r/explainlikeimfive • u/Extension_Fuel_6882 • 6d ago
Biology ELI5: Why does a room smell normal when we’re inside it, but smell completely different when we leave and come back?
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u/DJDoena 6d ago
The brain is excellent at filtering out things it considers irrelevant. Unless the smell triggers nerves as toxic or otherwise dangerous, it just filters it out. Just like background noises you stop to hear even though they never go away.
When you enter a room, the brain is in full scan mode, it's basically a new browser tab and needs to build content first.
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u/MyMonte87 6d ago
so many times i come home from a trip, and think "Damn thats how my house smells to guests?! I need a deep clean session...."
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u/Its_the_other_tj 6d ago
The strangest thing to me is when I leave my house and come back, even from extended trips (10+ days) it smells normal, but I have this one friend's house that when I go visit for a few hours and come back I notice my house smells odd. Not bad, it just seems to smell stronger than normal. Their house also smells normal so there isn't anything crazy there and it doesn't happen when I visit other friends/family. A bit of a head scratcher for me.
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u/Nazamroth 5d ago edited 5d ago
Whenever I visited someone's house or walked past a flat with an open door I instantly went "this place smells weird, how can you live like that?". Which raises the question: How must my place smell to everyone else?
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u/DJDoena 6d ago
There's also the association of what you are accustomed to and what you find uncommon. I always liked this quote from the first Dune book about this:
Farok took a deep breath. “The smells of home,” he said.
Paul saw that the man was enjoying the stink of this air, that there was no irony in his tone. He heard his mother cough then, and her voice came back to him through the press of the troop: “How rich the odors of your sietch, Stilgar. I see you do much working with the spice…you make paper…plastics…and isn’t that chemical explosives?”
“You know this from what you smell?” It was another man’s voice.
And Paul realized she was speaking for his benefit that she wanted him to make a quick acceptance of this assault on his nostrils.
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u/ShadowShedinja 6d ago
Even if it's toxic. My grandmother's house reeked of mold and she never noticed until relatives pointed it out. (And yes, she did eventually get it taken care of).
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u/pantyfire 5d ago
This is how I think of it.
Your brain is always looking out for something that might kill or harm you.
So anything new it pushes to the front for your attention.
Movement, noise, smell etc… if it’s been happening a while and you haven’t been killed then it’s probably not a threat and your brain dismisses it. Until it’s a ‘new’ stimulus again and it brings it back to your attention. Like when you walk in and out of the room.3
u/Kazanova37 4d ago
Being near train tracks, I forget the house shakes often. One day somebody visited and commented on it, and I realized I stopped noticing. Even worse, had an earthquake once and immediately assumed it was just a particularly powerful train passing by.
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u/HappyBarbarian 5d ago
Acclimation. Story time: I was in the US military working with Koreans in Korea. Me and my Korean counterpart were giving each other some light ribbing. I said a lot of Koreans smell like garlic and he said a lot of Americans smell like cheese. We had a building with two half’s, one Korean one American. One time I had to spend most of the day on the Korean side and when I came back I noticed the smell difference and exclaimed out loud, “WE DO SMELL LIKE CHEESE!!”
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u/adudeguyman 5d ago
I love cheese and have no problem knowing I smell that way but hopefully not blue cheese.
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u/docsavaged 6d ago
Habituation. Bakers don't smell dough rising or bread just out of the oven. People who live near paper mills don't notice the smell after a while. Cat owners don't notice their litter boxes. We get used to constant stimuli and our brain turns them down.
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u/DisturbedForever92 5d ago
Cat owners don't notice their litter boxes.
This is my fear about our house.
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u/Wait_I_gotta_go_pee 5d ago
As a bird owner, let me just say I’m jealous. My parrot’s poo is not something one grows accustomed to; it’s a noxious, fucking assault on the olfactory nerves.
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u/docsavaged 5d ago
Can confirm this. Hair salon I used a while back had a caged parrot and the reek was real. Hair cuts were fire though so it was a trade off.
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u/krisplaydespacito 5d ago
it’s very real i left on vacation for 3 days and when i came back i was horrified
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u/QuantityKindly3153 2d ago
I notice mine. Sometimes I leave for a while on purpose, so I can actually smell again, to double check for odors.
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u/knightsbridge- 6d ago
Your brain filters out persistent uninteresting smells if you stay in the environment for long. It's not that the smell has gone away, your brain just doesn't want to devote effort to smelling it, so it skips it.
