Reader Sarah writes in to ask, “What is it that causes that distinctive 'old person' smell? Whatever it is, it seems to be common to all elderly people. Is it inevitable or is there something you can do to avoid it?”

Ever notice that your grandparents and their house had a dull, kind of sweet stink to them? You’re not alone. Old people really do have a chemically-distinct odor.

Like other body odors, this “old person smell” is produced when chemicals from the skin glands get broken down into small odorous molecules that waft away into the air. The specific chemical that gives old folks their unique odor, scientists suspect, is a compound called 2-nonenal. Created by the oxidative breakdown of other chemicals over time, it produces what’s described as an “unpleasant greasy and grassy odor” in people and is also responsible for some of the “cardboard” flavor of stale beer.

In 2000, Japanese researchers found that people’s concentration of 2-nonenal increased with age. They had 22 people, ranging in age from 26 to 75, wear odor-collecting shirts to bed for a few nights and then analyzed the molecules that adhered to the cloth. They found more 2-nonenal in the shirts worn by people over 40 years old than they did in the younger subjects. And in the over-40 crowd, the concentration of 2-nonenal increased significantly with age, with the oldest subject producing almost three times as much as the middle-aged subjects.

The researchers didn’t see any other odor compound increase with age like that, and think that the “deterioration of body odors” in the elderly, as they politely put it, can be pinned on 2-nonenal. But why does the compound increase as a person ages? The researchers also noted the presence of more omega-7 unsaturated fatty acids in the shirts worn by the older subjects, and think that the 2-nonenal comes from the breakdown of these fatty acid chains. The reason the fatty acids increase with age, meanwhile, is still unclear. The researchers speculate that it might be because of age-related changes to metabolism or changes in the amount of some other chemical in skin secretions.

Another big question still hanging in the air is what purpose, if any, an age-related change in smell serves. Humans and some non-human animals can tell the difference between older and younger individuals by smell, and some animals are known to be more attracted to the odor of older individuals and have more success mating as they age. One possible explanation for this is that older individuals may have some genetic advantage that allowed them to survive longer and makes them more attractive mates, and that distinct age-related odor is an advertisement for their genetic quality. It’s not clear that this is what actually happens, but if it is, it’s hard to imagine smell having much of an effect with humans when we place such high value on the physical attractiveness and other qualities of youth.

The fact that old person smell is usually thought of as unpleasant doesn’t seem to be a stumbling block here, though. Research subjects who didn’t know the source of the smell rated old person odors as less intense and less unpleasant than odors from younger people—suggesting that the smell on its own isn’t bad, but is perceived that way in certain contexts.