13 Surprising Facts About the Armpit

The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

Tucked away in that damp crevice between your arm and torso, the armpit—a.k.a. the axilla—is often the source of unpleasant odors and embarrassing wetness, and a hairy font of annoyance. But it's also an important juncture that protects important lymph nodes and soft tissue. Mental Floss spoke to microbiologist Alex Berezow, a senior fellow of biomedical science with the American Council on Science and Health, about this often overlooked spot. Here are 13 things we learned.

1. YOUR ARMPITS ARE CHOCK FULL OF LYMPH NODES.

In the small hollow of each armpit are a surprisingly large number of lymph nodes, approximately 20, in two clumps, though you can't usually feel them unless they're swollen. (One clump is closer to the surface than the other.) These lymph nodes are actually an important part of your body's immune system and serve to filter toxins out of tissues. They also produce a variety of immune cells known as lymphocytes that fight infection. In some kinds of breast cancer, these affected lymph nodes may have to be surgically removed.

2. THEY PRODUCE A DIFFERENT KIND OF SWEAT FROM OTHER BODY PARTS.

Not all sweat is created equal. In fact, your skin has two types of sweat glands that help to cool you down: eccrine glands and apocrine glands. Eccrine glands cover most of the surface of your body, and are responsible for that thin sheen of sweat on your brow and extremities during heat and exercise. However, your armpits are abundant in apocrine glands (also found in your groin). These glands are copious in places with more hair follicles, and the sweat they secrete tends to be thicker.

3. YOUR PITS ARE TEEMING WITH BACTERIA.

Your skin is home to many different kinds of bacteria, some of which are quite beneficial, collectively known as a microbiome. This microbiome can vary depending on the body part—so the bacteria on your hand can be vastly different from the moist, warm, dank environment of your armpits.

"Because of oil and sweat secretion, the armpit provides a nice home for many different kinds of bacteria," Berezow tells Mental Floss. Compared to other parts of our skin, armpits are rather densely populated, he explains. Not only that, but armpit microbiomes vary from person to person. "One study showed, after sampling nine people, that there were three types of armpit bacterial communities: One was dominated by Betaproteobacteria, a second by Corynebacterium, and a third by Staphylococcus. So one person's armpit bacteria won't necessarily be the same as somebody else's."

4. IT'S NOT YOUR SWEAT THAT STINKS.

"The secretions our armpits make don't stink. Bacteria break down the compounds, and those breakdown products stink," says Berezow. The bacteria that live in the moist crevices of your armpits interact with your sweat, which contains volatile fatty acids and odorous steroids (among other compounds). That creates a product known as thioalcohols, whose oniony, meaty scents you're likely familiar with if you've ever been stuck in a crowded elevator, subway, or gym at peak workout time.

5. SCIENTISTS ARE WORKING ON A DEODORANT THAT WOULD KILL ONLY SOME BACTERIA…

The researchers plan to engineer a deodorant that would kill only the stink-producing bacteria, instead of the entire armpit microbiome. That's because some good bacteria also live under there, like those that help protect you against fungal infections.

6. …BECAUSE REGULAR DEODORANTS CHANGE YOUR ARMPIT MICROBIOME.

…and not necessarily for the better. "Deodorants change the composition of the microbiome," Berezow says. He cites a study that found "antiperspirant reduces the number of bacteria in our armpits, but interestingly seems to encourage a greater diversity of microbes." He adds, "deodorant seems to increase the number of bacteria compared to people who don't wear deodorant."

Scientists have also found that the pits of people who usually use antiperspirants or deodorants, but stopped for a couple of days as part of the study, grew crowded with an overabundance of Staphylococcaceae—the bacteria that causes staph infections. The individuals who habitually did not use products were dominated by the friendlier—and yet stinkier—Corynebacterium. We just can't win. 

7. WHY DON'T YOUNG KIDS' PITS STINK?

While teenagers often exist in a funk so tangible you can almost see it, most children do not begin to have stinky pits until their tweens. A process called adrenarche begins around age eight for some kids (but often even later) in which the adrenal glands start to secrete hormones called androgens. While these are typically thought of as male hormones, both boys and girls produce them in different quantities. At this stage, not only can sweat start to take on its pungent stench, but children can begin to grow armpit and groin hair. Not much is understood about adrenarche, except that it may be a necessary step in order to trigger puberty. Which may explain why middle school locker rooms do tend to get whiffy.

8. WOMEN'S PITS SMELL LIKE ONIONS AND MEN'S LIKE CHEESE.

Researchers from Firmenich, a company in Geneva, set out to understand the subtle nuances in body odor to better market deodorant products to consumers. In their 2009 study, published in Chemical Senses, they discovered that your unique bouquet may be different depending on whether you're a cisgender man or woman. Women's sweat contained higher levels of an odorless sulphur-containing compound that produces a pungent oniony thioalcohol when combined with the bacteria in the underarm. Men's sweat held higher levels of a fatty acid that produced a "cheesy" scent when the bacteria of the armpit came in contact with it.

9. WOMEN DIDN'T ALWAYS SHAVE THEIR ARMPITS.

Since women were socialized to keep most of their bodies covered for centuries, exposing an armpit was an unlikely event in a public place before 1915. However, an ad in Harper's Bazaar changed everything when it suggested that in order to engage in "Modern Dancing," women should first remove their "objectionable" underarm hair. By the Roaring Twenties, many women's pits were as hairless as the day they were born.

10. SOCIAL EXPECTATIONS SHAPE OUR COMFORT WITH ARMPIT HAIR.

Despite armpit hair being as natural as the hair on our heads—and everywhere else it grows—women's armpit hair tends to be controversial. A feminist scholar set out to explore some of the reasons for this in a 2013 study in the Psychology of Women Quarterly and found that social expectations play a huge role in women seeing body hair—on themselves and on other women—as "disgusting" or simply socially unacceptable. Even women who purposely grew their pit-hair out to flout societal expectations felt self-conscious showing armpit hair in social settings.

11. …AND SO MIGHT OUR ANIMAL NATURE.

The 2013 study, conducted by a professor at Arizona State University, suggests that this revulsion with armpit hair may be a Western aversion to our primal roots as animals. Other animals send out chemical signals called pheromones to attract mates. We still don't know whether pheromones exist in humans, but plenty of evidence indicates we are highly sensitive to each other's biochemicals. If pheromones do exist, body hair around the groin and armpits could be a likely place to find them. But as "civilized" people, we believe the process of finding a partner lies in our hearts and minds—not in our armpits. Maybe one day we'll find out it's all of the above. 

12. YOUR ARMPIT LYMPH NODES MAY WARN YOU OF BREAST CANCER.

Most of the time a swollen lymph node in the armpit is little more than a sign of a cold or flu virus attacking your body. However, it can also be an early symptom of inflammatory breast cancer, an aggressive form of cancer that is best treated when caught as early as possible. Other areas that may swell in this cancer are your breast itself, and around your collarbone. If you have these kinds of sudden swellings, it's a good idea to see a doctor.

13. SOME PEOPLE GET THEIR ARMPITS BOTOXED.

A condition known as hyperhidrosis—excessive sweating—can be frustrating for those who'd like to be able to simply wear clothing they don't drench. According to Dr. Sonam Yadav, medical director of a cosmetic dermatology clinic in New Delhi, India, Botox is used to treat underarm sweating (yes, here in the U.S. too). Yadav tells Mental Floss, "It works by regulating the synergy between the neuromuscular junction and the sweat glands."