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Learn · Part One — Learning to smell · Lesson 0 of 11

What the sense of smell is, and how the nose smells

Smell works through about 400 receptor types and a combinatorial code. Most of "taste" is smell, and every nose is different.

The sense of smell, olfaction, is the ability to detect molecules light enough to travel through the air and reach the nose. Together with taste it is a chemical sense. The eyes and ears measure light and sound waves; the nose measures the very molecules that touch it.

This article follows a smell from the moment it leaves an object to the moment you can name it, and then explains why two people smelling the same thing can receive two different signals.

The path of a smell

  1. Molecules rise. An object smells when it releases molecules into the air. The lighter and more volatile a compound, the sooner it reaches the nose. The article on boiling points explains why this order decides what you smell first.
  2. They reach the olfactory area. Most air passing through the nose goes down to the lungs. Only a small share rises to the olfactory epithelium, a small strip of lining in the roof of the nasal cavity. That is why people take short, strong sniffs when they want to smell closely.
  3. They meet receptors. Here there are millions of olfactory neurons. Each one carries one type of receptor, and each receptor type catches certain molecular shapes.
  4. The signal goes to the brain. It travels to the olfactory bulb and then to the olfactory cortex. Unlike sight, hearing and touch, this route does not pass through the thalamus before reaching the cortex, and it connects closely with areas of the brain that handle emotion and memory. That anatomy is often used to explain why a smell can bring a whole memory back. The precise mechanism is still being studied.

Olfactory neurons have another unusual feature: they are replaced continually throughout life. Most other neurons in the body are not.

About 400 receptor types, and a code

Humans have about 400 working olfactory receptor genes. In 1991 Linda Buck and Richard Axel discovered this gene family, and in 2004 they received the Nobel Prize in Physiology or Medicine for it.

What matters is how the receptors work. There is no receptor for "lemon" or "rose". One molecule activates several receptors at once, each to a different degree. One receptor, in turn, responds to many different molecules. The brain reads the combination of receptors switched on, the way you read a word from its letters. A few hundred letters can spell a very large number of words.

The code is sensitive to the three-dimensional shape of a molecule. The two mirror-image forms of carvone share a formula and a boiling point, yet one smells of spearmint and the other of caraway seed. The article on isomers goes further.

How many smells can people tell apart

The figure of "one trillion smells" appeared in 2014, from an experiment that mixed odour blends and asked people whether they could tell them apart. Within a year, other groups showed that the calculation depended heavily on its mathematical assumptions, and that the same data could give wildly different numbers. The honest answer today is that nobody knows. We only know it is far larger than the ten thousand repeated through the twentieth century.

What most people lack is not the ability to discriminate but the words to name. The article on smell vocabulary takes this up.

Most of "taste" is smell

Smell has two ways in. The first is breathing in through the nostrils. The second comes from the back of the mouth: as you chew and swallow, vapour from the food rises into the nasal cavity. The brain usually cannot separate the two, and it files the smell arriving by the back route under "taste".

The tongue detects only a few basic tastes: sweet, salty, sour, bitter, umami. The rest of flavour is smell. Pinch your nose shut and eat a piece of mango or sip some coffee: you still sense sweet or bitter, but you can no longer tell it is mango or coffee. That is also why food tastes flat when your nose is blocked.

Everyone has a different nose

Olfactory receptor genes are among the most variable genes from person to person. The consequences can be measured:

  • Coriander leaf. For part of the population, coriander leaf smells of soap. A genetic variant near a cluster of olfactory receptor genes is associated with this perception. The coriander entry in the dictionary says more.
  • Androstenone, a compound found in sweat and in boar meat. Given the same sample, some people find it foul, some find it sweet and slightly floral, and some can barely smell it. Variants of a receptor called OR7D4 explain part of this difference.
  • The detection threshold for the same compound can differ several-fold between two people. The article on detection thresholds explains what that figure measures.

So when two people describe the same essential oil differently, neither is necessarily wrong. They may truly be receiving two different signals.

The nose changes over time, and we often do not notice

Within a few minutes, the nose stops reporting a constant smell. That is olfactory adaptation, and it wears off once you leave the smell for a while.

Slower change is much harder to notice. A 2002 study in the United States tested the sense of smell of nearly 2,500 people aged 53 to 97. About a quarter had impaired smell, and among those aged 80 and over the figure was nearly two thirds. Yet only about one in ten participants reported that their sense of smell had declined. In other words, most people with impaired smell did not know it.

Sudden loss of smell is a different matter. If you suddenly cannot smell anything, or familiar smells suddenly turn strange, see an ear, nose and throat doctor.

Training the nose

The sense itself changes little, but the ability to name what you smell can grow a great deal. Perfumers do not have different noses. They have a set of familiar reference smells and a vocabulary attached to each. Two articles to read next: how to smell something three times in three minutes and reference smells already in your kitchen.

Sources

  • Buck and Axel (1991). A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65(1), 175–187.
  • Malnic, Hirono, Sato and Buck (1999). Combinatorial receptor codes for odors. Cell 96(5), 713–723.
  • Bushdid, Magnasco, Vosshall and Keller (2014). Humans can discriminate more than 1 trillion olfactory stimuli. Science 343(6177), 1370–1372.
  • Gerkin and Castro (2015). The number of olfactory stimuli that humans can discriminate is still unknown. eLife 4, e08127.
  • Rozin (1982). "Taste–smell confusions" and the duality of the olfactory sense. Perception & Psychophysics 31(4), 397–401.
  • Keller, Zhuang, Chi, Vosshall and Matsunami (2007). Genetic variation in a human odorant receptor alters odour perception. Nature 449, 468–472.
  • Eriksson et al. (2012). A genetic variant near olfactory receptor genes influences cilantro preference. Flavour 1, 22.
  • Murphy et al. (2002). Prevalence of olfactory impairment in older adults. JAMA 288(18), 2307–2312.

Further reading for this part

A shared list for the whole of Part One — Learning to smell, not a note for every sentence. We state what each source was used for.

  1. Asifa Majid and Niclas Burenhult, 2014. Odors are expressible in language, as long as you speak the right one A study of the Jahai of the Malay Peninsula, a group with an abstract smell vocabulary. It underpins the central argument of Part One: the gap lies in vocabulary, not in the nose.
  2. Caroline Bushdid et al., 2014. Humans can discriminate more than 1 trillion olfactory stimuli The trillion figure is widely quoted. We cite it together with its critique, because Meister (2015) and Gerkin and Castro (2015) showed problems with how it was extrapolated. It is our example of how an attractive number can spread without anyone checking it.
  3. Những cây thuốc và vị thuốc Việt Nam A Vietnamese-language source for the species names, parts used and sensory descriptions of most of the medicinal plants in the dictionary. Where it and other literature disagree — as with garden mint — we give both.

One thing must be said plainly. We have not read every work above in the original; most of the knowledge comes through secondary literature and reviews. We say so rather than let the list suggest a depth of research we have not reached.

Last updated: October 2026