Short answer: Nonenal, or 2-nonenal in full, is a smell molecule that forms when certain oils on the skin react with oxygen and break down. Japanese researchers reported it in human body odour in 2001 and found it only in people aged 40 and over. It has a greasy, grassy smell. It is not an illness and not a sign of poor hygiene. Daily washing, fresh clothes and an aired home are the most useful everyday steps.
Perhaps you saw the word on a soap label, or noticed a change in your own scent or a parent's. This guide explains in plain language what 2-nonenal is, where the smell comes from, what scientists have actually measured and what is worth doing about it.
Nonenal in plain terms
2-nonenal belongs to a family of chemicals called aldehydes. The "non" part of the name refers to its backbone of nine carbon atoms. Molecules like this form when fats react with oxygen, a process called oxidation. It is the same process that makes a bottle of cooking oil smell rancid after too long in the cupboard.
Nonenal is not unique to people. Brewers know trans-2-nonenal as the cause of the stale, cardboard-like taste in beer that has been kept too long. A team at the Université catholique de Louvain in Belgium showed how the compound is released as beer ages (Noël et al., 1999, Journal of Agricultural and Food Chemistry).
What does the 2-nonenal smell like?
The researchers who first found it in body odour called it unpleasant, greasy and grassy. A later Japanese paper on kareishu, the Japanese word for the smell of ageing, compared it to musty old books in the rainy season or old-fashioned hair pomade. The same paper said it seems to come mainly from the chest and back (Tatsuguchi et al., 2012, Journal of Japan Association on Odor Environment).
So it is not the sharp smell of a sweaty gym kit. It is quieter and more stale, and it lingers. Because it comes from skin oil, it soaks into fabric that touches the skin for hours: pillowcases, shirt collars, the headrest of a favourite armchair. Many people notice it there before they notice it on a person.
How 2-nonenal forms on the skin
Your skin constantly produces a thin, oily film called sebum. Among its ingredients are omega-7 fatty acids, such as palmitoleic acid. The research points to a chain of events roughly like this:
- Some of the oils on the skin's surface react with oxygen and become lipid peroxides, fats that have already started to oxidise.
- These peroxides set off a chain reaction that breaks down the omega-7 fatty acids.
- One product of that breakdown is 2-nonenal. It is volatile, which means it drifts off the skin into the air as a smell.
The 2001 study found that omega-7 fatty acids and lipid peroxides both increased with age, and that people with more of them also gave off more 2-nonenal. That would help explain why the compound turned up only in older participants.
The 2001 study that put nonenal on the map
The best-known paper is by Shinichiro Haze and colleagues at the Japanese cosmetics company Shiseido: Haze et al., 2001, Journal of Investigative Dermatology 116(4): 520–524.
| Question | What the paper reports |
|---|---|
| Who carried it out? | Researchers at Shiseido's product development centre in Yokohama, Japan |
| Who took part? | People aged 26 to 75 |
| How was odour measured? | Body odour was collected and analysed by gas chromatography and mass spectrometry, laboratory methods that separate and identify chemicals |
| Main finding | 2-nonenal was detected only in participants aged 40 or older |
| Skin oils | Omega-7 fatty acids and lipid peroxides rose with age and went hand in hand with the amount of 2-nonenal |
| Lab tests | 2-nonenal formed only when omega-7 fatty acids were broken down |
| Conclusion | 2-nonenal may be involved in the way body odour changes with age |
Notice the word "may". The authors were careful: they suggested nonenal could play a part in age-related odour, not that it is the only cause.
Not every study finds nonenal
That caution turned out to be wise. Researchers at the Monell Chemical Senses Center in Philadelphia sampled skin odour from the upper back and forearm of 25 healthy adults aged 19 to 79. They identified many odour compounds, but they did not detect 2-nonenal with any of their methods. Several other compounds were higher in the older group, including nonanal, a close chemical relative of nonenal. The authors wondered whether the Japanese finding might partly reflect diet, such as a long habit of eating a lot of fish and other seafood (Gallagher et al., 2008, British Journal of Dermatology).
Other Japanese work has backed up the age link. Researchers at Tokai University placed a small sampling device on the back of the neck and found that the amount of 2-nonenal given off by the skin increased with age, in women as well as men (Kimura et al., 2016, Journal of Chromatography B).
The fair conclusion is that nonenal is a credible part of the story, but probably not all of it. Skin oil, sweat, skin bacteria, diet, clothes and the home all play a part in how a person smells.
Is nonenal harmful?
No. Nonenal is not a disease. It is part of the way skin changes as we get older. Interestingly, when 41 young volunteers rated body odour samples from three age groups, the oldest group (aged 75 to 95) was judged less intense and less unpleasant than the younger groups. It was also the only group the volunteers could reliably match to the right age (Mitro et al., 2012, PLoS One). An older person's smell, in other words, can be recognisable without being stronger.
A sudden or marked change is different. The NHS suggests seeing a GP if you notice a change in how your body odour usually smells, or if you suddenly start sweating much more than usual. In the US, speak to your doctor.
What helps with nonenal smell?
- Wash every day with soap and water. Soap helps wash away the oily residue that carries the smell.
- Give the back of the neck, behind the ears, the chest and the upper back proper attention. The nape of the neck has dense networks of oil and sweat glands, which is why the Tokai University team chose it for their measurements.
- Change clothes often and choose natural fabrics such as cotton, wool and silk, as the NHS suggests.
- Change pillowcases often and open the bedroom window every day.
- Be wary of big promises. No soap or spray removes nonenal for good, because skin makes fresh oil every day. A steady routine does more than any single product.
For all the likely causes, read old person smell: what causes it and what helps. For laundry and soft furnishings, see how to get the smell out of clothes and the home. And if you are curious about soaps made with persimmon extract, does persimmon soap work? sets out what is and isn't known.
Frequently asked questions
At what age does nonenal appear?
In the 2001 study, 2-nonenal was detected only in people aged 40 and over. That does not mean everyone over 40 notices it. It varies a great deal from person to person.
Can I smell my own nonenal?
Usually not very well. Our noses get used to smells that are always around us. A trusted partner, or a pillowcase you have slept on for a few nights, is often a better guide.
Is nonenal the same as sweat smell?
No. Fresh sweat has little smell of its own; the odour comes mainly from skin bacteria breaking it down. Nonenal comes from skin oils oxidising. Both can be present at the same time.
What does kareishu mean?
Kareishu (加齢臭) is the Japanese word for the smell that can come with age. Much of the research on nonenal comes from Japan, and so does a long tradition of using persimmon, which you can read about in our article on kakishibu.