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What Does Nonenal Smell Like? Every Published Description

Almost everyone asking this question is trying to match a smell they can already detect to a name. The research makes that oddly hard — and the reason it is hard turns out to be the most useful thing on the subject.

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Ask what 2-nonenal smells like and almost every page on the internet gives the same four words: unpleasant, greasy and grassy. The phrase is real and comes from a real paper. It is also the entire contribution of body-odour science to the question, it was written by chemists reading a gas chromatogram rather than by anyone describing a person, and it is not quite what people are asking.

What they are asking is narrower. They can already smell something — on a parent, in a coat, on themselves after a long day — and they want to know whether it is this thing, because that decides what to do about it. Answering it properly means pulling apart two bodies of research that have never been introduced to each other.

The only description that came from body odour research

In 2001, researchers at Shiseido sampled body odour from 22 people aged 26 to 75 and analysed it by headspace gas chromatography and mass spectrometry. One unsaturated aldehyde stood out, and they described it as having an “unpleasant greasy and grassy odor” (Haze et al., 2001). That is the sentence. There is no sensory panel behind it, no descriptors rated by participants, no comparison against reference smells — it is a short characterisation in a chemistry paper, by authors at a company that later sold products aimed at this odour. We go through its strengths and limits in our index of the nonenal research.

It is worth saying what the phrase gets right. “Greasy” and “grassy” sound like an odd pairing until you look at the mechanism in the same paper: 2-nonenal is generated when omega-7 unsaturated fatty acids in skin oil are broken apart by oxidation, and oxidising fat is exactly what produces green, fatty aldehydes. The description and the chemistry agree.

The rest of the published descriptions come from food chemistry

This is the part almost nobody mentions, and it is more useful than the 2001 phrase. 2-Nonenal is a well-studied odorant — just not in dermatology. Flavour chemists study it because it is commercially annoying, and their work comes with sensory panels and quantified odour thresholds attached.

Two results matter. In a 2020 study, premium extra virgin olive oil was compared against certified rancid off-flavour oil from the International Olive Council. Thirteen odorants showed odour activity values above 100 in the rancid sample, (Z)-2-nonenal among the most odour-active, and the authors proposed it as a chemical marker for olive-oil rancidity (Neugebauer et al., 2020). In work on soy drink, (E)-2-nonenal is listed among the “representative green odorants” behind the drink’s green off-flavour; when fermentation cut those compounds, 60 per cent of a 23-person panel no longer perceived the off-flavour at all (Nedele et al., 2021).

One honest caveat, because isomers matter: the olive-oil marker is the cis form, while the soy-drink work and the human body odour work both report the trans form. They are not interchangeable, but the family character is consistent, and the same molecule is a dominant volatile in cucumber — which is where “grassy” and “green” keep coming from.

So the best published answer is not the 2001 phrase at all. It is this: oil that has gone off, with a green, cucumber-like edge when there is only a little of it. You are not looking for a loud, dirty, unwashed smell. You are looking for a pantry item past its date.

Three smells people offer instead, and why each one is a different problem

People reaching for a comparison overwhelmingly reach for one of three things, and all three are chemically somewhere else. The confusion is costly, because each responds to a different intervention.

The ammonia case is the clearest. That note is largely manufactured after urine leaves the body: in one study of stored human urine, fresh samples sat at pH 5.2 to 5.3, stored urine rose to pH 6.1 with ammonium up 32 per cent “due to urea hydrolysis”, and one-week-old urine reached pH 8.9 (Andreev et al., 2017). The two molecules also leave by different doors — ammonia is tiny and water-loving at XLogP −0.7, so it rinses; 2-nonenal at 3.1 stays in oil and fabric. We cover that smell separately in our guide to a strong urine smell in an elderly person.

Sour and oniony is the classic underarm pathway: bacteria working on apocrine secretions to release thioalcohols, with a dedicated transporter moving the precursors into the axilla (Minhas et al., 2018). Bacteria are central there and entirely absent from the 2-nonenal mechanism, which is why a leave-on acidic deodorant and a rinse-off tannin soap are not competing products.

Mothballs are simply mothballs. Naphthalene and 1,4-dichlorobenzene are aromatic hydrocarbons skin does not produce; they migrate into stored wool and cotton and stay for years. The association is circumstantial, and no soap touches it.

Whether anyone can pick this smell out of a line-up

Readers are right to be sceptical here, and some are very sceptical. Under a widely shared post about 2-nonenal this month, one commenter in r/todayilearned read the whole thread and began to wonder whether they were simply incapable of smelling it.

Several people in the same thread recalled a television segment in which samples from different ages could not be told apart, and offered that as a debunk.

Exactly one experiment bears on this, and it is better than the television version. Researchers at the Monell Chemical Senses Center collected body odour from three separated donor groups — aged 20 to 30, 45 to 55 and 75 to 95 — and had 41 young participants rate and sort it (Mitro et al., 2012). Participants could discriminate between the age categories, the effect carried mainly by the oldest group’s odour and surviving removal of variance explained by intensity. They could also correctly attach the age label to the oldest group’s samples — and to no other group’s. A reader in the same thread, describing years of volunteering with older people, drew the distinction more usefully than the debunk did.

That matches the data. The smell is distinctive, and identifiable as old age, but only at the far end of the age range.

That reframes the question. “Old person smell” is treated online as an unusually bad smell. What has been measured is an unusually recognisable one. Only the second claim has evidence behind it.

