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Not mothballs, but a purely human scent.
Scientists have discovered that humans can distinguish between a young person and an old person by relying on that person's smell alone.
A team from four academic institutions in the US and Sweden collected body odors from 41 donors in three age groups: young, from 20 to 30 years old; middle-age, from 45 to 55 years old; and old-age, from 75 to 95 years old.
For five nights, donors wore a T-shirt with nursing pads sewn in the armpits. These shirts would then be sealed in plastic bags by day to avoid contamination by outside odors.
After their daily activities, donors would then take a shower using odorless shampoo and soap and wear the shirts again. The shirts were also previously washed with odorless detergent.
The donors' odors were collected so that 41 participants could undergo two tests.
The first was to choose which among two odors from different age groups belonged to the older person. The samples were presented one after the other.
The second was to classify six odors into young, middle-age, and old-age categories. The odors were presented all together.
"Participants were able to discriminate between age groups, as well as group the old-age body odors together significantly more often than expected by chance," according to the research as published in PLoSONE.org.
The latter is a peer-reviewed online journal dedicated primarily to scientific studies, PLoS being the acronym for Public Library of Science.
While participants rated male and female odors from younger donors differently in terms of pleasantness and intensity, they rated male and female odors from older donors similarly.
"These perceptual differences clearly demonstrate that body odors have age-dependent odor characteristics," the article said, citing the study entitled The Smell of Age: Perception and Discrimination of Body Odors of Different Ages.
The research was conducted by the team of Susanna Mitro, Amy Gordon, Mats J. Olsson, and Johan N. Lundström. The scientists come from the Monell Chemical Senses Center, Swarthmore College, and University of Pennsylvania, all in the US, as well as the Karolinska Institute in Sweden.
"The concentration of lipids present on the skin surface begins to decline to pre-pubescent levels with older age, returning to childhood levels around age 80, suggesting that older men and women share skin chemistry features important for body odor production that are not uniform between the sexes at younger stages of maturity."
It was also found that participants rated odors from older donors as less intense and unpleasant as compared to those from younger donors.
"These studies suggest that elevated levels of certain chemicals are a potential biomarker for old age," the study said.