Material guide: alpha-ionone — raise the concentration and it can get harder to detect

Most materials follow a simple rule: add more and it reads more clearly. Not this one.

The short version

• It runs toward violet and orris, sweet and woody with fruit behind. It and beta-ionone are two siblings of the same family pointing different directions.
• 115 suppliers, near the top of the database.
• A 2025 study across ten log-spaced concentrations found the human detection curve for it is not monotonic: at two particular concentrations, detection performance reverses downward.
• Storage advice is refrigerate, rare in this series. The GHS classification carries H334 (may cause allergy or asthma symptoms or breathing difficulties if inhaled), also rare.

The basics

CAS	127-41-3
Formula	C₁₃H₂₀O
Molecular weight	192.30
Type	Naturally occurring, also synthesised
Odour	Sweet, woody, floral, violet, orris, tropical, fruity
Odour family	floral
Strength	Medium (the database explicitly recommends evaluating at 10% or less)
Longevity	112 hours (blotter, neat)
Appearance	Colourless to pale yellow clear liquid
Boiling point	237–238 °C @ 760 mmHg; 131 °C @ 13 mmHg
Flash point	Above 110 °C
Specific gravity	0.927–0.933 @ 25 °C
Refractive index	1.497–1.502 @ 20 °C
logP	3.00 (est)
Natural occurrence	Raspberry, blackberry, blackcurrant, carrot, plum, sweet acacia, champaca concrete 0.03% and others
Storage	Refrigerate in tightly sealed containers
Regulatory	FEMA 2594, JECFA, CoE, FLAVIS
Suppliers	115
Corpus rank	25th (99 of 953 formulas, 10.4%, median dose 3.00%)

In plain terms. A boiling point of 237 °C plus 112 blotter hours says it moves slowly and belongs to the heart and drydown. Take the refrigerate line seriously — it dislikes heat more than most.

The dips in the detection curve

Cameron and Doty, in Physiology & Behavior in 2025, took on a phenomenon that rarely gets acknowledged.

The usual assumption is that detectability rises monotonically with concentration. For some odorants, though, the curve carries reversals — the authors call them notches — where raising the concentration temporarily lowers detection performance. Such reversals are rarely acknowledged, few participants and odorants have been tested, and methodological concerns abound.

So they built it carefully. Ten log-based concentrations of alpha-ionone, presented in random order. Each trial used Snap & Sniff® wands to present odourless mineral oil and one concentration of alpha-ionone in rapid succession, counterbalanced, and asked which seemed stronger plus a confidence rating on a 9-point scale.

• Study 1: 24 participants, one 30-minute session of 60 trials.
• Study 2: nine participants, 600 trials each across ten 30-minute sessions.

Both studies consistently found notches near the 10⁻⁵ and 10⁻³·⁵ vol/vol concentrations.

The authors conclude that this definitively demonstrates reliable reversals in the human alpha-ionone psychometric function, and ask whether those reversals reflect the recruitment of receptive elements with differing response profiles.

For someone composing, this explains a genuinely annoying experience: turning it up can make it harder to judge, not easier. If you have ever thought "I clearly added more and it got less obvious", that is not necessarily your nose failing.

One further finding deserves separate mention: participants' trial-by-trial confidence judgments corresponded with their detection performance, but their self-rated sense of smell did not. People are accurate about "am I sure this time" and inaccurate about "is my nose any good". Individual differences covers another face of the same thing.

An unexpected second job

Yoon and colleagues, in Molecular Nutrition & Food Research in 2024, worked entirely outside perfumery.

They demonstrated expression of the olfactory receptor OR10A6 along with olfactory signalling components in A431 human squamous cell carcinoma cells — an ectopic olfactory receptor, sitting in tissue other than the nose. Activating OR10A6 with alpha-ionone as ligand inhibited proliferation and migration and induced apoptosis, working through the Hippo pathway (phosphorylation of LATS, YAP and TAZ). In cells with OR10A6 knocked down, those effects disappeared. In an A431 xenograft mouse model, injecting alpha-ionone suppressed tumour growth, with no effect in the knockdown tumours.

This is cell and mouse work, delivered by injection. It supports no health claim about smelling a perfume or using a product containing ionones. It is here for the same reason as the hydroxycitronellal piece: fragrance molecules frequently hold a second job in the body, and that job is unrelated to what they smell like.

Alpha and beta are two different things

Two materials from one family, pointing different ways.

	Odour	Blotter hours
Alpha-ionone	Sweet, woody, violet, orris, fruity	112
Beta-ionone	Violet, woody, cedar, raspberry	per record

We covered beta already, and its story is receptor genetics: a single variant explains more than 96% of the variation in sensitivity to it. Alpha's story is concentration — the same person's performance reverses at different levels.

Smelling the two side by side is worth doing, and it has to happen at the same dilution, or you are comparing concentrations rather than materials.

When you smell it

• Start at 1%, not 10%. The database's strength field already recommends 10% or below, and the study above says lower concentrations deserve more of your time.
• Run a concentration ladder once. Prepare 0.1%, 1% and 10% blotters and cycle between them with gaps. It is the cheapest way to see the non-monotonic effect for yourself.
• Don't explain your results with "my nose is off today". That study found self-rated ability does not match measured performance. Write results down instead of trusting the feeling.
• It holds through the middle. 112 hours puts it in the heart and drydown, which is why the median dose sits at 3%.

Buying and storage

With 115 suppliers it is easy to source. Two things to watch:

Isomers. Alpha-, beta- and methyl ionone are different materials with different CAS numbers and different smells. Alpha is 127-41-3. Product sold simply as "ionone" without a letter is usually a mixture whose ratio depends on the supplier.

Refrigeration. The database storage field says refrigerate outright, which is unusual across this series. It minds heat more than most, so decant into small bottles and chill them on arrival, with dates on the labels.

Regulatory and safety

The GHS classification lists a single statement, and it is an uncommon one: H334 — may cause allergy or asthma symptoms or breathing difficulties if inhaled.

That is a respiratory sensitisation statement rather than a skin one. As our piece on the health risks of the hobby noted, most fragrance materials concentrate their GHS classifications on skin and eyes, with respiratory statements less common. This one has it, so ventilate when evaluating neat material and keep your nose off the open bottle.

For acute toxicity, the database records only an intraperitoneal mouse LD50 of 2,277 mg/kg. Oral and dermal values are not recorded.

Register listings are FEMA 2594, JECFA, CoE and FLAVIS. For use limits, check current IFRA Standards — here is how.

→ Alpha-ionone in the database: full physical data, 115 suppliers and suggested blends. → Search by odour: try "violet orris woody".

References

E. L. Cameron, R. L. Doty, Non-monotonic psychometric functions for α-ionone in young adults, Physiology & Behavior, 290, 114749 (2025). PMID 39549869. doi:10.1016/j.physbeh.2024.114749

Y. E. Yoon et al., A Food Odorant, α-Ionone, Inhibits Skin Cancer Tumorigenesis by Activation of OR10A6, Molecular Nutrition & Food Research, 68(15), e2400085 (2024). PMID 39021302. doi:10.1002/mnfr.202400085