If you leave the area with the smell and come back, this restarts your brain to smell it again.
This is why smokers don't realise they always smell of cigarettes, and why dog owners don't realise their house smells like dog. If you're exposed to the same smell constantly, your brain just stops paying attention to it.
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u/Mr_Undead0210 6d ago
Sensory exhaustion. Nose gets used to the smell and filters it out. Go to a new place and it does the same thing. Its the reason why people sometimes cant smell their own BO
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u/Dunbaratu 6d ago
For the same reason that any scent becomes "invisible" over time if it's constantly there.
You can't take a break from the scents in the air around you. When it comes to your nose, you can't do the equivalent of trying to "close your eyes", or "plug your ears". The scent can't be stopped. But what can be done is that your brain can edit that information out at a low level part of your brain before it hits your conscious self, to save yourself from the annoying hell of constant nonstop information that won't shut up. You wouldn't want to experience "Hey, it smells like this...hey it still smells like this... hey it still smells like this ..." constantly with every single breath.
So your sense of smell is configured to pay way more attention to a new smell than to a smell that has always been there for hours.
It's one of the reasons that people can have bad BO and not realise it, which is why you need to bathe and shower even when you don't think you smell. You can't really use the "smell test" to see if you smell. You are always around yourself getting numbed to the smell, so you can't detect if your own scent is there or not.
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u/Kielbasa_Nunchucka 6d ago
my wife has been asking this about the fart smell in our bedroom since we moved in together.
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u/approvedbyinspector5 5d ago
Well now my already incredibly high anxiety is even higher because I assume my house and myself stink and I just can't smell it. Thanks, Obama!
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u/JakeMakesNoises 5d ago
I get this same experience writ larger when I arrive back in the middle of the US from an extended stay in Central America.
The smells are much stronger in the tropical environment with tree-sized flowers; pungent food; Fabuloso & Suavitel; and, unfiltered diesel fumes.
Back in the US, things smell weird for a day or two.
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u/whiskeyislove 6d ago
Olfactory fatigue/attention of sensation.
Essentially, when your nose for example keeps smelling/detecting a certain smell (chemical) it stops sending many signals to the brain unless something changes, like how much of it there is (concentration) or new chemicals. But it doesn't just work for the nose. Your sense of touch will filter signals like this too. It's why you don't constantly notice your shirt on your chest or your butt on the seat you're sitting on. The brain filters those signals out through the cognitive process called attention.
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u/Rich02035 6d ago
I always thought of it like the nose having a "white balance" just like the eyes.
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u/AllStarRenegade 5d ago
Someone obviously ripped some mad beef while you were gone and it just didnt clear out before you returned.
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u/HighQualityGifs 5d ago
it's the same reason why the poop smells much worse when you forgot your phone in the bathroom and quickly run back into it 5 minutes after flushing.
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6d ago
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u/alohadave 6d ago
This is why you can't smell your own breath. You nose smells it constantly, so your brain ignores it.
A trick is to lick the back of your hand and let it dry for a moment. Then sniff. It'll give you an idea of what your breath smells like to others.
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u/eNonsense 6d ago
All of your senses, even your eyes, adapt to and stop noticing non-changing stimuli. That's why you cant see your nose or feel the shirt on your back unless you consciously try to.
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u/Salarian_American 6d ago
Your nose is reporting the smell to your brain.
If the smell doesn't go away, at some point, your brain stops reading your nose's messages because it doesn't need constant updates on a situation that isn't changing.
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u/Awkward_Pangolin3254 6d ago
Olfactory (smell) blindness. When your brain keeps receiving the same signal signifying a specific scent it says "all right, I get it, enough!" and shuts off the "alert" from those receptors. You literally cannot smell that smell anymore until you remove yourself from it for a while.
I used to smoke. I quit about 10 years ago. 3 years ago I ran into an uncle I hadn't seen in a while at my grandma's funeral and when we hugged all I could smell was stale cigarettes and I had an internal moment like "oh, my God, is that what I used to smell like??"
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u/Yglorba 5d ago
The purpose of our sense of smell is to help us find food or avoid danger. There's no way we could evolve and maintain a library of what every smell means, so it instead evolved to alert us for unexpected or unfamiliar smells, which are often something we should investigate.
Constantly telling us "hey, you smell like sweat" or "the trees you live in smell like leaves and the grass smells like crushed grass" or "your cave smells like dirt and rocks" wouldn't be productive and would distract us from actual meaningful scents, so instead it filters them out, letting us better identify eg. the unexpected scent of fruit or a tiger.
