Smells Humans Are Ridiculously Good At Detecting

Right now, you’re breathing in. As you inhale, air rushes past millions of sensory receptors, activating the part of your brain responsible for smell. And yet, there’s one scent you’ll never notice: the very nose you’re breathing through, because humans are smell blind to themselves. Today, Professor Hannah Fry and VSauce's Michael Stevens explore the mechanics of perinasal chemosensation, otherwise known as smell. They explore how the Victorians sidelined the sense, why we’ve underestimated it ever since, and whether tuning out most of odours our noses detect is a flaw, or a powerful strategy that's central to survival ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton Video & Social: Bex Tyrrell Assistant Producer: Imee Marriott Producer: Becki Hills Senior Producer: Lauren Armstrong-Carter Head Of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 0:02Michael Stevens?

  Dinosaurs walked the Earth 180 million years ago. But you know, cancer was part of their story, too. Scientists have found tumors in ancient fossils.

• 0:13Hannah Fry?

  Well, that is a part of the reason why cancer is a big, big part of our story, right? It's the other side of evolution. It's the most complex disease that we face. There are more than 200 types of cancer in total, each with distinct characteristics, challenges and mysteries.

• 0:28Michael Stevens?

  And that complexity demands scale. Cancer Research UK is the world's largest charitable funder of cancer research, with more than 4,000 scientists, doctors and nurses working across more than 20 countries in the search for answers. And then sharing their discoveries beyond borders.

• 0:45Hannah Fry?

  And the impact of this collaboration is clear, because over the last 50 years, the charity's pioneering work has helped to double cancer survival in the UK. That is more people who are living longer, better lives.

• 0:57Michael Stevens?

  Fossils can show us the past, but research is shaping the future. And for more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit cancerresearchuk.org/restisscience.

• 1:14Speaker 02

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• 2:44Speaker 03

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• 3:20Hannah Fry?

  Hello, I'm Hannah Fry.

• 3:22Michael Stevens?

  I'm Michael Stevens.

• 3:23Hannah Fry?

  And this is The Rest Is Science. We're talking smell today, Michael.

• 3:28Michael Stevens?

  Smell? Yeah. Okay, well, let me ask you this, Hannah. What do you smell right now?

• 3:35Hannah Fry?

  Like, honestly, not very much. I'm in my house. Normally, I have a scented candle on in the corner. Don't at the moment. I sort of can't really smell anything. I mean, this is like, it's the inferior sense. There's not, there's not much going on. I can't smell anything.

• 3:48Michael Stevens?

  Whoa. The inferior sense? How, your nose can hear what you're saying.

• 3:53Hannah Fry?

  It's gonna testify against me one day.

• 3:55Michael Stevens?

  We're gonna talk about that in this episode. Is, is our sense of smell inferior in some way? But I want you, the listener, to also answer this question to yourself. What do you smell right now? I can smell, I can kind of smell the soda I have in this mug over here. It's pretty weak. But otherwise, yeah, it's, it's a pretty odor-neutral environment that I'm in. I think we live in very odor-neutral environments, or we think we do.

• 4:24Hannah Fry?

  Because I wonder if we swapped, if we did like, kind of, we invented wormholes for the purposes of this discussion and then just immediately swapped. You kind of came into my house, I came into your studio. I reckon in the first instant of walking in there... Neither of us would think that it smelled of nothing. I mean, I like to imagine that you think that mine smells of roses and beautiful perfume and like, you know, window cleaner and like, you know, fabric softener and all of the lovely smells.

• 4:53Michael Stevens?

  Yeah, and it probably would, right? It would smell like something. It would not smell like the nothing it smells to you. My favorite game to play is to go on a vacation and then come back and stop before I go into my house and just prepare myself to smell myself. I can smell what my house smells like to guests because I've been gone for weeks. And for like a half a minute, I'm going to be able to smell what they smell.

• 5:22Hannah Fry?

  That's a really fun idea.

• 5:23Michael Stevens?

  You gotta do it.

• 5:24Hannah Fry?

  I also have a game that I play when I go on holiday, which is if it's going to be a really nice holiday, I will buy a particular scented candle and I will light it when I'm on that holiday, say this on my honeymoon, for example. And then in the future, whenever I want to feel like I'm back there. So I have like four or five different scents that it's like, put on that candle and transported back to that place.

• 5:49Michael Stevens?

