean, are we tired of swiping our armpits?

SARAH EVERTS: Yeah. Well, it’s interesting because this is actually a relatively new phenomenon. For most of human history, we have either lobbed on perfume if we were anxious about our BO, or we’ve washed with soap and water or just water and then lobbed on perfume.

There’s this way in which the last 100 years deodorant and antiperspirant manufacturers have put the fear of sweat in all of us. Deodorants are actually just antiseptics, and so they kill the population of bacteria in your armpit that eats your apocrine sweat and turns it into stinky odors. Whereas antiperspirants cut off the food supply by blocking your pores, so they close the buffet, so that these bacteria go hungry and can’t make the stinky odors.

But there are researchers trying to find different new strategies to fight odor. So some are looking at instead of killing the bacteria, blocking the enzymes that the bacteria are using to make those stinky smells. So it would be kind of like a live and let live situation, but just don’t do that one thing.

IRA FLATOW: What about manipulating the microbiome? Maybe some probiotic that’s in the deodorant that competes with the bacteria, right?

SARAH EVERTS: Yeah. So that’s not a totally new idea, but you could imagine people trying it out. And there’s one really fascinating researcher who’s studied what’s called armpit transplantations. But instead of lifting the skin, effectively, it’s just like lifting the bacteria from one armpit and putting it into somebody else’s armpit.

So say, you produce pretty stinky odors. It could be because you have a higher proportion of Corynebacterium in your armpit than, say, Staphylococcus. And say, I produce less. Maybe I could donate my armpit microbiome to you.

He’s only had success with twins, like very close family members. Because, of course, the bacteria living in your armpit– they are living there because they love your skin, right? They love all the weird little components that you produce in your sweat, your eccrine sweat, that salty stuff. They love that ecosystem, and they’re probably pretty well established.

So it’s actually very hard to disrupt a person’s microbiome if they’re healthy. Maybe putting less smelly bacteria in your armpit might be a solution, but so far, it hasn’t worked.

IRA FLATOW: Thank you very much for this book. It’s a great read. Sarah Everts, author of The Joy of Sweat– The Strange Science of Perspiration. Thank you for taking time to be with us today.

SARAH EVERTS: Oh, it was such a pleasure.

IRA FLATOW: And to our listeners, if you want to read an excerpt from the book, no sweat. Just go to sciencefriday.com/sweat.

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