A pungent blend of onions, cheese, and cat urine with hints of… is that…wet goat? Most of us don’t need more than one whiff to identify that generally unpleasant, characteristic smell we call body odor. But it’s a surprisingly complex phenomenon influenced by our genetic makeup, age, diet, and hygiene.
So what is this odor, exactly? Where does it come from? And can we do anything about it? To start, you just need two things to produce that familiar scent: your armpit’s own secretions and the bacteria that feed on them. Most people associate body odor with sweat, and it’s an important piece of the puzzle.
Your body has millions of sweat glands, and they come in two major types: eccrine glands are found all over your skin and secrete mainly water and salt. Apocrine glands, on the other hand, develop at puberty in your armpits and a few other places on your body. The sweat they secrete is full of proteins and fats.
By themselves, these secretions are usually odorless. That’s where bacteria come in. Every square centimeter of our bodies is covered with thousands of bacteria. Many microorganisms thrive in moist environments, like our armpits. There, you can find about a million bacteria per square centimeter, one of the highest concentrations anywhere on the skin. Lurking in this throng of microorganisms are species of Corynebacteria, Staphylococci, Micrococci, and others.
When these bacteria feed on the proteins and fats in apocrine sweat, they turn the odorless compounds into new ones that can smell very unpleasant. Some of the worst offenders may be sulfur-containing chemicals; those give body odor its oniony aroma. Carboxylic acids are in the mix, too, adding notes of cheese. These molecules waft up from the armpit and can be sucked directly into our noses, where they’re trapped and detected by an array of specialized receptors.
Those can recognize odor molecules at concentrations of less than one in a million. So what determines how strong your body odor might be? It depends on the resident microbial populations in your armpit, and the nutrients that your glands provide them with. Your genes help determine what compounds you produce, and in what quantity, so everyone has a slightly different set. In fact, a gene variant that virtually eliminates body odor is common in people of East Asian descent.
Adrenaline increases the ratio of apocrine to eccrine sweat, so body odor can be more intense when you’re nervous. Bacterial composition and concentration also varies between individuals and plays a part. Even what you eat can have a small effect on how you smell. So how can we deal with body odor? Washing the armpits with soap and water helps but won’t remove all the bacteria since many are buried in deeper layers of the skin. Deodorants, however, inhibit bacterial activity and mask odors at the same time.
Antiperspirants work by forming tiny gel plugs that block sweat glands, drying out the armpits. While we continue to battle body odor, scientists are trying to understand it. We don’t know why the brain often interprets these particular odors as off-putting.
But some researchers have proposed that secretions from the armpit could have a positive function, too, like cementing social bonds and providing a means of chemical communication. We don’t know yet if that’s the case. For now, body odor seems to be just another smelly part of the human condition.