Why Can't You Smell The Inside Of Your Nose?

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1:20Hello, I'm Hannah Fry. I'm Michael Stevens. And this is The Rest is Science. We're talking smell today, Michael. Smell? Yeah. Okay. Well, let me ask you this, Hannah. What do you smell right now? Like, honestly, not very much. I'm in my house. Normally I have a scented candle on in the corner. Don't know at the moment. I sort of can't really smell anything. I mean, this is like, it's the inferior sense. There's not much going on. I can't smell anything. Whoa. The inferior sense? Your nose can hear what you're saying. It's going to testify against me one day. We're going to talk about that in this episode.

1:58Is 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 a pretty odor neutral environment that I'm in. I think we live in very odor neutral environments or we think we do. because I wonder if we swapped if we did like kind of 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 uh of roses and beautiful perfume and like you know window cleaner and like you know fabric softener and and all of the lovely smells.

2:53Yeah. 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. That's a really fun idea. You got to do it. 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.

3:35So 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's like put on that candle and transported back to that place. 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. Oh, that's a fantastic idea. Well, hold on a second, right?

4:11If 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. Yeah. What is the inside of your nose smell like? Well, if we're going to get to the bottom of these questions, I think we should first turn to an assumption which is still quite popular. The fact that smell is the inferior scent. Should we start there? Yeah. Give me your case. This is essentially Victorians being dismissive of anything they didn't consider highfalutin, right?

4:46So there's a French guy, Paul Broca. Broca? Broca. I say Broca. Okay, so Broca, Broca. In the Victorian era, he like started 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.

5:25They'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. Right. 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. That's so Victorian. Very Victorian, isn't it? 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. You can smell the difference between coffee grounds and like soil after rain, you know, pencil shavings or like wet dog.

6:04If 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.

6:39Yeah. 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. Trillions. If you give them two smells and you say, are these different?

7:19They 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. 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 it's effectively like being able to tell the difference between a left and right-handed glove you know that's essentially how good we are at it have you ever smelled ethyl more captain no you have if you've ever smelled a gas leak because propane, natural gas, these things are odorless, but that's dangerous.

8:17If 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, when 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 Ethel Mercaptan crashed near my house and it smelled like there was a gas leak, 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, oh, don't light a match.

9:02It's gas. And I'd be like, actually, that's the Ethelmer Captain 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, Michael, is that actually gas or is it the odorant? Oh, look at me. I'm Michael. And I've never forgotten that. 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. Yeah. Just imagine this head with this beard, but on like a kid body. Well, but I bring up Ethelmer Captain because we are really sensitive to it.

9:40We can, we can detect 0.009 parts per billion. That's the equivalent of like three drops in an Olympic size swimming pool. Okay. You could put three drops in 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? Because even though we can detect small trace amounts, it doesn't get our attention.

10:24We 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. 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, these huge long pipes that would go for hundreds and hundreds of miles. And because they were pumping in natural gas, they had Ethel McApton 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.

11:06And a lot 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 were circling over where there was a leak in the pipe, right? And so it 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. I think they hadn't honed it down to being Ethel McAptain at that point.

11:40They just knew that vultures were circling. And this group of scientists were like, OK, 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 tarpaulin 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 tarpaulin 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 tarpaulin and it turns out that ethel macaptan is like this compound that is released when bodies decompose 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 gas leaks.

12:31That 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 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 can detect something. But if we're just normally breathing, there needs to be a lot of odorant around for us to notice it. 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.

13:22The 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 sight or sound. If I'm in a room and there's a sound, I'm like, ugh, the workers on the roof are loud or the refrigerator is loud. It's really, in my awareness, in a spatial location. Same with a light, the source of a light.

14:08It'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 because we just don't have the same conscious interior map of smells that we have for our other senses. 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.

14:48With 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. That's right. There is no rainbow of smells. Yeah. Where we're like, oh, make that a little bit more XYZ. And funny enough, 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 sense of like sweeter, more bitter.