  That's brilliant. You know, there was an artist who created necklaces that contained a little ampule of a very unique smelling compound. And if something ever happened in your life that you wanted to remember forever, you could break the ampule and have this new scent that would be tagged to that memory for the rest of your life.

• 6:07Hannah Fry?

  Oh, that's a fantastic idea. Well, hold on a second, right? If the inside of your house smells like something to everybody else and to you for the first few moments when you arrive back from holiday, I wonder what else is constantly around you that also has a smell that you can't smell?

• 6:25Michael Stevens?

  Yeah, what does the inside of your nose smell like?

• 6:29Hannah Fry?

  Well, if we're going to get to the bottom of these questions, I think we should first turn to an assumption which is, which is still quite popular. The fact that smell is the inferior scent. Shall we start there?

• 6:39Michael Stevens?

  Yeah, give me your case.

• 6:40Hannah Fry?

  This is essentially Victorians being dismissive of anything they didn't consider highfalutin, right? So there's a French guy, Paul Broca, Broca? Broca.

• 6:52Michael Stevens?

  I say Broca.

• 6:53Hannah Fry?

  Okay, so Broca Broca, in the Victorian era, he like starts looking at particularly the olfactory bulb in the brain, which is actually incredibly small, particularly compared to other animals. And so he said, look, this is kind of pathetic for us. And actually, the Victorians, the sort of the English scientists at the time, they were very happy with this. They were like, this means that we've transcended the kind of animal-like senses, right? Like, we've got sight and hearing, that's why we can do art and science and have intellect and, you know, smell and taste, they're sort of much more animal-like, right? It's more like about kind of dirt and lust and like savages. Those are the things that have, that care about smell.

• 7:34Michael Stevens?

  Right.

• 7:34Hannah Fry?

  We've evolved out of it. And that essentially is the reason people are very snobby about smell and have been for a long time.

• 7:41Michael Stevens?

  That's so Victorian.

• 7:43Hannah Fry?

  Very Victorian, isn't it?

• 7:44Michael Stevens?

  But at the end of the day, we're made of dust and lust, you know? And pretending that we aren't is like the classic predicament of our modern age, Hannah.

• 7:56Hannah Fry?

  Hmm. You can smell the difference between coffee grounds and like soil after rain, you know, pencil shavings or like wet dog. If you like spend even a moment thinking about this, which perhaps the Victorian should have done, we are so good at smelling that you can put a ripe banana, right, hide it in a drawer or like in a cupboard or something, and then get a person to go in and ask them to guess what has been hidden. And people will get it right. We are incredibly sensitive to this chemical that gets released by ripe fruit. We're so sensitive to it, we can detect a few parts per trillion. We get way, way, way more sensitive to this than dogs are, by the way. Dogs don't really care about ripe bananas. Humans, honing on it, absolutely want to know.

• 8:39Michael Stevens?

  Yeah. Okay, so there are things about our olfaction system that are pretty impressive. So I want to direct our attention to a really great paper from 2010. It's, the whole thing is a great read. And they address the fact that there's this problem in the way we think about smell. We think that, oh, dogs are great at smelling and like cats are really good, but humans are bad. It turns out that humans aren't bad. All mammals have like the same kind of olfactory system. Humans can differentiate seemingly trillions of different smells.

• 9:16Hannah Fry?

  Trillions?

• 9:17Michael Stevens?

  If you give them two smells and you say, are these different? They can tell. Down to tiny percent changes that it's different. However, that doesn't mean that we can identify trillions of different smells. We have to be paying attention to smell.

• 9:34Hannah Fry?

  We can also, we can tell the difference between spearmint, the smell of spearmint, right? And then caraway, right? If you can imagine how distinctive those two are. Those are actually, when you look at the molecular structure of those two, that the molecules that give them their distinctive flavors, they're actually totally identical to one another. It's just that one is the mirror flip of the other. So essentially, it's effectively like being able to tell the difference between a left and a right-handed glove, you know? That's essentially how good we are at it.

• 10:05Michael Stevens?

  Have you ever smelled ethyl mercaptan?

• 10:07Hannah Fry?

  No.

• 10:08Michael Stevens?

  You have if you've ever smelled a gas leak. Because propane, natural gas, these things are odorless. But that's dangerous. If there's a leak, no one can tell unless they've got special instruments. So they add a chemical called ethyl mercaptan to propane, natural gas, the stuff that comes out of your stove, your gas stove. That stuff doesn't normally smell. So when you smell it, or you think you're smelling gas, you're smelling the odorant that's added so that we can detect it. And I want to give you a little side story. When I was a kid, a truckload of ethyl mercaptan crashed near my house and it smelled like there was a gas leak.

