How does the nose smell?

The bit of nose you can see is only the doorway. Behind it, two tunnels, the nasal cavities, run about 10 cm back into your head. A thin wall, the septum, splits them into left and right: bendy cartilage at the front, bone further back. We have cut the left side away so you can look into the right one.

On the outer wall of each tunnel hang three curled shelves of bone covered in soft, wet lining: the inferior, middle and superior turbinates (doctors also call them conchae). They make the air swirl past a big, warm, wet surface, which is how your nose warms, wets and cleans each breath before it reaches your lungs.

Smelling happens in just one small place: a yellowish patch high in the roof, the olfactory epithelium. It covers a few square centimetres in all, roughly a postage stamp on each side. Millions of smell nerve cells live there. Their fibres pass up through tiny holes in a sieve of bone, the cribriform plate, to the olfactory bulb, the first stop in the brain.

Around the nose sit air-filled spaces in the skull bones, the sinuses: in your forehead, your cheeks, between your eyes and behind your nose. At the back, both tunnels meet in the nasopharynx, which leads down your throat.

In quiet breathing most air rushes along the lower tunnels, and only a small share, roughly 5 to 15% in computer models, curls up to the smell patch. A sniff pulls air in faster and sends more of it up there.

Your lungs are delicate. They like air at body temperature and nearly full of water vapour, whatever the weather outside. Your nose does that job in a fraction of a second.

The turbinates are packed with blood vessels and covered in wet mucus. As air swirls past, heat flows from the warm lining into it and water evaporates into it. Measurements inside people’s noses found that room air at about 25 °C reaches the back of the nose at about 34 °C and about 90% relative humidity. Even freezing air is warmed to around 30 °C. Breathe through your mouth and much less of this happens, so the airways further down, towards your lungs, have to finish the job.

The mucus is also flypaper. Dust, pollen and germs stick to it; particles bigger than about 10 micrometres rarely get past the nose and throat. Tiny hairs called cilia beat more than ten times a second, sliding the whole mucus blanket back towards your throat at a few millimetres a minute, where you swallow it without noticing.

Here is a surprise: in most people the two sides take turns. One side swells a little and the other opens up, then they swap every few hours. This is the nasal cycle, and it is why one nostril often feels more blocked than the other.

Zoom into the smell patch about a thousand times. It hangs from the roof of your nose, so air is below it. A thin layer of mucus covers it, and millions of olfactory sensory neurons poke tiny hairs, cilia, down into that mucus.

A smell is a cloud of molecules light enough to float in air. They drift up, dissolve in the mucus and bump into the cilia. The cilia carry receptors: proteins shaped to grab only certain molecules. When one grabs a molecule, its neuron fires an electrical signal up its axon, through the holes in the cribriform plate, to the brain.

In 1991 Linda Buck and Richard Axel found the genes for these receptors. Humans have about 400 working kinds, and each neuron uses just one kind. But you can tell apart far more than 400 smells, because each molecule fits several receptor types, strongly or weakly. The smell is the combination that fires, like a barcode. Change the strength of a smell and the barcode changes too, which is why some smells seem different when faint.

Shape matters. Carvone comes in two forms that are mirror images of each other, like your left and right hands. One smells of spearmint; its mirror twin smells of caraway. Same atoms, different fit, different barcode.

The smell neurons for one receptor type are scattered all over the smell patch. Yet their axons all find their way to the same one or two tiny knots in the olfactory bulb, called glomeruli. So each smell’s barcode becomes a map of lit-up spots on the bulb. In mice each receptor type has about two glomeruli; human bulbs have more than 5,500 glomeruli, roughly 16 per type, and the numbers vary a lot from person to person.

From the bulb, mitral cells carry the signal along the olfactory tract to the piriform cortex, which helps work out what the smell is, and, just as directly, to the amygdala, a centre for feelings, and the entorhinal cortex, the doorway to the hippocampus, where memories are made. You can explore these parts in BrainClear.

Every other sense stops first at a relay station deep in the brain, the thalamus, before reaching the thinking cortex. Smell’s first stop in the cortex skips it (a second route does loop through the thalamus later, on the way to the orbitofrontal cortex behind your eyes). That short, direct wiring into the brain’s feeling and memory centres is one reason a whiff of rain, jasmine or a grandmother’s cooking can bring back a whole scene, full of feeling. Writers call it the Proust effect, after a novel in which the taste and smell of a cake dipped in tea unlocks a childhood memory.

Your tongue can detect just five basic tastes: sweet, sour, salty, bitter and umami (savoury). You can find out how in TongueClear. So how can you tell a mango from a banana, or good coffee from bad?

When you chew and breathe out, air from the back of your mouth carries the food’s smell molecules up behind the palate, through the nasopharynx, to the smell patch. This is retronasal smell, smelling from the inside. Your brain blends it with taste and feels it all “in your mouth”, so we call the whole thing flavour and usually say “taste” when we mean smell. People often quote “80% of flavour is smell”; nobody has measured it that neatly, but experiments show most foods are hard to recognise once smell is blocked.

Hold your nose and the air in it stops moving, so the smell never arrives: a mango is just “sweet and sour”. That is also why food seems dull when you have a blocked nose from a cold.

Chilli burn and mint cool are neither taste nor smell. They come from a third nerve, the trigeminal nerve, whose sensors are also triggered by heat (for capsaicin in chilli) and by cold (for menthol in mint). Finally, smell gets tired: if a smell stays the same, your nose and brain turn it down within minutes. This is adaptation, and it is why you stop noticing your own home’s smell.

This chapter shares general facts only. If your sense of smell changes, see a doctor: they can check what is going on.

A blocked nose. In a cold, the lining and turbinates swell with blood and mucus. Air can’t reach the smell patch high in the roof, so smells and flavours fade. They usually come back as the cold clears.

Losing smell. Losing your sense of smell completely is called anosmia. In 2020 it became one of the best-known signs of COVID-19: studies early in the pandemic found it in roughly 40 to 50% of patients, far fewer with later variants. The virus mainly infects the supporting cells around the smell neurons. Most people recover over weeks to months, though a small share, around 1 in 20 of those affected in one large analysis, report lasting problems.

Getting older. Smell slowly weakens with age. In one US study, about a quarter of people over 53 had a measurable loss, and more than 60% of those over 80. Many don’t notice, which matters because smell warns us about smoke, spoiled food and gas.

The smell of gas. Cooking gas (LPG) and natural gas have almost no smell of their own. So companies add a tiny amount of a very smelly chemical, usually ethyl mercaptan, which most noses notice at around one part per billion. After a gas explosion killed about 300 people at a school in New London, Texas, in 1937, laws began to require it. See how a kitchen chimney deals with fumes in ChimneyClear. If you ever smell gas: don’t switch anything electrical on or off, no flames, open doors and windows, turn off the cylinder if you safely can, get out, and call for help (in India, the LPG emergency line is 1906).