Science Figures Interpreted and Analyzed by RealClearScience
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)