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Which Molecules Make Our Armpits Stink?

By Alex B. Berezow

Your armpits stink, but it's not your fault. That sweaty, oily, hairy cavity is home to one million bacteria per square centimeter. It's a population comprised of dozens of different species! These microbes convert your body's odorless secretions into the gag-inducing stench which fuels the multibillion-dollar deodorant industry. Thus, gaining a better understanding of these potent purveyors of putrefaction is important for both economic and olfactory reasons.

Which malodorous molecules are the guilty parties? In a review article in Trends in Microbiology, German researchers list the offensive organics:

In the table above, the odor-causing molecules are listed along with their precursors, as well as the bacteria and enzymes responsible for transforming these precursors into fetid fumes.

The sulfur-containing molecules (panel A) are the worst, giving armpits their characteristic nauseating, onion-like smell. In panel B, 3-hydroxy-3-methylhexanoic acid has a cumin spice-like odor, while 3-methyl-2-hexenoic acid has been described as hircine, which means "of or characteristic of a goat." (So, when somebody tells you, "Take a shower because you smell like a goat," they are being quite scientific in their description.) Panel C depicts two possible pheromones, androstenol, which is musky, and androstenone, the smell of which differs depending on whom you ask. Isovaleric acid (panel D) has a cheesy, sweaty foot smell, as does propionic acid (panel E). Acetic acid is vinegar.

So next time you have to sit by a smelly person on the bus, instead of holding your breath, just breathe normally and appreciate the lesson in organic chemistry!

Source: Eugenie Fredrich, Helena Barzantny, Iris Brune, and Andreas Tauch. "Daily battle against body odor: towards the activity of the axillary microbiota." Trends in Microbiol 21 (6): 305-312. (2013)

Alex Berezow is the editor of RealClearScience.

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