Part Four teaches you to doubt the seller. This article doubts something else: claims that sound very scientific and that sellers and writers on scent copy from one another. Our report on odour molecules and emotion traced 20 such claims back to their sources. Here are the seven you are most likely to meet, and what the original data actually say.
1. “After 20 minutes your nose stops smelling anything”
We have not been able to trace the 20 minutes to any source. Measured in people, the perceived intensity of a steady smell falls to about 25–40% of its starting level within roughly four minutes, then levels off; it does not reach zero. Some people still smell it after 15 minutes. And the hot, pungent sensation carried by the trigeminal nerve rather than by smell does not fade this way; the sting actually grows the longer you breathe it in.
Why you can’t smell your own home covers this adaptation.
2. “Diluted indole smells floral”
This holds for only some people. When the concentration of indole was raised fourfold, 12 of 27 people changed their judgement, from pleasant to unpleasant or the reverse; the other 15 did not change. For skatole, p-cresol, civetone and many other materials said to be foul when strong and lovely when weak, we have not found any quantitative data in people.
The general effect is real, as concentration decides the nature of a smell describes. The numbers show only that it does not happen to everyone.
3. “A pleasant smell has an ideal strength, and too much turns it unpleasant”
This inverted-U curve is often drawn in books, but its source is a textbook chapter, not a study with data. Three studies with data show that for pleasant smells, pleasantness rises with concentration and then levels off; it does not fall. Unpleasant smells are different: the stronger they are, the more steeply pleasantness drops.
4. “The more notes, the more layered the experience”
People, perfumers included, can pick out at most about three components in a mixture. Training and experience do not raise this limit. A formula of 30 materials is not perceived as 30 notes.
5. “Adding a material adds a smell”
Often the reverse. The intensity of a two-compound mixture is about 34% lower than the sum of its two parts. Once there are some 20–30 components of equal intensity spread across the families of smell, different mixtures start to smell alike. In tests, people told apart two mixtures from the same cluster of smells 67.5% of the time, but two widely spread mixtures only 42.4% of the time. Coverage matters more than count.
6. “Molecular structure decides whether a smell is pleasant”
A model predicting pleasantness from chemical structure reached a correlation of 0.71 with the average score of a group, but only 0.41 with the scores of individuals. Put another way, structure explains about half the variation in group averages, and only about 17% for any one person. One of the authors of that very model wrote that prediction at the level of the individual is not feasible.
7. “An oil’s effect is the effect of its main compound”
This can be wrong even in direction. In a double-blind trial, lavender oil tended to raise blood pressure, while pure linalyl acetate, one of its two main compounds, lowered it. With citrus, when pure limonene was tested, systolic blood pressure rose and participants felt more restless, the opposite of the calming reputation of this family. Sandalwood is the same: the study usually cited to call sandalwood calming in fact measured rises in pulse rate, skin conductance and systolic blood pressure.
A mixture is not the sum of its parts, and the result for one substance does not carry over to another.
Five kinds of error
All 20 claims in the report fall into five kinds of error:
- a result for a mixture attributed to a single molecule;
- a result in mice attributed to people;
- a result for one route of use attributed to another, for example from a preparation taken by mouth to one breathed in;
- a group average attributed to every individual;
- a figure that cannot be traced to any source.
When you meet a claim about scent that sounds very scientific, asking these five questions in turn is enough to sort most cases. The same questions apply to this site.
The underlying evidence is in our research section: Results: 20 misattributed claims and Results: emotion, concentration, detection thresholds and valence.
Sources
- From odour molecule to emotion: the evidence chain across 52 compounds, Table 2 in the chapter on 20 misattributed claims. The table gives the real source of each figure and what the original data say; the full reference list is in the report’s appendices.