• 10:51Michael Stevens?

  But actually, it was just the odorant. And every day on the school bus, we would drive through this area where you could smell that odorant. And the kids would all go, ah, don't light a match, it's gas. And I'd be like, actually, that's the ethyl mercaptan odorant that's added. And then it became, well, I became the odorant boy. Like, they just mocked me. Whenever we smelled it, they'd go, oh, Michael, is that actually gas? Or is it the odorant? Oh, look at me, I'm Michael. And I've never forgotten that.

• 11:20Hannah Fry?

  I like to imagine that a young Michael Stevens was just exactly the same as you are now, just smaller, just literally identical, just a slightly smaller version.

• 11:31Michael Stevens?

  Yeah, just imagine this head with this beard, but on like a kid body.

• 11:35Hannah Fry?

  Exactly.

• 11:35Michael Stevens?

  Well, but I bring up ethyl mercaptan because we are really sensitive to it. We can, we can detect point oh oh nine parts per billion. That's the equivalent of like three drops in an Olympic-sized swimming pool. Okay, you could put three drops in. An Olympic pool is big. I could do that to one pool and not another. And you could tell me which pool had it and which one didn't. But this brings us back to where we started, Hannah. When you put this odorant, the ethyl mercaptan, in gas, you don't put in 0.009 parts per billion. You put in 0.5 parts per million. That's 57,000 times more than we need to detect. Why?

• 12:20Michael Stevens?

  Because even though we can detect small trace amounts, it doesn't get our attention. We have to be slapped in the nose by a smell to notice it. Like, we can listen, but we only pay attention if it's screaming.

• 12:33Hannah Fry?

  That point about how we have to be kind of slapped in the nose in order to notice it, that's not true of other creatures. So there's this story about, in the 1930s in Texas, they were building these gas pipelines across country, right? These huge, long pipes that would go for hundreds and hundreds of miles. And because they were pumping in natural gas, they had ethyl mercaptan added to it. And what they noticed was that every now and then there would be these turkey vultures that would circle above the pipes. And all of these gas engineers initially were like, what on earth is going on there? Like, why are these vultures obsessed with this particular section on the pipe? And they would go and investigate. And time after time, they would find that the vultures

• 13:18Hannah Fry?

  were circling over where there was a leak in the pipe, right? And so it kind of became like folklore, if you like, among gas engineers in this particular area that you look out for vultures and that would tell you where a leak was. And then in the 1960s, a couple of scientists were like, this sounds nonsense. That like, I think they hadn't honed it down to being ethyl mercaptan at that point. They just knew that vultures were circling. And this group of scientists were like, okay, are vultures really hunting based on smell? So what they did is they got a dead pig, they hid it in some woods, under a bit of topolin. And then in another separate set of woods, they just constructed the same experiment, but you know, there wasn't a dead pig underneath the topolin.

• 14:01Hannah Fry?

  And like, you know, almost immediately, these turkey vultures managed to find the dead pig, but weren't interested at all in the other bit of topolin. And it turns out that ethimacaptin is like this compound that is released when bodies decompose.

• 14:17Michael Stevens?

  Oh, wow.

• 14:18Hannah Fry?

  So it's essentially the smell of death that attracts vultures, but that we are deliberately putting in as this like smell signature in order to sort of save us from, from, from gas leaks.

• 14:32Michael Stevens?

  That makes sense that we would evolve to be really sensitive to an odor released by decaying flesh. It's a good danger warning. For vultures, it's a dinner bell. But either way, it's really, it's really salient. You need to know if it's around. But humans in particular have to be punched really hard to notice a smell. If we are specifically told to try to smell something, we take a different kind of a sniff than usual, not a breathing sniff, but a smelling sniff. And then we, we can detect something, but if we're just normally breathing, there needs to be a lot of odorant around for us to notice it.

• 15:14Michael Stevens?

  And there are a couple of really fascinating consequences of this. And they all point to why we think smell is such an inferior sense. The second thing, though, is what I found the most fascinating, which is that it's also related to attention and it's the fact that humans have remarkably bad olfactory coordinate systems in our brains. We can find bananas, but we can't find a lot of stuff the way other animals can. When a room smells, the room smells. We don't think, oh, man, that area of the room is smelly. The way that we would with

• 15:53Michael Stevens?

