I AM standing in a bright and airy converted barn in the English countryside sniffing vials of pure armpit odour. The contents of these five tiny bottles are so pungent they actually knock me back. I’m getting top notes of cheeses – stinky as they come – lots of sulphurous onion and a hit of ammonia. The least offensive has a citrusy undertone. The bottles are provided by Camille Ferdenzi of the French National Centre for Scientific Research (CNRS) in Lyon, whose work includes recruiting volunteers to sniff sweaty T-shirts. Clearly, studying human smells isn’t for the squeamish.
Our bodily scents provide a channel of communication that evolved to help us survive and thrive, and in recent years Ferdenzi and others have revealed this language to be far richer than we realised. We have now discovered that each person’s scent is unique – not even identical twins smell exactly alike. Each of us also has a one-of-a-kind nose for smells. What’s more, we have learned that scents wafting from our bodies and wisping into our nostrils help us to forge family bonds and draw us to partners, divert us from danger, illness and aggression, and even allow us to sniff other people’s happiness.
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Yet throughout history and across cultures, people have scrubbed, perfumed and deodorised to disguise their natural smells – perhaps never more than today. “Every day, we control our olfactory image,” says Ferdenzi. If these smells are such a powerful form of communication, our aversion to them is puzzling. And recent evidence suggests we are getting less stinky and losing the ability to detect certain scents. What the smell is going on?
The role of smell in our lives begins before we are born. Many odour chemicals from foods can cross the placenta, says Benoist Schaal at CNRS in Dijon, giving fetuses a taste of what their mother is eating. That may help explain why, after birth, babies find the smell of human milk – also flavoured by their mother’s diet – intrinsically attractive. Schaal’s team has even found that a mother’s odour helps her newborn’s brain to identify faces, suggesting that from an early age, even mainly visual processes incorporate smell. He speculates that babies may have an early sensitive period for their mother’s smell, which fortifies the drive to bond, like the “imprinting” we see in birds and some other mammals.
“There is no doubt that we can smell fear, stress and anxiety in others”
The scent of newborns makes quite an impression on other people too. The smell of a new baby’s head , similar to when we receive a treat or even take a drug. That is a potent way for babies to remind us to look after them.
Newborn smell evaporates away by about 6 weeks of age, but as we get older, a vast range of things influence our personal bouquets: diet, age, fertility, illness, even state of mind. So individual are our scents that some have tried to use “odourprints” , or “nose witnesses” to . And it isn’t just how you smell to others that is unique, but your sense of smell too (see “Can you smell that?”).
But what exactly constitutes your particular aroma, the scent of you? The realm of human smells is vast, and most of our knowledge about it is confined to the underarms. That is what we generally negatively think of as body odour (BO), although armpit odour is unlike other noxious smells. “It is not a typical bad or ‘off’ odour, even if we perceive it as such,” says Andreas Natsch, a scientist at fragrance firm Givaudan. In other words, it doesn’t contain the small volatile chemicals associated with things like sour milk, decay and faeces – although our breath and foot smells may.
Instead, armpit odour has three key components: thiols, steroids and a diverse set of acids. All three are secreted as odourless precursors and then converted into smelly compounds by microbes living on our bodies, says Natsch. We know genetics plays some part, and that it is the microbes, not the precursors, that determine how ripe you get. But we still don’t know all the steps in the process.
Getting sniffy
In general, people with lots of bacteria in the genus Corynebacterium, which are more common in men, tend to smell pungent, whereas those with lots of Staphylococcus are less smelly. Men also tend to sweat more, releasing more precursors to feed those musky microbes – another reason they are generally the stinkier sex.
It may come across as rude, but recoiling from someone’s oniony underarms is a long-evolved reaction. “There’s a high sensitivity of the human nose to these odorants,” says Natsch. “This indicates that they had some function in human history, even if today they are just malodours.”
Until recently, it was thought that we could detect only about 10,000 scents. In 2017, this idea was by John McGann at Rutgers University in New Jersey. Comparing the neuroanatomy of 24 mammals, he found that humans can smell up to a trillion different odours – similar to a dog or a rat.
Still, the notion that smell is less important for us persists. “We humans don’t typically walk up to strangers and overtly sniff them,” says Noam Sobel at the Weizmann Institute of Science, Israel. “Or do we?” His team has shown that we sniff our hands a lot and are twice as likely to do so after shaking hands with a stranger. Handshaking is just one of the subtle ways we sample someone else’s scent. “People constantly sniff themselves and others,” says Sobel.
