It's something that we're all aware of when visiting an elderly relative, or somewhere with older people living there, but the musty odour that can be smelled in those places actually has a scientific explanation behind it, and it's perfectly normal.
So, it's often - pretty unfairly - referred to as 'old person smell' but, while there are loads of common misconceptions that persist about it, it's actually nothing to do with poor hygiene, and is just something that is inescapable and happens to us all as we get older.
That's a chemical compound that is created when omega-7 fatty acids on the skin break down through oxidisation.
The human skin starts producing more of those fatty acids as the body's natural anti-oxidant defences start to deteriorate at around age 40.
That process can be added to by hormonal changes such as the menopause, as well.
2-nonenal is not water soluble, and therefore can stay on the skin no matter how much you wash it, so it's pretty much guaranteed to be something that happens to everyone.
As the skin grows older, the natural oils deoxidise more easily, meaning that there is more 2-nonenal on there.
Hence, the older you get, the more likely it becomes.
In the same way that some people produce more body odour, it is possible to reduce the amount of 2-nonenal through a healthy lifestyle and diet.
However, as we've established, it's non-water soluble so it's nothing to do with poor hygiene, though that will help.
Things like exercise, avoiding stress - like that's possible - and not smoking or drinking too much alcohol will all help out. As will drinking plenty of water and getting sufficient sleep.
You can pretty much take as much personal care as you like, but many soaps, shower products and deodorants will have no effect on removing 2-nonenal.
Obviously, it's still important that elderly people - and those that look after them - keep themselves and their houses clean, and even opening the windows to recirculate air can be of use.
However, as people get older, it often becomes more difficult to do these sorts of things.
In short, the truth about the 'old person smell' is that it is perfectly normal and just a side-effect of ageing.
If more people were aware of what was happening, there might be less stigma and unpleasantness attached to something that is simply a normal part of the ageing process.
The more of that we can do, the better.