  Or sound. If I'm in a room and there's a sound, I'm like, ugh, the, the, the workers on the roof are loud or the refrigerator is loud. It's, it's, it's, it's really in my awareness in a spatial location. Same with a light, the source of a light. It's very much about a point in space where the light is coming from. But smells are just present. We don't have binocular smell like we do binocular sight. Even though we have two nostrils, we don't do a good job of going, oh, yep, okay, let me go right to where the source of the smell is. It's just a stinky place. And there's a theory that we don't rely on smell much

• 16:33Michael Stevens?

  because we just don't have the same conscious interior map of smells that we have for our other senses.

• 16:40Hannah Fry?

  Yeah. If you think about color, right? You can put color on a spectrum. We sort of have like a map, as it were, of like how one color relates to another. With smells, like sure, we might be able to smell like the, you know, the scent of ground, the smell of pencil shavings, but it's not like we're able to relate one to another.

• 16:59Michael Stevens?

  That's right. There is no rainbow of smells.

• 17:01Hannah Fry?

  Yeah.

• 17:02Michael Stevens?

  Where we're like, oh, make that a little bit more XYZ. And funny enough, if you, if you read up on smells, you'll find three systems of organizing odors listed. And none of them have a link to an article. Like, none of them are common enough or clear enough that it's expounded anymore. It's just like, oh yeah, some guy tried to create a spectrum of smell or a coordinate system of smell in the, in the sense of like sweeter, more bitter. I don't know. It's, it's hard for us even to describe them and compare them the way we can talk about color.

• 17:36Hannah Fry?

  So is the idea then that in other animals like dogs, for instance, which we know to have this absolutely phenomenal sense of smell, not just in sensing it, but also in, in mapping it physically, right? Like.

• 17:46Michael Stevens?

  And, and paying attention to it.

• 17:48Hannah Fry?

  And paying attention to it. I had this story once. Um, I, uh, I was talking to someone who was an ex-army dog trainer. And he told me this story about how during major conflicts, when you have these military zones, you have people who are often trying to smuggle vast sums of cash over these borders. And it could be really pernicious, right? It can be really dramatic. It can be used to buy arms, can be used to bribe people, all that kind of thing. So, so of course the military want to try and stop these like. The chunks of cash getting smuggled. So what they did is they took all of these dogs and they they took them to a lab and they were like, right, we're going to teach these dogs how to smell money. And like, money doesn't have like a distinctive odor, but there's lots of things like the ink, the kind of the the um the oil that's left in people's

• 18:32Hannah Fry?

  hands. There's like a kind of catalog of different smells that might be associated with it. And the dogs in the training were really good. Like, they were amazing at finding these these words of cash, no matter where you'd hide them. And exactly as you're describing, the dogs are able to like spatially pinpoint precisely where the smell is coming from, rather than just that the smell exists and is there. Anyway, they then took the dogs out into the field and tried it again. And all of a sudden, the dogs were absolutely rubbish, right? Like where they like got hit after hit after hit in the lab, take them out to the military zone and they just weren't able to do it. Even when they had people who were subsequently caught smuggling cash, the dogs had like not managed to let it go. And they were really wondering what on earth was going on until they finally realized

• 19:16Hannah Fry?

  that the difference between the lab conditions and the military zone was that when in the lab, the the money had been wrapped in cling film in order that people didn't steal it, it didn't go everywhere. And they had inadvertently taught in this extremely expensive program, these dogs to detect cling film. I mean, that's how good these dog smell is.

• 19:36Michael Stevens?

  Yeah.

• 19:37Hannah Fry?

  Does cling film really have that strong a scent?

• 19:39Michael Stevens?

  No, I I think money and cling film, I couldn't smell the difference. But research has shown that we can. We just choose not to.

• 19:47Hannah Fry?

  Just choose not to.

• 19:48Michael Stevens?

  Yeah. And we just don't sniff enough. That's humans problem, Hannah. Um, a rat, especially, like every breath they take is a sniff. There's a difference between a like, I'm breathing through my nose and I'm smelling through my nose, right? Well, multiple times a second, a rat.

• 20:08Michael Stevens?

  Is smelling. It's like bringing odorant molecules from the environment up into its olfactory bulb to think about and experience. But we only do that every few seconds. Most of human breathing is just for the air. And then every so often, we might go take a bigger one that really gets odorants on that olfactory bulb. But because there's such a long gap in between them, we don't often notice changes. It's a bit like if you looked at two pictures where, you know, there's only like find the seven differences. There are similar pictures otherwise. If you look at one and then you don't look at the other one until a week later, you're not going to do a very good job. If you if you look at them simultaneously, it becomes easier.