What are we trying to suss out with all of this sniffing? Despite popular enthusiasm for the idea, there is still no good evidence for specific pheromones that drive sexual attraction (see “Love potion”). But many researchers, including Tristram Wyatt at the University of Oxford, think we probably do produce pheromones – chemicals that have evolved to trigger a specific behaviour in the individual who smells them. He believes the first will be discovered between mothers and their babies, where we already know scent plays a role in bonding and that mothers can recognise their infant’s smell .
Body odour undoubtedly plays a broader role in kin recognition, but studies show surprisingly inconsistent results. “Kin recognition is based on both familiarity and a genetic component,” says Ilona Croy at the Technical University of Dresden, Germany. She notes that not all mothers recognise the smell of their children but, if they do, they much prefer it to the smell of other kids. “Familiarity seems to be more important,” she says.
“Handshaking is just one of the subtle ways we sample someone else’s scent. We constantly sniff each other”
The genetic component of kin recognition by smell is puzzling. It is thought to rest on the human leukocyte antigen (HLA) set of genes, which varies a lot between individuals and codes for proteins associated with immunity. It has been assumed that humans are sexually attracted to the smell of people whose HLA is dissimilar to our own, based on reams of research suggesting this is the case for many animals. That would be an evolutionary boon for offspring because the more diverse a person’s HLA, the stronger their immune system. But studies in humans have been disparate and controversial, and a 2017 meta-analysis failed to confirm that HLA drives sexual attraction, even though it did show that we . “This is just a small part of partner choice,” says Jan Havlíek at Charles University, Czech Republic, whose own studies reveal that the HLAs of real couples are actually more similar than those of randomly chosen ones.
Beyond bonding, our bodily odours convey a whole other world of meaning, particularly where emotions are concerned. For example, a meta-study published in 2017 leaves no doubt that in the body odours of others. Recently, researchers from Sobel’s lab during their first jump and found it contained 29 volatile compounds not present before. Similarly, using sweat from cage fighters, Havlíek and his colleagues discovered that people can identify winners and losers, suggesting that smell helps us recognise dominance.
Sobel’s team has also found that , possibly through a volatile chemical called hexadecanal, which modulates aggression in mammals. Some work has even shown that .
Love Potion
A host of online products claim to offer sexual allure in a bottle. They tend to contain steroids found in armpit odours, including androstadienone and estratetraenol, suggested as the first human pheromones in 1991.
They are no such thing, says Tristram Wyatt at the University of Oxford. Pheromones trigger specific behaviour when smelled, but as yet there is no robust evidence for any particular ones with a role in human sexual attraction.
“They were plucked out of thin air,” says Wyatt, then patented by a fragrance company to add to perfumes. Many papers have been , but the research is like an echo chamber. “There’s no more evidence now than there was then,” he says.
Less research has been done on whether body odour conveys positive emotions, but it has been shown that the . Ferdenzi believes that the smell of positive emotions, even in strangers, may help explain why happiness is contagious. In not-yet-published work, her team has shown that the smell of happy people produces feel-good physiological responses in others.
Clearly BO evolved over millennia to be a subtle and multifaceted channel of communication, but the fact remains that it often grosses us out. Why? One reason is obvious: certain bodily odours are intended to repel. This is true of smells connected with disease. We can’t all be super-sniffers like Joy Milne, the woman who can detect the scent of Parkinson’s disease, but anyone can including diabetes, pneumonia, cholera and certain cancers from specific smells given off in sweat, breath, urine and faeces.
Even without medical training, we find an individual’s sweat more intense and less pleasant when they are ill, says Mats Olsson at the Karolinska Institute in Sweden. His experiments reveal that, via sweat, we subconsciously detect when someone else’s immune system is ramped up in response to an infection, and .
Being repulsed by particular body odours can help us avoid other dangers too. “Kin recognition serves two main purposes: one is bonding, the other is incest avoidance,” says Croy. That would explain the finding that parents come to . The scent of aggression may put us on alert too. Bettina Pause at Heinrich Heine University Dusseldorf, Germany, has found a distinctive response in the sensory processing part of women’s brains when they sniff sweat from aggressive men, something she believes evolved to help women avoid violence.
Sickly smell
An obvious reason why, unlike other mammals, we work hard to wash away our smells is that we understand the link between poor personal hygiene and disease – we know that washing can increase our chances of survival. In fact, anthropologists argue that our changing attitudes towards personal hygiene coincided with the rise of urbanisation around the industrial revolution, says Asifa Majid at the University of York, UK.
