Material guide: indole — fecal when strong, floral when weak, but not everyone makes the turn

The jasmine absolute guide left one sentence unopened: that faint animal shadow across a white floral is usually credited, in the lore, to nitrogen-containing molecules like indole. This post is that shadow in person. It is the most divisive material in the series, and the only one where the database pushes the evaluation concentration all the way down to 1%.

What matters

• Indole runs to a few percent in the absolutes of jasmine, orange flower, narcissus and champaca (table below). Every good jasmine you have ever smelled came with it. The pure material is a crystal; concentrated, it reads pungent, mothball-like, fecal, and the database's strength field says evaluate at 1% or less.
• "Fecal when strong, floral when weak" is trade folklore, and in 2024 somebody put it in an MRI scanner. The interesting part is the recruitment: the study lined up 12 people who switch and 15 who don't. Whether you make the turn is itself an individual difference.
• Another paper from the same year identified Olfr205 as a new indole receptor. The key is a hydrogen bond between indole and a residue called A202 on a transmembrane helix.

The basics

CAS	120-72-9
Formula	C₈H₇N
MW	117.15
Odor	animal, floral, naphthyl, fecal, pungent, musty
Odor type	animal
Strength	high (database note: evaluate at 1% or less)
Longevity	400 hours (measured at 50% in DPG)
Appearance	white to amber yellow crystalline powder (est.)
Melting point	51.00 to 54.00 °C
Boiling point	253.00 to 254.00 °C
Flash point	> 110.00 °C
Shelf life	36 months or longer (cool, dark, sealed)
Regulatory	GRAS, JECFA, CoE, FEMA GRAS, IFRA

In plain words: CAS is the material's ID number. Two rows deserve a second look. The first is strength: most materials in this series say "10% or less", this one says 1%, and that number is the warning by itself. The second is longevity — those 400 hours were measured at 50% in DPG, not neat like galaxolide's 400 hours, so don't compare the figures across posts. A melting point of 51 °C means what arrives in the post is a crystal, not a liquid.

The name is worth a line too. Indole is a portmanteau of indigo and oleum, because it was first isolated by treating indigo dye with fuming sulfuric acid. Industrially it comes mostly from the 220–260 °C distillation fraction of coal tar, and the main synthetic routes start from aniline.

How much of it is in the flower

The database's natural occurrence field lists the numbers plainly:

Source	Indole content
Jasmine oil (Italy)	4.21%
Jasmine concrete (Egypt)	3.84%
Jasmine concrete (India)	1.07–1.85%
Orange flower absolute (Morocco)	2.6–9.9%
Neroli oil CO₂ extract	0.45%
Narcissus absolute	1.51% / 6.30%
Champaca (michelia) absolute	7.20%
Hyacinth absolute	0.05–0.08%
Ylang ylang CO₂ extract	0.13%

Look at the width of the orange flower row: 2.6% to 9.9%, nearly a fourfold spread. Origin, cultivar, harvest and extraction all move that number, which is why two samples both labelled "orange flower absolute" can carry very different amounts of animal character. Buying an absolute has never meant buying a fixed recipe.

It doesn't only live in flowers. The same field lists coffee, tea leaf, wine, cheese, egg and fish. Bacteria break down tryptophan and out comes indole; it occurs in human feces, and that is where the bad reputation started.

The turn, seen in the brain

"Fecal when strong, floral when weak" is common knowledge in perfumery. In 2024, Zou and colleagues took it into an MRI scanner in Cerebral Cortex:

Indole is often associated with a sweet and floral odor typical of jasmine flowers at low concentrations and an unpleasant, animal-like odor at high concentrations. However, the mechanism whereby the brain processes this opposite valence of indole is not fully understood yet. ⋯ 12 conversion individuals and 15 nonconversion individuals participated in an event-related functional magnetic resonance imaging paradigm with low and high indole concentrations in which valence was manipulated independent of intensity. The results showed that neural activity in the right amygdala, orbitofrontal cortex and insula was associated with valence independent of intensity. Furthermore, activation in the right orbitofrontal cortex in response to low-indole was positively associated with subjective pleasantness ratings.

About that 12 versus 15: it is a recruitment design. The researchers deliberately assembled one group that converts and one that doesn't, so the split cannot be read as "44% of people switch". What it does establish is that non-switchers exist, and in numbers large enough to fill a control group. The individual differences post covered the finding that any two people differ functionally in more than 30% of their odorant receptors. Indole may be the most dramatic display case for that difference.

One detail is easy to skip past: the experiment manipulated valence independently of intensity. That matters, because as concentration rises, "unpleasant" and "too loud" blend together, and ratings alone can't tell you whether someone found it disgusting or merely overwhelming.

