How we actually smell: a short tour of the nose and the brain

A founder’s plain-language guide to how the sense of smell really works, from a molecule in the air to a feeling in your mind, written by the founder of a Singapore fragrance house.

We treat smell as the simplest of the senses, the one we notice least until a durian clears a lift or a bakery stops us in the street. It is, in fact, one of the strangest and most sophisticated things the body does, and understanding it changes how you think about every perfume you have ever worn. This is a short, jargon-light tour of what actually happens between a scent molecule drifting off your skin and the memory or feeling it stirs, and why that machinery makes smell so unlike sight or sound.

It begins with a molecule in the air

Everything you smell is a physical thing. A scent is not a wave or a vibration in the way sound is; it is actual molecules, small and light enough to lift off a surface and float into your nose. When you smell coffee, tiny amounts of coffee are literally arriving at your nostrils. This is why warmth intensifies smell, since heat sends more molecules airborne, and why a cold, still room smells of less.

Those molecules are drawn up to a patch of specialised tissue high inside the nose, where the actual detecting happens.

The nose reads scent by combination, not by list

Here is the first surprise. You do not have a separate detector for coffee, for rose, for petrol. You have a few hundred different types of smell receptor, and every odour is recognised by the particular combination of them it switches on.

The discovery of these receptors was significant enough to win Linda Buck and Richard Axel the Nobel Prize in 2004.[¹] What their work revealed is a system a little like an alphabet. A few hundred receptor types, in different combinations, can spell out an enormous range of distinct smells, just as twenty-six letters spell every word in the language. It is why a perfumer can build so much from a finite palette, and why the true number of smells a person can tell apart is genuinely hard to pin down. The often-repeated figure of ten thousand is folklore rather than science, and honest estimates vary wildly.[¹]

The strange route to the brain

The second surprise is where the signal goes, and it is the key to everything that makes smell feel different.

When the receptors fire, they pass the signal to the olfactory bulb, a structure just behind the nose, and from there it takes an unusually direct road. Most of your senses send their information first to a relay station called the thalamus, which sorts and forwards it to the thinking parts of the brain. Smell largely skips that queue. As the neurologist Christopher Hawkes describes, olfactory signals reach the cortex without depending on the thalamic relay the other senses use, and they connect directly to the temporal lobe and the limbic system, the brain’s seat of emotion and memory.[²]

That single quirk of wiring explains so much. It is why a smell can move you or unsettle you before you have named it, why fragrance reaches feeling faster than thought, and why scent-triggered memories arrive with such emotional force. Sight and sound are processed, then felt. Smell is very nearly felt first.

A few more things worth knowing

The system has other unusual features. The smell-detecting neurons in your nose are among the very few nerve cells in the body that regenerate through life, replacing themselves every few weeks, which is part of why smell can often recover after a cold or a knock.

Smell is also tightly bound to the health of the brain. Because the olfactory pathway runs straight into vulnerable limbic regions, a fading sense of smell can sometimes be an early sign of certain neurological conditions, which is now an active area of research.[²] Losing smell, as many discovered in recent years, is also quietly devastating in daily life, draining the flavour from food and the emotional colour from the world, a reminder of how much this underrated sense carries.

Why this matters for perfume, and for us

All of this is not trivia. It is the ground everything we do stands on.

Because smell works by combination, composing a perfume is closer to writing music than mixing paint, arranging a limited set of materials so that, together, they read as something whole and new. Because odour molecules of different weights lift at different rates, a fragrance unfolds over time rather than arriving all at once, which is why a scent has a beginning, a middle and an end. And because smell wires straight into emotion and memory, a perfume is never only a pleasant surface. It is an instrument for feeling and recollection, whether we intend it or not.

Understanding the machinery is what lets us design with it rather than against it. When we compose a scent for a person, a brand, or a space, we are working with a sense that is physical, combinatorial, and wired directly to the heart of how people feel. Respecting how it actually works is the difference between a fragrance that merely smells nice and one that does something.

Frequently asked questions

How does the sense of smell work, simply put? Scent molecules float off a surface into your nose, where they are detected by a few hundred types of receptor. Each smell is recognised by the specific combination of receptors it activates. That signal goes to the olfactory bulb and then, unusually, straight to the brain’s emotion and memory centres, largely bypassing the relay station other senses use. That direct route is why smell feels so immediate and emotional.

How many different smells can humans detect? Genuinely more than the old textbook claims, but the exact number is uncertain and much debated. The commonly quoted figure of ten thousand is folklore rather than a real measurement. Because we detect smells by combining a few hundred receptor types, the range is very large, but honest scientific estimates vary widely, so no single number should be trusted too firmly.

Why is smell so closely linked to emotion and memory? Because of how it is wired. Unlike the other senses, smell reaches the brain without fully relying on the thalamic relay, connecting directly to the limbic system, including the structures that handle emotion and memory. This means a scent can trigger a feeling or a memory almost instantly, often before you have consciously identified the smell.

Can you regain your sense of smell after losing it? Often, yes, at least in part. The smell-detecting neurons in the nose are among the few nerve cells that regenerate throughout life, which is why smell frequently returns after a cold or minor injury. Recovery is not guaranteed and can be slow, and persistent smell loss is worth discussing with a doctor.

Why does understanding smell matter for perfume? Because it shapes how a perfume is built. Smell works by combination, so a scent is composed from a limited palette like notes in music; molecules lift at different rates, so a fragrance unfolds over time; and smell wires into emotion and memory, so a perfume affects feeling, not just the nose. Designing with this machinery, rather than ignoring it, is what separates a fragrance that smells nice from one that genuinely moves people.

Notes

[¹]: Avery Gilbert, What the Nose Knows: The Science of Scent in Everyday Life (Crown, 2008), on Linda Buck and Richard Axel’s Nobel-winning discovery of the olfactory receptors, the combinatorial way smells are encoded, and the fact that the popular “ten thousand smells” figure is folklore rather than a real measurement. Summarised in our own words.

[²]: Christopher H. Hawkes, The Neurology of Olfaction (Cambridge University Press), on the olfactory pathway reaching cortical and limbic regions without depending on the thalamic relay used by other senses, and on the links between the olfactory system, the limbic brain and neurological disease. Summarised in our own words.

Jason Lee is the founder and CEO of Scent by SIX, a Singapore artisanal fragrance house that designs and makes its own fragrances, composes bespoke and private-label scents for brands and spaces, and runs ambient scenting and perfumery workshops. The studio’s work includes Batik Flora for Singapore Airlines, the TCM collaboration with Eu Yan Sang, the Hikaru collection with the Singapore Association for Mental Health, and the Care Range with Caregivers Alliance. Jason writes weekly on fragrance design and on what scent can do for Singapore.

Tags for Medium: Singapore, Olfaction, Neuroscience, Perfumery, Made In Singapore