15:31I don't know. It's hard for us even to describe them and compare them the way we can talk about color. 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 mapping it physically, right? And paying attention to it. And paying attention to it. I had this story once. 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.

16:06And it can 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 of course the military want to try and stop these like big chunks of cash getting smuggled. So what they did is they took all of these dogs and 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 odour, but there's lots of things like the ink, the kind of the oil that's left in people's hands. There's like a kind of catalogue of different smells that might be associated with it.

16:36And the dogs in the training were really good. Like they were amazing at finding these wads 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.

17:11And they were really wondering what on earth was going on until they finally realized that the difference between the lab conditions and the military zone was that when in the lab, the money had been wrapped in cling film in order that people didn't steal 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 dogs smell is. Does cling film really have that stronger scent? No, I think money and cling film, I couldn't smell the difference. But research has shown that we can. We just choose not to.

17:47Just choose not to. Yeah. And we just don't sniff enough. That's human's problem, Hannah. 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 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.

18:28But 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 there's only like find the seven differences. They're 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 look at them simultaneously, it becomes easier. 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.

19:11But for smells, we basically live in a constant state of change anosmia. 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 yeah yeah you know actually also we uh 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 oh i bet they can because they're just they're so much more attentive to smells 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 uh See if I can do that.

19:57This episode is brought to you by Cancer Research UK, who over the past 50 years have helped double cancer survival in the UK. You might have heard of BRCA genes. These are the ones that made headlines when Angelina Jolie revealed that she carried a faulty version. Yeah, BRCA genes are part of our DNA. They help to repair cells and keep them healthy. The risk comes when BRCA genes are faulty. And about 1 in 400 people inherit a faulty version, increasing the risk of some cancers. Yeah, now this discovery came from Cancer Research UK scientists who came across the BRCA1 and BRCA2 genes, a breakthrough that changed how doctors prevent, diagnose and treat cancer.

20:36And now we've got genetic testing that means that people who have faulty BRCA genes can take steps to prevent cancer or to receive tailored treatment. The discovery also revealed a weakness in cancer. By turning that flaw against the disease, researchers developed PARP inhibitors, targeted drugs that are now helping thousands of people. And all of this really points to a future where medicine is no longer just one size fits all. It's something that's informed by your own DNA. So for more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit cancerresearchuk.org forward slash rested science.

21:19All right, welcome back. Hannah, what did the rats tell you? 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. Right, and they could smell you coming. 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.

22:02But 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. 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.

22:44All 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. You've used up all your spoons, basically. 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. For one and a half minutes. 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, you know, when there's something weird in your mucus.

23:21And then you'll gradually get used to that. You'll notice when the smell is gone. So you only get these like brief windows of awareness of your own scent and then it's gone. 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. Another part of change anosmia is adaptation, that we wind up adapting to smells really quickly. 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.

24:03And then after that, a certain amount of time passes, you can no longer smell it. 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 with smell, it happens really quickly. 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.

24:35Yeah, 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. What? Explain that to me. So there was a study done famously on dirty t-shirts.

25:18I'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. Sure. And they preferred the smell of shirts that were the most different from the natural odor profile of the woman. Okay. Okay. So you are looking for someone who smells different to you. That sort of makes sense. It makes sense for perhaps like an adaptation to avoid inbreeding. 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.

26:06You know, I don't know. It's a bit difficult to finish that sentence without claiming that some humans really smell. But they do. 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. Please don't. I beg you not to. 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 spent a lot of yesterday trying to find the best way to smell your own scent. And we just don't know how because you could like shower. OK, showering makes it hard to smell yourself.

26:52But as soon as you get out, you've altered your own smell. You can leave your house for weeks and come back and the scent of your house kind of gives you a sense of what you smell like. 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 smell things better after sniffing coffee beans versus lemon juice or just plain air. Let me go back to that deodorant thing, though, because I'm intrigued by that. Wait, so people prefer deodorant that smells more like them.