In 1934, a British physician named BA McSwiney stood before his colleagues at the Royal Society of Medicine and lamented that most folks didn’t concern themselves with the chemical composition of human perspiration. Instead, they focused solely on the mechanisms by which the evaporation of sweat from the skin’s surface allowed the body to cool itself.

But McSwiney knew that there was more to sweating than just evaporative cooling. Under certain conditions “the loss of constituents of blood-plasma by continued sweating may be considerable”. In other words, other stuff leaves the body in our sweat. But what kind of stuff, and is its loss a good thing or bad?

Some substances in our sweat we probably wouldn’t want to lose. Take chlorides. These compounds – chlorine atoms, often attached to sodium ones to form salt – are important for maintaining the body’s internal pH balance, regulating the movement of fluids in and out of cells, and transmitting impulses across nerve fibres. It’s normal for some chlorides to leak out of the body as we sweat, but there are some instances in which a person might lose too many. Imagine working for several hours in a hot place, for example. Most of us would know to drink water to stay hydrated. But sweat too much and drink too much and you might start to show symptoms of water poisoning. In those circumstances the body just can’t replace the chloride lost in sweat fast enough.

Also mixed in with sweat is urea, the substance for which urine is also named. By at least one estimate, between 0.24 and 1.12 milligrams of the stuff is dissolved in every cubic centimetre of sweat. That might not sound like much, but given that a person sweats some 600 to 700 cubic centimetres worth of liquid each day, sweat is responsible for up to 7% of someone’s daily elimination of urea. (For comparison, that much sweat would just about fill up a can made for pineapple chunks.)

Then there’s ammonia, proteins, sugars, potassium and bicarbonate. Not to mention trace metals like zinc, copper, iron, nickel, cadmium, lead, and even a tiny bit of manganese. For some of those metals, sweat is an important mechanism for excreting them from inside of the body.

Not all of the things that leak out in our sweat are chemical in nature

Sweat exits the body through one of two types of glands. Apocrine glands are found in the armpits and nostrils and on the nipples, ears and parts of the genitalia. Much more common, however, are eccrine glands, millions of which are distributed over most of the rest of the human body – everywhere except the lips and the genitals. When the body and skin get too warm, thermoreceptors send a message indicating as much to the brain. There, the hypothalamus – a small cluster of cells that controls our hunger, thirst, sleep, and body temperature – sends a message to the apocrine and eccrine glands, which begin pumping out sweat.

There is also a third type of sweat gland, first discovered in 1987. It’s only been found in the same places that apocrine glands show up, but because researchers couldn't classify them as apocrine or eccrine, they became known as apoeccrine glands. Some think that they are eccrine glands that become somehow modified during puberty.

Tool for communication

Not all of the things that leak out in our sweat are chemical in nature. Everybody has, at some point or other, started to sweat because they ate something spicy, and most people are familiar with emotional sweating due to fear, shame, anxiety, or pain. It’s no wonder that it’s the palms, forehead, and foot soles that are so commonly associated with emotional sweating: eccrine sweat glands there are clustered far more densely, up to 700 per square centimetre, than they are on, say, your back, where there are just 64 per square centimetre.

It turns out that emotion-induced sweating is an important tool for communication. In fact, the scents that we detect in sweat can tell us a lot about how others are feeling.

In one experiment, a quintet of Utrecht University psychologists collected sweat samples from 10 men as they watched videos designed to evoke feelings of fear (excerpts from The Shining) or disgust (excerpts from MTV’s Jackass). In order to avoid odour contamination, the volunteers agreed to forego smelly foods, alcohol, smoking, and “excessive exercise” for two days prior to their sweat donation session.

Then, 36 women were asked to see whether they could detect any emotional cues hidden in the sweat samples. The researchers found that when women were exposed to fear-derived sweat samples, their own facial expressions suggested fear as well. And when they were exposed to disgust-based sweat samples, their faces mirrored that emotion too. (Sweat collection pads that remained unused served as controls; these didn't cause the participants to show any predictable sort of facial expression.)

People who sniffed the sweat of scared skydivers became aroused in response to angry faces

That suggested to the researchers that sweat appears to be an effective means of transmitting an emotional state from one person to another. Importantly, the facial expressions the women made while sniffing the sweat were completely independent of their subjective perceptions of the odours’ pleasantness or intensity. So they might show a look of disgust even if they reported a particular sweat sample as smelling pleasant.

Similar patterns have also been seen in other experiments. In 2006, Rice University psychologists discovered that women exposed to sweat samples collected from fearful donors (this time the sweat came from both men and women) performed better on a word association task than women exposed to sweat produced by people watching neutral videos, or by sweat pads that contained no sweat at all. The fear-related cues gave them a heightened awareness of their environment.

And in 2012, psychologists and psychiatrists from the State University of New York extracted sweat from the t-shirts of 64 donors. Half of the donors jumped out of an aeroplane for the first time, while the other half exercised really hard. People who sniffed the sweat of scared skydivers became aroused in response to angry faces, but also to neutral and ambiguous ones. Psychologists refer to it as vigilance; the freefall-invoked sweat induced participants to pay attention to whatever possible subtle social cues that they might otherwise have overlooked. Those who sniffed the sweat of exhausted exercisers only became more alert when viewing angry faces, as would be expected under any circumstance.

Yet another experiment conducted by German psychologists and neuroscientists found that sweat from anxious men (who participated in a high ropes course) caused women to make riskier decisions – after spending more time deliberating on their choices – in a computer game designed to assess risk-taking behaviours.

Our ancestors took advantage of the olfactory data constantly flowing into their noses

None of these studies indicate whether people are aware that other people’s sweat has altered their own cognition or behaviour, but they do suggest that sweat might, in some cases at least, communicate important information about our internal mental states. They also suggest that we use the information contained in other people’s sweat to better understand our surroundings.

Perhaps that’s not surprising. Our species may be adapted to verbal and linguistic communication, but language is a fairly new item in our social toolkit. It seems reasonable to imagine that our ancestors took advantage of the olfactory data constantly flowing into their noses – and that they passed the skill down to us.

Indeed, people seem better able to identify emotions in virtual humans on a computer screen when the animated characters visibly perspire. And not only that, but the addition of sweat seems to allow people to perceive the intensity of a displayed emotion. Sweat, in other words, isn’t just a smelly signal, but a visual one too.

Sweat, in the end, is more than just the body’s air conditioning system. It just might be an emotional weather vane as well, a tool used for broadcasting our innermost feelings to our friends and family.

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