• 20:51Michael Stevens?

  We have this thing called change anosmia, where we cannot detect changes in the odors around us unless they're really extreme. Now, this is based on change blindness, a famous thing that we've known about for a long time when it comes to vision, that we often don't pay attention to a scene and a little bit of it can be changed and we won't notice. But for smells, we basically live in a constant state of change anosmia.

• 21:16Hannah Fry?

  You really have to be properly smacked in the nose. The spot, the difference picture has to really fundamentally change for you to notice it effectively.

• 21:25Michael Stevens?

  Yeah, yeah.

• 21:26Hannah Fry?

  You know, actually, also, we think of rats as smelly, but maybe we're sort of conflating the smell of rats with the smell of sewers. Do you reckon rats can smell sewers or smell themselves?

• 21:37Michael Stevens?

  Oh, I bet they can because they're just they're so much more attentive to smells.

• 21:41Hannah Fry?

  Well, given that in London, you're never more than two meters away from a rat, I'll just nip downstairs, see if I can find one and ask them during the break, shall I? I'll see if I can do that.

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• 23:34Michael Stevens?

  All right, welcome back. Hannah, what did the rats tell you?

• 23:37Hannah Fry?

  Nothing. They ran away as soon as I approached, which is, you know, one of the good things about being much, much larger than them.

• 23:45Michael Stevens?

  Right, and they could smell you coming.

• 23:47Hannah Fry?

  They could. I saw them sniffing the air pointedly. Actually, on the subject of sewers, there was a kid in my class at school whose dad worked in sewers. And he would always say that you would only smell for the first couple of minutes and then after that just smell like nothing. What is weird is that even when you try, you know, like there's lots of things that you pay attention to, don't pay attention to, they sort of blend into the background. But when you can actively make a choice to pay attention to something, that's interesting that even when you try and smell the inside of your nose, you can't.

• 24:26Michael Stevens?

  I know. It's called olfactory fatigue. You cannot try to smell, even if you try, because you've adapted to it to the point at which your neurons cannot even respond to that smell anymore. One way this happens is called acclimatization, where the actual vesicles in your nerve cells that contain neurotransmitters that send them across the synapse, that has to happen for you to have the smell experience, but they've run out of vesicles. You just used up way too many because you can't stop smelling the inside of your nose. All that's happened, they've run out of the ability to transmit messages, and so you don't get it anymore. Even if you try, they don't have the letters, they don't have the paper to write the letter to your brain on.

• 25:08Hannah Fry?

  You've used up all your spoons, basically.

• 25:11Michael Stevens?

  You've used them all up. So you can try, and you're just never going to smell your nose because you smelled it, I guess, like when you were born.

• 25:21Hannah Fry?

  For one and a half minutes.

• 25:24Michael Stevens?

  Yeah, it ran out, it hasn't been able to replenish. And now, there will be changes because the inside of your nose can smell weird when you have a cold or when there's something weird in your mucus. And then you'll gradually get used to that. You'll notice when the smell is gone. So you only get these brief windows of awareness of your own scent, and then it's gone.

• 25:45Hannah Fry?

  If I am ever lucky enough to attend the birth of another human, the main thing I'm going to be thinking about is as they immediately are born into the world and take their first breaths, it's going to be lucky you.

• 25:57Michael Stevens?

  Another part of change anosmia is adaptation, that we wind up adapting to smells really quickly.

• 26:06Hannah Fry?

  So like when you go into a fish shop, as you first walk in, or even not to make a direct comparison between the two, Michael, but when you return home after your holiday, you can smell it in the first few moments, and then after that, a certain amount of time passes, you can no longer smell it.

• 26:22Michael Stevens?

  Well, yeah, but it's called sensory adaptation. It doesn't just happen with the nose. It happens with vision and sound as well. You can get used to the sound in a place. You visit someone's house and there's this noise, and you're like, doesn't that annoy you? And they're like, I don't hear it anymore. But with smell, it happens really quickly.

• 26:40Hannah Fry?

  So it's about efficiency then essentially, it's about our brains finding like the most efficient way to process information and ignoring anything that's not important to us in the moment.

• 26:49Michael Stevens?

