Most people consider body odour to be a foul-smelling repellent. If you've ever found yourself at nose-level of someone's exposed armpit on a crowded, sweltering bus or streetcar, you've probably experienced its pungent power.
Dr. Chris Callewaert, however, is drawn to it.
As a postdoctoral researcher at the University of California, San Diego, Callewaert studies the microbial ecosystem of the human underarm, which has earned him the nickname "Dr. Armpit" from his colleagues. He examines the various strains of bacteria that live there, with the goal of combatting the ones responsible for undesirable smells.
Callewaert's area of interest, which was sparked by a curious and sudden change in his own natural scent, is not a particularly popular one. His quest to cure body odour, after all, doesn't quite carry the same urgency or prestige as finding a remedy for obesity or cancer.
While his fellow microbiologists gravitated toward studying microbes in the human gut, "I was the only one working on the armpit," he says.
Yet the underarm is a complex and bustling place that is ripe for exploration. Most of us have billions of bacteria living in each armpit, Callewaert says. And researchers such as him are now discovering that the smells we generate under there may hold important information about the types of bacteria we carry, which could lead to new and more effective ways to improve our natural bouquet. At the same time, scientists are finding more reason to pay attention to body odours; the scents we exude can contain powerful clues about our health, emotional state and identity.
In recent years, studies have suggested that moods such as fear, disgust and happiness can be communicated through volatile compounds in human underarm sweat. Other research suggests illnesses, from infections to cancer, can be detected through body odour. And some scientists hypothesize that we each have a distinct "odour print," such as fingerprints, that could be used to verify who we are.
The science of body odour is still relatively new, explains Dr. Gun Semin, whose findings that humans can convey emotions of fear and disgust through "chemosignals," or a type of body odour, were published in 2012.
Until recently, the human sense of smell was widely underestimated, says Semin, a research professor of psychology at the psychological and behavioural institute ISPA-Instituto Universitário in Lisbon. For a long time, he says, it was believed that humans lacked the ability to process cues from body odours. But that has changed as scientists have gained a better appreciation for the sophistication of our sense of smell, after 2004 Nobel Prize-winners Richard Axel and Linda Buck discovered how odorant receptors and the human olfactory system work.
Now, Semin says, "there is substantial and very good research showing you can identify gender, you can identify age, you can identify illness, you can identify a whole range" of characteristics through people's body odour.
Yet for many of us, body odour is something we try to get rid of with antiperspirants, deodorants and perfumes. Our struggles to be odour free, however, may come at a cost.
"If you go back 100 years ago or so, before we had deodorants … we all had a particular smell," says Dr. Gavin Thomas, a professor in the biology department at the University of York in England. "That must have been important in how we chose mates and lots of things."
For Callewaert, the path to becoming "Dr. Armpit" started when he suddenly began noticing he smelled unusual. Previously, he says, he was able to go more than a week between showers without developing any discernible body odour.
But suddenly, only a few hours after showering, he could detect a distinct smell. Nothing unbearable, he says, "but it was very noticeable to me."
When a trip to the doctor revealed nothing wrong with his health, the microbiologist suspected he had somehow experienced a transfer of new bacteria in his underarms "in a bad way."
Then, just as unexpectedly as he acquired it, Callewaert happened upon a cure for his body odour. One day, he says, while painting his house, he put on an old cotton T-shirt, which he believes helped enrich the population of the "good," non-smelly bacteria in his armpits and out-compete the "bad," odour-producing strains. After wearing the T-shirt, his scent returned to normal.
The experience prompted Callewaert to experiment with underarm microbial transplants. He takes individuals who have strong, undesirable body odour and thoroughly cleans the skin and hair follicles of their armpits. Then he asks donors, typically relatives who do not have offensive body odours, to stop washing for several days to ensure he can collect large amounts of their underarm bacteria.
