In short
An odor threshold is the lowest concentration at which you can detect a smell. You can explore your own by smelling a ladder of dilutions, weakest first, alongside a solvent blank. The result is personal and approximate, but it teaches you directly why a tiny amount of a powerful material can change a whole formula.
What you will learn
- Tell detection from recognition
- Build a dilution ladder and test it with a blank
- Explain what a personal threshold result does and does not show
Before this lesson: How to Smell Raw Materials on a Strip
Why strength is not the same as amount
Two materials used at the same weight can differ enormously in how strongly they smell. Some are perceptible at vanishingly small concentrations, while others need a great deal more. That is why a perfume formula can contain one material at 20% and another at 0.01% and have both matter. For the formal explanation, including how thresholds are published and used, read the reference guide on odor thresholds in perfumery. Here, you will experience the idea for yourself.
What a home test can and cannot show
| It can show | It cannot show |
|---|---|
| Roughly where you stop detecting a material on paper | A true threshold value in air, which requires controlled apparatus |
| The gap between detecting and recognizing | Anyone else's threshold, as people differ widely |
| Whether you have a blind spot for this material | How strong the material will be in a formula |
A strip is not a measured concentration in air, and your nose varies from day to day. Treat the result as training rather than as a number to publish.
Building the ladder
A ladder steps down by a factor of ten. Each rung is one part of the rung above plus nine parts of solvent, by weight:
| Rung | Take | Add solvent | Pure material in it |
|---|---|---|---|
| 10% | 1 g of neat material | 9 g | 1 g |
| 1% | 1 g of the 10% | 9 g | 0.1 g |
| 0.1% | 1 g of the 1% | 9 g | 0.01 g |
| 0.01% | 1 g of the 0.1% | 9 g | 0.001 g |
If you have not yet weighed anything, finish your first weighing practice and dilution basics in Level 3 first, and then come back. Serial dilution is what makes such low concentrations possible, because a hobby scale cannot weigh 0.001 g directly.
What to expect
Results vary, but most learners notice two things. First, they can detect something well before they can say what it is. Second, a material can change character as it weakens. Indole is the classic example, moving from harsh to floral. That change is one reason perfumers smell materials at several strengths, as you will in later levels.
Practical exercise: Run a dilution ladder
Choose a material with a clear, familiar smell that you can comfortably smell, such as vanillin or ethyl maltol. Some materials need warming or time to dissolve, so check that yours is fully dissolved. Use perfumer's alcohol, and keep to small amounts.
- Make the ladder down to 0.01%, and label each small vial with its strength and date.
- Label five strips underneath with a code. Ask someone to dip them, or shuffle them yourself so that you do not know the order. One must be solvent only.
- Let the solvent flash off for about 30 seconds.
- Smell each strip, with a pause between. Write one of three answers: nothing, something or I know what it is.
- Reveal the code. Note your lowest something and your lowest I know what it is. Check what you wrote for the blank.
- Repeat on another day, and see how the answer moves.
You have done this well if the blanks were included and coded, your dilutions can be traced to a weighed stock, and you recorded both days, even where they disagree.
Common mistakes
- Going from strong to weak. Smelling the strongest first fatigues you for the weak ones. Go weakest first when you know the order, or rest generously when you do not.
- Skipping the blank. Without it, you cannot tell imagination from detection.
- Treating your result as the material's threshold. It is your result, on paper, today.
- Using a dirty pipette between rungs. A trace of a stronger rung ruins a weak one. Use a clean one each time, and work from weak to strong when you reuse tools.
Safety
Alcohol is flammable, so work away from flames and with ventilation. Wear gloves when you handle neat material, and do not put these dilutions on skin. Keep vials labeled and away from children, pets and food.
Key terms
- Detection threshold
- The lowest concentration at which a person can tell that a smell is present, without necessarily naming it.
- Recognition threshold
- The concentration at which a person can identify what the smell is. It is higher than the detection threshold.
- Serial dilution
- Making each dilution from the one before, such as stepping from 10% to 1% to 0.1%.
- Solvent blank
- A strip dipped in solvent alone, used to check whether you are really smelling the material.
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.