Natsch points out that there is a much longer history of personal hygiene in East Asia, which might help explain the curious fact that many Asian people have naturally odourless armpits. Thanks to a , 95 per cent of ethnic Chinese people and some 70 per cent of East Asians don’t produce chemical precursors of armpit odours. We don’t know when the mutation happened, says Natsch, but an aversion to BO would have helped it succeed.
All this goes some way to explaining our distaste for bodily odours. However, there are plenty of scents that we love – and not just from those irresistible babies. In the right circumstances, we appreciate even the most pungent human odours. We are less disgusted by the nasty odours of our nearest and dearest – the so-called , for example. And we positively enjoy the smell of someone we are sexually attracted to when dancing with them. Sometimes context doesn’t even matter, says Ferdenzi, recalling that, after her first study, one of the volunteers asked if she could have the contact details for sweaty T-shirt No6.
Even as we are learning more about the subtle scent cues we send and receive, genetic studies now indicate that humanity as a whole is losing touch with its olfactory communication channel – and not just because we are evolving to stink less. A significant proportion of people have a selective anosmia, or loss of smell, for the major compounds found in armpit odour, says Natsch. For example, up to 40 per cent are unable to smell androstenone, a putative sex pheromone. “There seems to be an accelerated evolutionary loss of the ability to smell body odours,” he says.
Missed messages
Intriguingly, modern languages may reflect this loss. Most, like English, lack a decent lexicon for smell. We have relatively few words to describe specific scents compared with languages such as Umpila, which is spoken by a traditionally hunter-gatherer community in Australia. “Maybe in that kind of environment smell is a much more salient signal. That’s why you end up with a language for it,” says Majid. Hunter-gatherers also don’t share our enthusiasm for disguising their bodily smells, she notes.
That loss goes both ways. Having a lexicon of smell enhances people’s appreciation of this unsung sense. “Once you have it in language, it’s more part of your conscious awareness,” says Majid. Without this awareness, we are probably missing out on some of the subtle signals that body odour transmits about others. On the other hand, with the evolution of language, perhaps humans didn’t need to rely on smell as much as other mammals because we could verbalise some of the messages it carries. But Ferdenzi, for one, is unconvinced. “Language and body odour are totally different ways of communicating,” she says. “There is this subliminal effect of olfaction that we absolutely cannot control.”
Whatever the reasons for our changing relationship with human smells, we may be paying a price. Anosmia – which affects around 5 per cent of people – is an extreme version, but we know that it can undermine quality of life and increase risk for depression. And smell fortifies our relationships in ways we may only appreciate once the people, or their scents, are gone. When Thomas Hummel at the Technical University Dresden asked anosmics which odours they missed most, many said it was .
But there are things we can do. We can all learn to smell more “mindfully”, according to Hummel. In his efforts to help people with anosmia, he found that simply sniffing odours consciously and regularly radically . “It alerts people to smell smells,” he says.
Even if we could do more to appreciate their influence or stop taking them for granted, sometimes the power of human scents is undeniable. “It’s not just an ancient sense we don’t use anymore,” says Ferdenzi. Just ask sweaty T-shirt No6.
Can you smell that?
Our noses are home to about 6 million smell receptors of some – compared with just three kinds for vision. Genetic variations affect the way these work, and there are five such variations, on average, per receptor. Most odours activate several receptors, but a change in a single receptor is often enough to alter the way we perceive particular smells, .
It isn’t easy to measure something as subjective as smell. Having people describe aspects of certain odours such as strength, pleasantness and quality has revealed that we can perceive similar compounds very differently. “There’s a lot of variation among people in odours that have been tested,” says Casey Trimmer, a scientist at fragrance company Firmenich.
That’s just the first level of smell perception. “A whole other level happens in the brain,” says Trimmer. How you perceive smells depends on exposure, expectations and associations, but there are probably some universals. Asifa Majid at the University of York, UK, has found that Westerners and hunter-gatherers from Malaysia display . “That suggests there may be some shared element to what we find pleasant and unpleasant,” she says.
Having a particularly acute sense of smell – which is – also influences our perceptions. “As you find an odour more intense, you tend to find it less pleasant,” says Trimmer. She thinks that may be caused by overstimulation, but notes that some odours, such as vanillin, don’t offend even at high concentrations.
