You know that smell in retirement homes and your grandmother's house? Mothballs and stale air may not be entirely to blame.

In a new study, researchers have confirmed for the first time that older people have a recognizable body odor that can't be fully explained by grooming, diet, or other environmental quirks. In fact, the study found, this "old person smell" is distinctive enough that young adults can more often than not identify an old person by body odor alone.

This isn't totally surprising. Scientists have known for years that a broad range of animal species-including mice, deer, otters, rabbits, and monkeys-undergo body-odor changes in adulthood, which may help the animals select suitable mating partners.

Humans have found better ways of screening potential mates, but like other animals, we may have once used age-related signals gathered from body odor to choose partners, avoid sick people, or distinguish kin from non-kin, says Johan Lundström, Ph.D., the lead author of the study.

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Lundström, a senior neuroscientist at the Monell Chemical Senses Center, a nonprofit research institute in Philadelphia, began wondering about the effects of age on body odor when he noticed that the so-called old person smell seems to be consistent across cultures.

As a child in Sweden, he often accompanied his mother to the nursing home where she worked, and he remembered a unique odor throughout the building. "I never smelled it again until years later, when I came to the U.S. and gave a talk at a retirement home," he says. "As soon as I walked through the door, that exact same scent hit me."

Later, when he described his experience to a Japanese colleague, he learned there's even a word for this smell in Japanese: kareishū.

Lundström and his colleagues designed their study, which appears this week in the journal PLoS ONE, to explore whether there's a biological basis for the smell.

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First, they instructed 41 men and women who fell into one of three age groups (20 to 30, 45 to 55, and 75 to 90) to sleep for five consecutive nights in a t-shirt containing special underarm pads. Then, after cutting the pads into pieces and placing them in glass jars, the researchers asked a separate group of 41 young adults to smell two samples back-to-back and determine which came from an older person.

The young adults had trouble distinguishing between the young and middle-age groups, but they were much more successful at singling out samples from the oldest group. In addition-contrary to the stereotype of the old person smell-the volunteers generally rated the samples from the oldest group as being less intense and less unpleasant than those from the youngergroups.

The telltale odor couldn't be chalked up to the older participants' lifestyle or environment. The researchers asked all of the people who provided underarm pads to avoid scented soaps and shampoos, alcohol, tobacco, and certain foods and spices-all of which can affect sweat and body odor. (They also excluded any pads that were obviously contaminated by soap, smoke, perfume, or other odors.)

Although some of the participants in the older group were taking medications for chronic conditions such as high cholesterol and hypertension, none of these prescription drugs are known to alter body odor.

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The root cause of the old person smell is still a mystery, but the study notes that long-term changes to the skin glands may be involved. Lundström suspects it also may be related to an accelerated rate of cell decay. "As cells die at a faster pace, they might give off a different odor that is unique to people with old age," he says.

Another possibility is that the scent indicates an undiagnosed illness. Although the study participants were all outwardly healthy, some may have had underlying ailments that come naturally with oldage, Lundström says.

If this last explanation pans out, body odor could ultimately prove useful in identifying certain diseases, perhaps even before existing tests can, Lundström says.

Copyright Health Magazine 2011