How Does Smell Work? The Sense Behind Perfume

Smell begins when molecules evaporate from something — a flower, a cup of coffee, a perfume on your wrist — and travel through the air into your nose. High in the nasal cavity, they meet sensory nerve cells carrying olfactory receptors. Each kind of receptor responds to a range of molecules, and each molecule switches on several kinds of receptor, so what reaches the brain is a pattern, not a single signal. The brain reads that pattern as a smell, and it does so along pathways closely tied to emotion and memory. That is why a perfume can feel like a mood, or bring back a person, before you have found a word for it.

This guide follows a perfume from the skin to the mind, and links to the Journal’s articles on each step.

From the bottle to the nose

Only molecules that evaporate can be smelled. A perfume is a mixture of materials that evaporate at different speeds: the lightest rise first and fade first, the heaviest stay on the skin for hours. That is the whole basis of the familiar pyramid — see Top, Heart and Base Notes — and of the quiet final stage described in What Is the Dry Down of a Perfume?.

How far those molecules travel, and for how long, is what perfume lovers call sillage, projection and longevity; see Sillage, Projection and Longevity, Explained.

Two ways in: sniffing and eating

When you sniff, air carries molecules up through the nostrils. But there is a second route: when you eat or drink, molecules rise from the back of the mouth into the nose. This "retronasal" smell is why most of what we call flavour is really smell, and why food tastes flat when a cold blocks your nose. It also explains why gourmand perfumes — vanilla, caramel, coffee — can smell so convincingly of food; see Gourmand Perfumes.

Receptors and patterns

The genes for olfactory receptors form one of the largest gene families in the body. Their discovery, and the way the olfactory system is organised, earned Richard Axel and Linda Buck the Nobel Prize in Physiology or Medicine in 2004. Humans have several hundred working receptor types, each nerve cell carrying one of them.

Because smell works by patterns, a few things that puzzle perfume buyers make sense:

A "note" is a pattern, not a material. A perfume can smell of pear or rain without containing either, because a blend of other molecules produces a similar pattern. See Are Perfume Notes Real Ingredients?.
Small changes in a molecule can change its smell entirely, while unrelated molecules can smell alike. This is the ground that synthetic perfumery was built on; see How Synthetic Molecules Created Modern Perfume.
Strength is not the same as quantity. Some molecules are noticeable at tiny amounts, others need far more; a perfume’s balance depends on it.

Straight to feeling and memory

From the nose, signals travel to the olfactory bulb at the base of the brain, and from there to regions involved in emotion and memory. Smell reaches them by a shorter path than sight or hearing do. Whatever the full explanation, people commonly find that smells call up memories and feelings with unusual force — the subject of Why Smells Trigger Memories.

It also helps explain why our words for smells are borrowed from other senses — warm, bright, sharp, soft. See Why Some Scents Feel Warm.

We meet a smell as a feeling first, and find the word for it afterwards, if at all.

Why your nose stops noticing

The sense of smell is built to notice change. A smell that stays constant is turned down within minutes, which is why you stop smelling your own perfume while others still can. See Why Can’t I Smell My Own Perfume?.

Why no two people smell alike

People differ in their receptor genes, so the same molecule can smell strong to one person, faint to another and of nothing at all to a third — a "specific anosmia". Musks are the classic case; see What Is Musk in Perfume?.

Experience matters even more. Whether we like a smell depends largely on what we learned to connect it with; see Why We Love Some Smells and Dislike Others. And a perfume itself changes from one person to the next, for reasons on the skin rather than in the nose; see Why Skin Changes a Perfume.

Perfume marketing sometimes promises a shortcut past all this, in the form of pheromones. The evidence is examined in Do Pheromone Perfumes Work?.

What this means when you choose a perfume

Judge on skin, over time. A paper strip shows the opening; the pattern that stays with you develops over hours. See How to Test a Perfume Before You Buy It.
Smell only a few at once. After several perfumes the nose tires; step outside, or smell your own skin, before going on.
Trust your own reaction over a list of notes. Two people reading the same list can smell different things.
Give unfamiliar smells a second chance. Liking is partly learned, so a material that seems strange at first can become a favourite.

When it might be something else

A sudden or lasting loss or change in your sense of smell — food that tastes of nothing, familiar smells that seem wrong — is not ordinary adaptation. It can follow colds and other infections and has many other causes. If it persists, mention it to a doctor.

Common questions

How many smells can humans detect?

There is no reliable single number. Published estimates differ enormously and have been disputed, because "different smells" is hard to define. It is safe to say the number is very large, and that most people can learn to tell many more apart with practice.

Is smell connected to taste?

Closely. The tongue detects only a few basic tastes, such as sweet, salty, sour, bitter and savoury; most of what we call flavour comes from smell reaching the nose from inside the mouth.

Can you train your sense of smell?

You can train your attention and your vocabulary. People who smell materials regularly, and name them, become much better at noticing and telling smells apart — that is how perfumers learn.

Why do some perfumes give me a headache?

Some people find strong fragrances unpleasant or uncomfortable, especially in closed spaces. Wearing less, choosing quieter perfumes and testing in fresh air help; if reactions are strong or frequent, talk to a doctor.

Sources

The Nobel Assembly at Karolinska Institutet, press release on the 2004 Nobel Prize in Physiology or Medicine (Richard Axel and Linda B. Buck, "for their discoveries of odorant receptors and the organization of the olfactory system"), 4 October 2004.