"Let's go to grandpa's room and see if it still smells funny," said Tommy.
"Last time it smelled like cheese..." replied Lil.
Sure, these lines from "Rugrats," an animated series that aired on Nickelodeon in the 1990s, were meant to be funny. But they also may have a basis in real life, relating to the widely held notion that the elderly emit a particular -- and easily recognizable -- body odor.
While the smell has been described as stale, medicinal, musty or simply "old person," the Japanese have developed a more elegant term: kareishu. In one study, Japanese researchers traced the odor to a chemical compound called 2-nonenal. The compound is a byproduct of other chemical breakdowns and emits a "greasy" or "grassy" odor as the molecules exit the skin and are released into the air. Researchers discovered that 2-nonenal was the only odor compound whose presence became stronger with age.
In the study, in which 22 people aged between 26 and 75 were asked to wear odor-collecting shirts to bed, researchers analyzed molecules that stuck to the cloth. The amount of 2-nonenal was three times as prevalent in the oldest subjects as it was in the middle-aged group. The youngest members of the study produced the least amount of 2-nonenal.
Researchers speculate the increase in 2-nonenal may have to do with the breakdown of omega-7 unsaturated fatty acids, because these molecules were also present in the shirts worn by the study's oldest subjects. It's possible 2-nonenal may be produced as a byproduct of omega-7 as the compound deteriorates, possibly because of changes in metabolism as the body ages or changes in the amount of other chemicals that are released through the skin [source: Carroll].
Another study measuring participants' abilities to identify the age of people by sniffing their sweat samples found that it was nearly impossible to discern the differences in odor between the 20- to 30-year-old group and the 45- to 55-year-old age group. Participants could, however, easily identify the scent of old people (75- to 95-year-olds). Even so, the participants didn't rate the "old person" smell as particularly intense or unpleasant. It was simply uniquely recognizable [source: Sifferlin].
Although the biological purpose behind "old person" smell is still unclear, some researchers believe it's connected to a built-in age-detecting feature possessed by humans and animals. For instance, some animals can distinguish between older and younger animals by smell alone, and some animals are driven to mate with older animals. It's possible the ability to sniff out the smell of older animals meant that long-term survivors were revealed -- and these survivors had a genetic advantage that boosted their odds of survival and reproduction. In essence, the scent we've come to call "old person" smell could be an advertisement for superior genetic quality. It gives a whole new meaning to the term "silver fox" [source: Soniak].