Short answer
A material's odor threshold is the lowest concentration at which people can detect it. Thresholds vary across materials by many orders of magnitude, which is why a formula's percentages tell you so little about what it smells of: a material with a very low threshold can dominate at 0.01%, while one with a high threshold can be 30% of a formula and barely noticed. Perfumers use the idea informally — "this is a powerful material" — and sometimes quantitatively, as the ratio of a material's concentration to its threshold. Thresholds also vary between people, and some people cannot smell certain materials at all.
Detection, recognition and difference
Sensory science separates several thresholds:
- Detection threshold — the concentration at which you can tell that something is there.
- Recognition threshold — the higher concentration at which you can say what it is.
- Difference threshold — the smallest change in concentration you can notice.
All three are measured statistically, across panels of people, under controlled conditions, and published values for the same material can differ widely depending on method. Treat any single published number as an order of magnitude, not a constant.
Why it matters in a formula
Most of what surprises beginners about perfume arithmetic comes from thresholds:
- Strength is not proportion. A formula that is 40% Hedione and 0.05% of a damascone may smell more of the damascone. Fragrance accords explained works through why accord ratios are not intuitive.
- Small errors in powerful materials are big errors. A 0.01 g mistake in a low-threshold material can change a trial more than a 1 g mistake in a weak one. That is a reason to weigh such materials as dilutions; see how to dilute aroma chemicals.
- The pyramid depends on it. A material's place in the opening or the drydown depends on volatility and threshold: a low-volatility material with a very low threshold can be smelled from the first minute. Top, middle and base notes discusses where the pyramid simplifies this.
Odor value: concentration over threshold
Flavor and fragrance chemists sometimes estimate a material's contribution as its odor value (also called odor activity value): its concentration divided by its detection threshold. A value of 1 means it is just at threshold; 100 means it is a hundred times above. In a mixture, materials with the highest odor values tend to be the most noticeable.
It is a useful idea and a crude tool. Perceived intensity does not rise in proportion to concentration — doubling a material rarely doubles how strong it smells — and materials in a mixture mask, enhance and blend with each other. Use odor values to understand why a trace material dominates, not to predict a formula.
Perceived intensity rises slowly
For most odorants, perceived intensity grows much more slowly than concentration. In practice this means:
- a trial with twice as much of a material often smells only somewhat stronger in it;
- to make a change clearly noticeable, steps between trials need to be large — doubling or halving, not adding 10%;
- once a material is noticeable, small further increases change the balance less than you expect, until it suddenly dominates.
Choosing step sizes with this in mind is covered in how to adjust a perfume formula.
People differ
Thresholds vary from person to person, with age, health and practice. Some people have a specific anosmia: they cannot smell a particular material, or a class of materials, at all, while smelling everything else normally. Some musks and some woody-amber materials are often mentioned in this context. Two practical consequences:
- If a material seems to do nothing for you, ask someone else to smell it before dosing it higher.
- When a perfume relies heavily on one material, a share of its wearers may perceive it quite differently. Consumer testing catches this; see consumer fragrance testing.
Fatigue matters too: smelling a material repeatedly raises your threshold for it temporarily, which is why evaluation sessions should be short and spaced.
Using thresholds in the lab
- Note "powerful" or "weak" on each material's record, based on your own smelling in standard dilutions.
- Keep powerful materials as 1% or 0.1% dilutions and weigh them that way.
- When a material seems too weak to matter, increase it in large steps before concluding it does nothing.
- When a trace material dominates, halve it, and keep halving until you find its useful range.
Material strength is one of the first things worth learning about a palette, along with family and tenacity; the perfume formulation guide puts those pieces together.
Frequently asked questions
Where can I find threshold values?
Published scientific literature and some suppliers' technical data give thresholds for individual materials, often measured by different methods. They are useful for comparisons between materials, not as precise figures for your formula.
Why can't I smell a material that others find strong?
You may have a specific anosmia to it, or you may be fatigued. Rest, smell it fresh in a different dilution, and ask a colleague. Record it either way; it affects how you evaluate formulas containing it.
Written and reviewed by the RUŌOD Lab team. This article is general education about perfume formulation and record-keeping; it is not legal, regulatory or safety advice, and the examples are illustrations, not validated commercial formulas. How we write and check these guides.