He then applies the donor bacteria to the recipient's armpits, and asks the recipient to refrain from showering and from using deodorants or antiperspirants for a week to allow the new bacteria to incubate. (While deodorants and antiperspirants may eliminate unwanted odours for most people, Callewaert does not recommend them for people with strong body odour since they tend to wipe out existing bacteria populations, leading to lower overall numbers of bacteria but increasing their diversity. Diversity, he explains, is good for an ecosystem in general, but in the armpit greater diversity can lead to more odour.)
"So far, it's very successful in the short term," Callewaert says of his transplant procedure, noting he has tried it on about 18 individuals at this point.
Most of them have noticed a decrease in body odour within a month afterward, though a few have reported some return of their smell months later. Callewaert says he is now working on optimizing the technique.
Who's under there?
Callewaert notes most people have around 100 to 200 different strains of bacteria living in their armpits. So which are the smelliest? Not everyone agrees on the answer.
Recent research from Thomas and his team at the University of York suggests the bacterium responsible for that instantly recognizable body-odour funk is one called Staphylococcus hominis.
In collaboration with the company Unilever, the researchers found S. hominis is particularly effective at producing thioalcohol, considered the most pungent compound in body odour. Their findings were presented at the Society for General Microbiology's annual conference in 2015.
As Thomas explains, thioalcohols are so potent that one would be able to smell them at even extremely low concentrations of a few parts per billion in the atmosphere. When researchers are handling them, he says, "you have to work in a fume hood in the laboratory, because if they get out there, they stink the whole building up."
Thomas believes people produce an odourless precursor molecule that S. hominis converts into thioalcohol.
He explains people have two kinds of glands in their underarms, one that produces perspiration, which does not lead to strong body odour, and another called apocrine glands that only become active in puberty. Apocrine glands secrete a sticky material that evaporates on underarm hair follicles, and it's this substance that he says contains the precursor molecule for thioalcohol.
The underarm bacteria take up this odourless molecule into their cells, where certain enzymes convert them into the smelly compound, Thomas says.
In addition to the bacterial composition in your armpits, your genetics can also determine whether you experience body odour, he explains. Certain populations, particularly in East Asia, are known to have a genetic mutation which determines that they do not secrete the precursor for thioalcohol, he says.
A complex bouquet
But there's more to body odours than what our bacteria produce – and likely much more than the human nose can perceive.
"Probably a lot of [what makes up our individual odour prints] is due to microbes," says Dr. George Preti, an organic chemist at the Monell Chemical Senses Center in Philadelphia. But he notes, "There's a little bit of contribution from a lot of different sources," such as molecules emitted in our sweat or from our breath.
Which is why, even if we shower or change our diets or douse ourselves in deodorant, we still likely give off certain chemosignals. We may, however, need to be in close proximity for others to receive them.
Preti notes he is currently studying how to diagnose different diseases from the odours of various bodily fluids. For example, he says preliminary results suggest people with ovarian cancer have a specific pattern of volatile organic compounds that are emitted by their blood plasma. While trained medical-detection dogs were able to sniff out the cancer samples with more than 90-per-cent accuracy, neither he nor his postdoctoral researchers could detect any smell.
But just because we don't necessarily notice various bodily odours doesn't mean we aren't affected by them.
Semin says around half of the participants in his studies, when asked to smell samples of others' underarm sweat, report they don't smell anything. And most of the others say they smell something, but are not sure what it is.
Even so, his work has shown that participants who smell the underarm sweat of others, collected on sterile pads while they're in a state of fear, disgust or happiness, express subtle facial movements of the corresponding emotions of the sweat donors. A whiff of happy sweat, for instance, can elicit a smile, even when the participant detects no identifiable scent.
"If you were able to analyze the chemical composition, the biochemical signature of the odour, the types of things you could do with that are quite unlimited," Semin says. "Imagine that I give you a soap or a perfume that has the same biochemical composition as a happy sweat – I would be a rich man, probably."
Determining the biochemical composition of body odour, however, is a complicated endeavour, since any one person's scent can hold an array of information about everything from their age to their emotional state, he says.
"It's not very easy," he says. "The main thing is body odour conveys multiple pieces of information at the same time."