  Yeah, it is. It doesn't mean that we are receiving no olfactory data. It just means that it's all non-conscious. So there has been a lot of research recently on the non-conscious influence of smells around us, where we don't think we're smelling anything, but we are. We think that we enjoy a scent for all the wrong reasons. They've done a lot of studies on like deodorant smells, and it's been found that people prefer deodorants that are the closest in molecular structure and odor to the person's own natural cellular scent.

• 27:26Hannah Fry?

  What? Explain that to me.

• 27:28Michael Stevens?

  So there was a study done famously on dirty t-shirts. I'm starting with a different study, where they looked at what kind of sweaty t-shirts that were worn by men, straight women would prefer.

• 27:45Hannah Fry?

  Sure.

• 27:45Michael Stevens?

  And they preferred the smell of shirts that were the most different from the natural odor profile of the woman.

• 27:55Hannah Fry?

  Okay.

• 27:56Michael Stevens?

  Okay.

• 27:56Hannah Fry?

  So you are looking for someone who smells different to you. That sort of makes sense.

• 28:03Michael Stevens?

  It makes sense for perhaps like an adaptation to avoid inbreeding?

• 28:08Hannah Fry?

  Right. Is there an extreme? I mean, I do sort of wonder whether there's like there's a sweet spot here. Like you want someone who doesn't smell absolutely the same to you, but you also want someone who's not too far away from you either, you know? I don't know. It's a bit difficult to finish that sentence without claiming that some humans really smell, but they do.

• 28:28Michael Stevens?

  Right, right. We all smell. We all smell. The inside of our noses all smell. I think if we scraped some mucus out of your nose, I could probably sniff it and go, oh yeah, that's got a smell.

• 28:42Hannah Fry?

  Please don't. I beg you not to.

• 28:45Michael Stevens?

  I won't do it, but I'm just saying that like we have a smell and we just, we can't smell ourselves. And I, I spent a lot of yesterday trying to find the best way to smell your own scent. And we just don't know how, um, because you could like shower. Okay. Showering, uh, makes it hard to smell yourself, but as soon as you get out, you're, you've altered your own smell. You can leave your house for weeks and come back and, and the scent of your house kind of gives you a sense of what you smell like.

• 29:19Michael Stevens?

  Apparently that trick that perfumists use where they have someone smell coffee beans in between. In order to cleanse the olfactory palate, apparently that doesn't work. People aren't able to, to smell things better after sniffing coffee beans versus lemon juice or just plain air.

• 29:37Hannah Fry?

  Let me go back to that deodorant thing though, uh, cause I'm intrigued by that. Wait, so, so people prefer deodorant that smells more like them.

• 29:45Michael Stevens?

  That's right. So then years later, in a follow-up study, some researchers had women take showers and then put on deodorants. And some of these deodorants were built, like chemically made to mimic the, the chemical composition of the outside of that woman's cells. Which is a very particular thing. It's unique to each one of us. That's part of what makes, um, us smell like ourselves. And the women weren't told that this was the case. They were just told, oh, we're trying out different smells. Uh, but the, the, the, the, there were, there was no masking smell. It wasn't like it's lavender with some of you in it. It was just you or not you. And the women preferred the deodorants that had their

• 30:27Michael Stevens?

  unique genetic chemical composition.

• 30:31Hannah Fry?

  Cause it smelled more, I don't know, familiar.

• 30:34Michael Stevens?

  Like them, like familiar.

• 30:35Hannah Fry?

  This is a bit of me.

• 30:37Michael Stevens?

  Yeah.

• 30:37Hannah Fry?

  Because I am interested by that. And the reason why is that I think like when it comes to sort of hygiene, right, and like cleanliness products, I also think that there's something about how the kind of the more neutral a smell, we sort of don't really like that idea either. And I'm basing this on Fabreeze. In the 1990s, they came up with this actually really, really clever bit of chemistry where they came up with this this new chemical. It's called cyclo dextrin. And essentially what it is, it's this is this ring shaped molecule. What it does is it manages to trap odor, like the literal compounds inside of it. So that it's not just masking smells, it's not just covering them up with other scents that are much stronger. It's something

• 31:22Hannah Fry?

  that actually neutralizes them so that then when they go over your olfactory bulb, they don't.

• 31:27Michael Stevens?

  Bind to it.

• 31:28Hannah Fry?

  Exactly. You literally cannot smell them.

• 31:30Michael Stevens?

  So that's how it works.

• 31:32Hannah Fry?

