We’d all be lying if we didn’t admit that, when we walk into a retirement home or assisted living facility, or give our great-grandparents a tight hug, we didn’t notice a smell.

Not a bad smell, just a smell that’s associated with old people.

But why?

Like all body odors, it boils down to chemicals that waft away after skin glands are broken down. The specific chemical in the case of old-person smell is a compound called 2-nonenal. It’s created by oxidative breakdown of chemicals over time and is described as an “unpleasant greasy and grassy odor, also responsible for the “cardboard” flavor of stale beer.

In 2000, Japanese researchers found that people’s concentration of 2-nonenal increased with age. In the study, clothing worn by people over 40 years old tested higher for the chemical compound.

Not only that, but the concentration increased significantly as people aged, with the oldest person in the study – a 75-year-old – producing as much as 3x as much as the middle-aged subjects.

Researchers might have isolated the source of the smell, but they’re not sure exactly why it increases with age. They’ve also noted the presence of more omega-7 unsaturated fatty acids in the older subjects’ clothing, which is also likely a cause of the 2-nonenal, but they’re not sure why the fatty acids increase, either.

It could be because of age-related changes to metabolism or changes in other chemical secretions, but either way, researchers are also curious as to what purpose age-related smell might serve. One possible explanation is that older people might have a genetic advantage that allows them to survive longer, and therefore make more attractive mates, but it doesn’t seem that plausible in humans.

What might be most interesting about the study is that when people were asked to rate body odor smells without knowing the source, they rated old person odors less intense and unpleasant than odors from younger people – which means it’s some kind of context or preconceived notion that’s putting people off, not the smell itself.