How the nose catches it

The same year, Li and colleagues came at the molecule from the receptor side in Archives of Biochemistry and Biophysics:

Indole is widely present in nature and contributes significantly to the smell of flowers and animal excretion. However, the odor perception mechanism for indole is unclear, despite previous reports suggesting that it activates the Olfr740 family of receptors. In this study, we successfully identified another receptor, Olfr205, that is responsive to indole. Molecular model construction and binding pocket analysis predicted that the A202 residue in transmembrane helix 5 of Olfr205 forms a crucial hydrogen bond with indole, facilitating receptor activation. ⋯ Our mutant function assay showed that substitution of A202 in Olfr205 and G112 in Olfr740 with other amino acids significantly decreased the receptor response to indole.

Two caveats. Olfr is the mouse naming convention for olfactory receptors (human ones are written OR), so this is a mouse result and doesn't transfer straight to your nose. The second is method: they predicted which amino acid mattered with a computational model, then swapped that amino acid out to see whether the response dropped. Predict, then go and check — that step is where papers of this kind earn or lose their credibility.

What matters here is that more than one receptor catches this molecule. The sandalwood guide covered a famous starting point of receptor research, where the story was one molecule to one receptor. With indole, two receptor groups grab it from different angles and bind it in different ways. If a single molecule's odor description can hold both floral and fecal, multiple receptors are probably part of the physical reason.

When you smell it

Supplier Pell Wall's product page quotes Arctander, and it remains the sharpest piece of lore we have seen on this material:

Extremely diffusive and powerful odor, almost tarry-repulsive and choking when concentrated, but in concentrations lower than 0.1%, or in compositions, it shows powerful floral notes and pleasant radiation. Good tenacity, in spite of the volatility at room temperature.

In practice:

• Buy the 1% or 10% dilution, not the pure crystal. You won't need that much, and weighing out crystal powder is a job you will regret: the smell stays on the balance, the spatula and the draft shield for days, and it earns itself a dedicated set of tools.
• Evaluate at 1% or less. It diffuses hard enough to cover the blotter next to it, so don't leave other samples spread across the bench while you work with it.
• In a formula it is a trace player. The database's suggested pairings put it alongside hedione (methyl dihydrojasmonate), phenylethyl alcohol, geraniol, citronellol and heliotropin — all long-standing partners on the white floral and rose side.
• The fastest way to understand what it does is a side-by-side: build the same jasmine accord twice, add 0.05% of the indole dilution to one, leave the other alone. The difference shows up not in how bad it smells but in whether the flower is alive.

Regulatory and safety

Listed in GRAS, JECFA, CoE, FEMA GRAS (FEMA number 2593) and IFRA. As always, those listings carry no limit values; for anything going on skin, check the current IFRA Standards and the supplier's specification sheet.

The safety fields are more serious than for most materials in this series. The GHS classification includes harmful if swallowed (H302), skin irritation (H315), serious eye damage (H318), may cause respiratory irritation (H335), and very toxic to aquatic life with long lasting effects (H400, H410). The practical meaning of those last two is direct: leftovers and rinse waste do not go down the sink, they go to waste disposal. Wear gloves and eye protection when handling the pure material.

Buying notes

1. Start with the 1% dilution. Both natural isolate and synthetic versions are sold, and plenty of the 53 suppliers carry both; unless a formula has to claim all-natural, there is no difference worth paying for while you are practising.
2. Buy the smallest pack. It is a crystal with a 36-month-plus shelf life, and you will not get through a gram in a year.
3. To fill in the white floral line in one go, buy it with jasmine absolute, benzyl acetate and hedione: the fruity face, the airiness, the animal shadow. Smell all three side by side and the word "jasmine" changes resolution.

→ Indole in the database: full physical data, 53 suppliers, suggested pairings. → Jasmine absolute | Hedione | Search by scent: try "animal floral" and it comes back first.

References

L. Zou, Y. Qi, L. Shen, Y. Huang, J. Huang, Z. Xia, M. Fan, W. Fan, G. B. Chai, Q. Z. Shi, Q. Zhang & C. Yan, The neural representations of valence transformation in indole processing, Cerebral Cortex, 34(4), bhae167 (2024). PMID 38652554

G. Li, L. Wang, F. Ye, S. Li & H. Yu, Molecular determinants of olfactory receptor activation: Comparative analysis of Olfr205 and Olfr740 family member responses to indole, Archives of Biochemistry and Biophysics, 758, 110061 (2024). PMID 38880319