  That's how originally it worked. And so the idea is that, you know, you spray this stuff on like pet beds and like smoking rooms and it totally worked, right? So they did all of these experiments and they were like, look at this great, we found kind of the holy grail in a bottle. And then they launched it, sent loads of free samples to households who had like cats and, you know, teenage sons and like smokers and whatever. And people didn't use it because they sprayed it all around and they couldn't smell the difference. You know, they couldn't smell their own home. They couldn't smell the stinkiness of their own home. And it just smelled like nothing and nothing didn't feel clean. So instead, they like reformulated it, added perfume to

• 32:12Hannah Fry?

  it, added like a kind of a smell that doesn't need to be there, like a smell of cleanliness. And that was when it really took off as a product.

• 32:21Michael Stevens?

  Wow.

• 32:21Hannah Fry?

  I mean, it's the same as as like toothpaste, right? Like menthol or like minty taste. There's nothing, you know, you could have toothpaste that like, I don't know, tasted like bacon, right? Like it doesn't, it doesn't matter what it smelled like or tasted like. It's just we have like linked certain flavors in our minds to this idea of cleanliness and hygiene.

• 32:43Michael Stevens?

  Yeah, which I think to revisit the inferiority of smell brings us to maybe a point that I'm willing to agree with more, which is that we definitely think of it as being inferior, even though it doesn't need to be. Many studies have done this where they've taken cherry flavored liquid and they've changed its color.

• 33:07Michael Stevens?

  And people will decide that it has a different flavor than it really does. If you take a cherry flavored liquid and you dye it orange, people will say, oh, it's orange flavored. Because they're relying on vision, they're letting that trump what they actually taste and smell. If you, if you take that same cherry flavored liquid and you dye it green, they'll say, oh, it's, I think it's lime or kiwi, maybe. They just do not allow themselves to surrender to olfaction alone. They trust their eyes, nothing else.

• 33:41Hannah Fry?

  All right, then I accept that, you know, our smelling senses is weaker than our other senses because, because we're choosing to ignore it. But I think the question is, what would happen if we did pay attention? We have like this ability there, what if we did do more sniffing? So there was this, this really excellent and frankly hilarious study, which tried to ask that question, which was essentially like, could humans smell as well as dogs? So they got a group of 32 human subjects, human volunteers. They gave them a blindfold. They also, for some reason, gave them ear defenders. And then they tried to see if they could follow a trail left by some chocolate essence

• 34:22Hannah Fry?

  that a bit of twine had been dipped into and then dragged for 10 meters across a lawn. So these people, it's very undignified, this entire experiment, they basically were butts in the air, noses to the ground, like proper, you know, dog sniffing territory we are. And it turns out they actually were able to do it. They actually were able to follow this trail that had gone through the grass. They even tried, they even tried a 45 degree bend to see if they could work out at what point it turned the corner. And I mean, they were actually, I mean, look, they weren't as good as dogs would have been, and that's fine. But they were there sniffing, deliberately paying attention to their sniffing. They were wearing gloves as well, actually, which probably aligns with your theory of like minimized touch.

• 35:07Hannah Fry?

  You know, they're not, they're not exactly bloodhounds, but in most cases, they were able to follow the trail. And crucially, The more they practise, the better they got at it. Maybe it is partly a question of practice, but also I think that there are some people who are going to be better at this than others. And there are some people that are known as super smellers, essentially people who are like very, very good at paying attention to their olfactory system. And also very good at detecting even small amounts of molecules that the rest of us can't really distinguish between. So there's this incredible story. Joy Milne from Perth in Scotland, and she started to notice one day, she'd always been a good smeller, right? But she noticed one day that her husband had this like very particular scent about him.

• 35:52Hannah Fry?

  And she'd be like, have you washed? Are you using a different shampoo? Are you using something different? Why you, why do you smell like that? And he was like, nothing's changed. Like, you know, it's, it's, I am as I have always been. But over time, she was absolutely convinced that his smell had changed. Anyway, a little while later, her husband was diagnosed with Parkinson's. And a little while later, still, she attended a conference for people who were suffering from Parkinson's and their loved ones. And as she went into the room, she was like completely floored by the fact that all of the people who had Parkinson's in there had this exact same smell that she had detected on her husband. And she said that she was like drinking cups of tea, sort of talking to people and

• 36:36Hannah Fry?

  basically going up to them and sort of like surreptitiously sniffing them just to like check if, if they were also someone who had that smell. Anyway, at the end of the conference, she goes up to the, the academic who is giving the presentation and she says, you know that there's a smell associated with Parkinson's, right? Like, you know, asking it as a question. And this, uh, this medic was like, no, like, what are you talking about? This is absolutely nonsense. But he was so convinced by how like totally determined she was to tell him that there was this distinctive smell, that he, um, set up an experiment for her to really test whether she actually could smell Parkinson's. So this Parkinson's research, a guy called Tylo Knath from the University of Edinburgh. He was like, okay, this seems really unlikely, but I'll give it a go.

