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If you've ever associated a smell with old age, you weren't imagining it—it all has to do with the chemistry of our skin.

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If you got to spend time with your family back during the holidays, you might have noticed that grandpa smells a little… old. Not stinky or bad — he just had that distinctive old person smell which some people describe as grassy or greasy.

And you weren’t imagining it, either. Like baby humans, older humans do have a certain odor, and it’s likely caused by the chemical nonenal. More specifically, that smell is caused by a version of the molecule called 2-nonenal, and it occurs naturally on human skin.

Researchers have found that nonenal begins appearing around age 40, and that the amount of it increases as you age. It all has to do with the complex chemistry in your skin. See, your skin secrets fats to help form a barrier between your body and everything in the outside world.

It helps keep moisture in and bad things out. And these fats are found in the outer layer of your skin — the stratum corneum. Now, when those fats are exposed to oxygen in air, they start to break down and form other chemicals, also known as being oxidized.

But your body kind of needs those fats. So to help prevent this, your skin also produces antioxidants. But as you age, two things happen.

One is that your skin produces fewer antioxidants, so more fats get oxidized. And the other is that the types of fats your skin secretes also changes, which might have to do with how your hormones change over time. And when those new fats get oxidized, that creates nonenal.

As you get older, your skin produces more of these specific fats and fewer antioxidants. So you get more nonenal, and more Old People Smell. So what can you do about it?

Not much. Nonenal isn’t water-soluble, which means it won’t dissolve in water and it’s hard to wash off. Soap can remove non-water soluble substances, but it doesn’t remove all the fats from your skin.

If it did, your skin would be really dry and uncomfortable. Which means that, even after bathing, some nonenal is still left on your body. And while many commercial soaps contain deodorants to combat other bodily odors, most don’t yet contain deodorants targeted for nonenal.

So smelling like it does not indicate poor hygiene. Whether or not you like the smell probably depends on your personal experiences, your culture, and how you feel about getting older, but by itself, that smell is a totally normal part of aging. In fact, in a blind sniff test, the body odor of people over age 75 was described as “neutral” and rated as less unpleasant than that of young or middle-aged people.

And in general, you might even consider nonenal as the smell of a survivor, of experience... or of the most interesting people in the world. Thanks for asking, and thanks to all of our awesome patrons on Patreon who keep these answers coming. If you’d like to submit a question to be answered, you can go to [♪♩OUTRO].