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Two literatures that never meet

Here is why the question is so much harder to answer than it looks, and we think it is the most useful thing on the page.

Walk the published papers that mention both 2-nonenal and body odour. There are ten, unchanged in count and membership since September: headspace gas chromatography, a passive flux sampler strapped to skin, enzyme gas biosensors, a machine-learning sensor array, protein chemistry, lipid analysis, and one dietary intake trial in 14 people. Not one asks a human being to smell something and say what it is like. And Mitro’s perception experiment, the only study that did ask people, measures no compound at all — it is not in that set, because it never mentions 2-nonenal.

The gap is methodological rather than accidental. The compound is measured because instruments are precise: the experimental bio-sniffers report a detection limit near 0.23 parts per million (Iitani et al., 2023; Saito et al., 2021). Perception is studied with whole human odour samples, because that is what noses receive. Even the newest attempt to make malodour assessment objective went around the nose entirely, recording cortical blood oxygenation in 18 adults smelling a reconstructed “middle-aged female scalp model odour” — and the neural differences stopped being significant once perceived intensity and pleasantness were adjusted for (Inoue et al., 2026; the authors work for Shimadzu and Mandom).

Which leads to the sentence we would rather say than not say. No published study describes what 2-nonenal smells like on human skin at the concentrations a person actually encounters, so anyone whose product removes “that smell” is describing something that has never been characterised. That includes us. The strongest honest claim for a tannin soap is about the oil it washes off, not a named odour it eliminates, and there is still no peer-reviewed trial of persimmon soap of any kind — a title-and-abstract search for persimmon and soap returns nothing.

A third literature has joined, and it does not smell anything either

Since this page first went up, a field nobody on the body-odour side cites has started publishing on the molecule: textile engineering. It is worth knowing about, because of all the research on 2-nonenal it comes closest to treating the thing as a real-world nuisance — and it still never puts a nose anywhere near it.

A 2025 paper in Langmuir opens by calling 2-nonenal “a primary component of the characteristic ‘aging odor’” which “poses minimal toxicity but significantly affects quality of life and psychological well-being, necessitating its effective removal” — a franker statement of why this matters than anything in the measurement papers. The group then built a layered fabric, manganese oxide on polyester combined with amine groups, which generates reactive oxygen species from the static charge produced by rubbing, and reported 87.77% removal of 2-nonenal. A 2026 paper from the same university bonded microcapsules built around a light-activated modified beta-cyclodextrin onto cotton and measured adsorption and degradation of trans-2-nonenal specifically, which is the isomer the body-odour work reports rather than the cis form the olive-oil chemists use.

So a third discipline now measures this compound, names it as the age-related odour, and designs materials to destroy it. And in neither paper does a human being smell the treated fabric. The endpoint is a concentration read by an instrument, as it is in all ten of the body-odour papers. Three literatures, three sets of instruments, and the question at the top of this page is still unanswered by any of them — which is the clearest possible demonstration that the gap is methodological rather than a matter of nobody having got round to it. What those papers do settle is a practical point about fabric sprays, and we have worked through it in whether Febreze removes body odour: the approaches that measurably shifted 2-nonenal in cloth bonded the chemistry into the fibre and broke the molecule apart, which is not what a mist from a bottle does.

How to work out which smell you have

Since no test exists, the practical route is elimination, in this order.

1. Check for the sharp note first. Ammonia concentrates on bedding, underwear and upholstery rather than on skin, and worsens the longer something sits. It is a contact-time problem, not a soap problem.
2. Separate person from room. Spend a few hours out, come back and breathe once before you take your coat off. Adaptation to a constant, familiar odour is measurable and fast, and recovers when you leave it (Dalton, 2000) — which is why a smell that greets you at the door is usually the room rather than you.
3. Smell the fabric, not the body. Sleep on a freshly laundered pillowcase over a clean or protected pillow and smell it before opening a window. The compound is oil-soluble, so textiles are where it accumulates — and because it also becomes covalently bound to protein in skin (Ishino et al., 2010), a wash is a removal event rather than a cure.
4. Discount your own nose, kindly. In 2,491 adults aged 53 to 97, impaired olfaction affected 24.5 per cent overall and 62.5 per cent of those aged 80 to 97, and self-report caught only a small fraction (Murphy et al., 2002). The people most likely to be told they smell are least able to check.

If what remains is faint, stale and oily rather than sharp or sour — strongest on collars, pillowcases and the back of a chair — then the stale-oil description fits, and a rinse-off bar aimed at skin oils is a reasonable thing to try. Our comparison of the best persimmon soap options sets out what each bar does and does not claim, including ours.

Bottom line

The answer comes in three parts. Body-odour research offers four words, written by chemists at a company with a product to sell. Food chemistry, which has studied the molecule far more carefully, puts it in the family of rancid oil and cut cucumber. And the only experiment that asked people to rate aged body odour found it less intense and less unpleasant than middle-aged body odour — so the smell is not the loud, offensive thing it is sold as, just the one that is easiest to place.

If you cannot name what you are smelling, that is not a failure of attention. It is a fair reflection of a literature in which the instruments never smell anything and the noses are never shown a molecule. Sort the sharp from the sour from the stale, work on the fabric as hard as the skin, and treat any product promising to eliminate a smell nobody has described with the scepticism it has earned.

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