• 37:19Hannah Fry?

  So he sets up an experiment where he has 12 people wearing T-shirts. I mean, it's similar to your T-shirt experiment earlier, but six of them were just like ordinary men, uh, nothing interesting going on, and six of them had Parkinson's. And he wrapped up these T-shirts and he gave them to Joy to smell them. And she ended up getting eleven out of the twelve correct, correctly identifying every single person who had Parkinson's. And the only one that she got wrong was that she said that somebody had Parkinson's when they didn't.

• 37:49Michael Stevens?

  No, one false positive.

• 37:51Hannah Fry?

  And everyone was like, this is, this is really, this is like incredible, right? This is like absolutely amazing, even that she'd got that many correct. But then, eight months later, that person who she had sniffed Parkinson's on, uh, ended up being diagnosed with Parkinson's.

• 38:08Michael Stevens?

  Whoa.

• 38:09Hannah Fry?

  She'd got the full whammy, right? Like twelve out of twelve correct. And this like incredible feat, this just this one woman who had like detected with her nose, this, this like combination of molecules that are concentrated in, in sort of the oily sebum that in the skin of people who are going to develop the disease, has like opened out this whole new area of research of like electronic noses, essentially. We are so biased by vision, right? That most of our diagnoses that happens, happens through like visual means, right? You know, you, you take a test to a lab and they look at it on a slide. You kind of, you, you take someone into a medical scanner and you look at the, the, the X-ray or whatever it might be.

• 38:53Hannah Fry?

  You kind of go into a doctor and they take a look at your, your, your complaint. And this opens up a whole new world of like, maybe there are ways to diagnose disease that are much more around smell rather than about vision.

• 39:06Michael Stevens?

  Wow. Super smellers. I'm more of a super stinker.

• 39:13Hannah Fry?

  She'd be amazing as a perfumer, wouldn't she? Joanne Mill. She'd be like, she'd be all over the place.

• 39:19Michael Stevens?

  Well, yeah, she could do, I don't know why you would want this, but you, she could design a perfume to make you smell like you had all kinds of diseases.

• 39:27Hannah Fry?

  Or even like you smell like yourself. Like imagine if there was like a, a Michael Stevens perfume.

• 39:33Michael Stevens?

  Oh, I would love that. I would love a perfume that smelled like me. Yeah.

• 39:39Hannah Fry?

  You wouldn't be able to smell it, though.

• 39:40Michael Stevens?

  Ah, she, okay, that's the problem, because I want to know what I smell like. But we live our lives with this blissful ignorance, where we just focus on the things that we need to be focused on. You know, they call this the centipede's dilemma. There's this great poem. "A centipede was happy, quite, until a toad in fun said, 'Prey, which leg moves after which?'" "This raised her doubts to such a pitch, she fell exhausted in the ditch, not knowing how to run." And so, as soon as someone says, "Wait, think about what you're doing," or "Explain what you're doing," you go, "Oh, shoot." Like, I don't know, it's just, it's happening without me having to be aware of every little bit.

• 40:22Michael Stevens?

  But once you make me think about it all, once the centipede is asked, "What's wrong with your 34th left foot?", then it just goes, "Whoa, that's too much, I don't know."

• 40:31Hannah Fry?

  So when it comes to the question of what does the inside of your nose smell like, the answer is, you'll never know, but thank goodness you won't.

• 40:39Michael Stevens?

  Yeah. That's a good answer. It does have an odor. You can non-consciously detect it. Other people can consciously detect it, but you will never know what it smells like, and it's good that you won't.

• 40:53Hannah Fry?

  What a beautiful way to end this episode. If you have any questions for us, anything you want to send to our mailbag, you can email us, [email protected].

• 41:02Michael Stevens?

  And also, don't forget to like and subscribe. And if you want to hear more from us, go to therestis.com/science to sign up for our newsletter.

• 41:10Hannah Fry?

  Until next time.

• 41:11Michael Stevens?

  See you later.