In short
Smell begins when airborne molecules reach sensory neurons high in the nose, where receptors respond to them and send a pattern of signals to the brain. For a perfumer, the practical consequences are that your nose adapts quickly, that people differ in what they can smell, and that perception can be trained.
What you will learn
- Trace the path from a molecule in the air to a perceived smell
- Explain why two people can perceive one material differently
- Separate established findings from popular claims about smell
Before this lesson: Volatility: Why Top, Heart and Base Notes Exist
The path, briefly
- A molecule evaporates and travels into your nose with the air you breathe.
- It reaches the olfactory tissue, a small patch high in the nasal cavity, where sensory neurons carry receptor proteins.
- Receptors respond. Each neuron carries one receptor type, and an odor molecule can activate several types.
- Signals go to the olfactory bulb and on to other brain regions, where the pattern is interpreted as a smell.
Richard Axel and Linda Buck shared the 2004 Nobel Prize for discovering the odorant receptor gene family and how the system is organized. Their work in mice described about 1,000 such genes. Humans have fewer that work, and our overall article on how smell works, in the RUŌOD Journal, gives the general account. Here we care about what it means at the bench.
Sniffing matters, and it is not the only route
Air reaches the olfactory tissue in two ways: when you sniff in through the nose, and from the back of the mouth as you chew and swallow. That second route is why food tastes flat when you have a cold. Your perfumery work uses the first route, and so how you sniff is a skill, which the lesson on smelling raw materials covers.
Four facts that change how you practice
| What is known | What to do about it |
|---|---|
| Receptors and neural processing adapt to a constant odor | Smell briefly, step away, and rest. Olfactory fatigue gets its own lesson. |
| Identity comes from a pattern, and mixtures produce new patterns | Two materials together can smell like neither, which is what makes an accord possible. |
| People differ, sometimes greatly, for particular materials | Test your own blind spots, and get a second nose on your formulas. |
| Smell connects strongly to memory and emotion | Use personal associations as memory hooks, without assuming others share them. |
Differences between noses
A person with otherwise normal smell can have a much weaker response to particular materials. That is called specific anosmia, and some musks are a well-known case. It has a direct professional consequence: you could overdose a material you barely perceive and never know. It also explains why your opinion of one material can differ so strongly from someone else's, with neither of you wrong.
A general loss of smell is different. If your sense of smell has reduced or changed overall, particularly suddenly, that is a health question for a clinician, and this course is not a substitute for that advice.
Be careful with big claims
Popular accounts often say that humans can tell a trillion smells apart. A 2014 paper in Science gave that estimate, but a 2015 reanalysis in eLife found that the conclusion depends heavily on assumptions and thresholds, and that the true number is still unknown. It is a good reminder of how much we do not know about smell. Equally, the mapping from a molecule's structure to its odor is still not fully predictable, and Level 7 returns to that subject.
Practical exercise: Find your nose's blind spots
Gather as wide a range of smells as you can, including at least one clean musky product such as laundry detergent or a skin scent. If you have materials, include a musk and an ionone. Use strips, labeled on the back.
- Smell each item once. Rate its strength from 0 (nothing) to 5 (very strong).
- Note any item that seems almost odorless, or much weaker than you expected.
- If possible, ask someone else to rate the same set without seeing your answers.
- Compare the two lists. Mark any item where you differ by two or more points.
- Write down what this means for you: which materials must you never dose by nose alone?
You have done this well if both sets of ratings were recorded without comparing as you went, and your conclusion names actual materials you should not dose by nose alone.
Common mistakes
- Assuming your nose is the standard. A material you cannot smell may be loud to everyone else.
- Treating brain science as a recipe. Knowing how receptors work does not yet tell you how a blend will smell. Your nose still decides.
- Repeating smell statistics you cannot source. Many circulate, and some have been challenged.
Key terms
- Olfactory receptor
- A protein on a sensory neuron in the nose that responds to certain odor molecules. Each neuron carries one receptor type.
- Orthonasal smell
- Smelling by sniffing air in through the nostrils.
- Retronasal smell
- Smell that reaches the receptors from the back of the mouth while you eat or drink.
- Specific anosmia
- A reduced or absent ability to smell one particular substance or group, while the sense of smell is otherwise normal.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.