Think about how much you can determine about a person, solely from scent alone: You can detect an oily, fried odor wafting over from their clothes. There is a distinct smell of cigarette smoke from their breath, and the fragrance that is barely detectable on their clothes is the same laundry detergent that you buy. From this quick sniff, you could probably make some accurate guesses about this person and what they had recently been doing.

The human nose can detect the presence of a compound called 2-nonenal. This compound is both involved in and produced by the intake of lipids (fats), and it has an unpleasant grassy and greasy smell to it. As people increase in age, the concentration of this compound on their skin increases, yielding a distinct smell.

Close up of the side profiles of two women from the nose to the chin.
Photo by Angela Roma

Humans release a variety of different molecules that relate to their health, genetics, and diet. Body odor is something many of us strive to minimize. We often experience social rejection when our body odor is objectionable to others. Olfaction, or the human sense of smell, is sensitive enough to detect the presence of different molecules being released from the body. This may be an adaptive strategy to avoid disease.

People have a more unfavorable scent when they are ill. When the body activates the immune system, there is a resulting inflammatory response that affects body odor. This was demonstrated in an animal study, where experimental mice were given an injection of lipopolysaccharide (LPS), a bacterial endotoxin, and control mice were injected with saline. The control mice would avoid the experimental mice and their urine. It is even possible for humans to smell low-level immune-activation in urine!

In another study, 21 individuals (donors) were injected with LPS, causing their immune system to act as if it were responding to an infection, triggering an inflammatory response. The donors were both photographed and swabbed for their body odor before and after the LPS injection; these pictures and swabs were presented to study participants in order to gauge their reactions. Upon smelling the sick swab, participants ranked the face of the individual as less likeable. Without knowing it, the study participants were displaying aversion towards illness.

Baby sitting in front of flowers, smelling a daffodil.
Photo by Tetyana Kovyrina

There are some stereotypical scents that separate different stages of life. Think about the gentle fragrance of a newborn baby, or the pungent odor of a teenager who comes back from gym practice. There is the hard to describe “old person smell”, that is only ever used to describe elderly individuals. So, the next time your nose crinkles up at a smell, take a moment to appreciate that your nose knows.





Featured Image: Photo by Ketut Subiyanto


Peer Editor: Omar El Merhebi


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