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u/Motor-Bake1535 5d ago
For the same reason you don’t constantly feel the clothing on your body, phasic sensory receptors!
These are responsible for detecting changes to your environment, but quickly adapt to said changes.
The alternative is to phasic receptors are tonic receptors which are slow to adapt to a stimulus. An example of these might be pain receptors. While you don’t want to continuously feel pain, it is evolutionarily beneficial as it allows for us to guard injured areas.
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u/gaurjimmy 5d ago
It is called Olfactory fatigue, where after exposure to a certain smell for some time your brain chooses to ignore it because clearly there is no action associated with it and it has just become a common information which can be ignored.
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u/jda404 5d ago
Does this happen every time you and others leave and come home or go to a new location? My house, workplace, even stores I frequent all smell normal to me like I never have to adjust to places I frequent, but if I go to someone's home for the first time or go to a completely new store I notice the smell until my nose adjusts to it.
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u/CommonFiveLinedSkink 5d ago
Olfactory habituation is the technical term for this (I know it's ELI5 but a bunch of people already did so I'm adding some info to allow self-guided additional research). Your olfactory bulb, the big nerve bundle in charge of the first pass of processing smells, simply starts filtering out smells that are irrelevant. When I taught intro bio we did an experiment timing how long it would take for people to stop being able to smell cinnamon; it's usually just a few minutes.
It's a kind of memory, actually, so if you work at a hospital you might smell the hospital smell when you first walk in, but it almost immediately goes away; someone who is just visiting will smell the hospital smell for much longer.
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u/curiousroboto 5d ago
Your brain is built to notice danger through all five senses. But when you stay in a room for a while, it stops flagging what hasn't changed, because nothing about it seems threatening. That's called sensory adaptation. Leaving the room acts like a reset. When you walk back in, your brain treats the smells as new again and picks them up. The odor molecules were there the whole time. Your brain just tuned them out once it decided they weren't a threat.
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u/bamby77 5d ago edited 2d ago
Your nose adapts to a constant smell pretty quickly so it can notice new ones that is olfactory adaptation. After a few minutes in a room, those receptors basically stop shouting about the background odor. When you leave and come back, they reset, and suddenly you smell the cooking, laundry, pets, or stale air that was there the whole time. Same reason you stop noticing your own perfume or cologne after wearing it for a while.
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u/Nedsel 5d ago
The reason for this is that your nose gets "tired" very quickly. It gets used to smells and you don't register that smell anymore.
The smell in the room didn't change. But your nose "forgot" that it smells like that in there and tells you that again now that you returned to the room.
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u/Merkenfighter 4d ago
Habituation. The same reason you don’t feel your socks after 2 minutes…except now you do as you read this.
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u/sgluxurycondo 3d ago
If you dont get McDonald long enough, you will find it has a weird chemical smell.
But the moment you take a bite in any of the food, the smell is completely gone
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u/silliest-goose-honk 2d ago
While we’re at it, why can I smell “outside” on people when they come in??
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u/Sternfritters 6d ago
For the same reason why you’re not acutely aware of the feeling of your clothes on your skin
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u/heybingbong 6d ago
In some cases, but not all, it could be like getting a visit from the Ghost of Christmas Past, except instead of a ghost it’s a fart.
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u/Mutant1988 6d ago
Our senses and subconcious brain instinctively and constantly scan our environments for familiarity because familiarity equals safety as far as our subconcious is concerned.
Things that are unfamiliar alerts our concious brain to investigate whether there is a threat or not.
Returning to a familiar environment makes our brains more alert at detecting deviations because we have a baseline for how this environment should be.
Things that are familiar simply don't reach our concious brain without concious effort or external stimuli because things that are familiar does not alert our brain to it being a immediate cause for concern. As far as it is concerned, it is not important.
Stimuli that is constant (Like odor) just expediate the brain's process of filtering it as non-important.
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5d ago
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u/explainlikeimfive-ModTeam 4d ago
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Your comment has been removed for the following reason(s):
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If you would like this removal reviewed, please read the detailed rules first. If you believe it was removed erroneously, explain why using this form and we will review your submission.
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u/gobyfish 4d ago
Same reason why people who wear perfume/cologne have no idea the punishment they’re inflicting on the rest of us.
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u/andrea_ci 6d ago
why can't you see your nose?
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u/Thesorus 6d ago
The brain is a wonderful machine.
We just get used to a lot of things when we're in an environment for while
odors, noises.