Human sweat – 99 percent water with a dash of salts and a pH of around 4.5, is a much-maligned bodily fluid that plays a vital role in keeping us humans healthy and alive.
The word's become shorthand for hard work and discomfort but without it human life just wouldn't be the same: if our prehistoric ancestors hadn't been able to sweat they could never have stayed cool for long enough to chase down their prey on the ancient Savannah.
What happens when we sweat?
People sweat for all sorts of reasons; emotional sweating when we're stressed, scared or in pain, or the sweat we get on our forehead when we eat spicy foods. There's the night sweats that people suffer during menopause, and the sweating people experience when they are withdrawing from drugs.
But the most common encounter we have with sweat is for thermo-regulation, for cooling us down when we get too hot running for the bus, in humid weather, or when we exercise. When we heat up, the hypothalamus in our brain detects that our body and skin temperature is rising and using a neurotransmitter called acetylcholine it sends a signal to stimulate millions of eccrine sweat glands to release a salty liquid- sweat- via ducts onto our skin to evaporate and cool the body down.
These eccrine glands are spread out over most of your body but you have higher concentrations on the palms of your hands, the soles of your feet, your underarms and your forehead. The sweat from these eccrine glands is mostly water and salt and doesn't tend to smell.
There's another type of sweat gland called the apocrine gland that can be more problematic. These glands are concentrated in the underarm and groin and from puberty onwards secrete a more oily sweat full of proteins and lipids when you get hot or stressed out and anxious. Although apocrine sweat is sterile when it hits the skin's surface, the bacteria living on you love to eat this gooey goodness, and then excrete the volatile compounds that we sniff as body odour.
The role of our microbes in making us smell
"Hot and humid, the armpit populated by bacteria cursed with creating a noxious odor. That smell, however, has proved lucrative. Today more than 90 percent of Americans use some sort of armpit cosmetic, creating a worldwide deodorant bonanza worth $18 billion." Terrence McCoy in The Washington Post
It's tough to talk about sweat for any length of time without acknowledging the way it smells (thanks apocrine sweat glands!) and the challenges brought about by body odour. And our embarrassment and sensitivity around the subject comes with a hefty price tag attached. Deodorants and antiperspirants, the first line of defence for most of us, form an 'armpit cosmetics' market worth an estimated NZ$25 billion annually.
But the truth is that nobody's exactly sure why one person might smell worse than another. The only thing that is clear is that it's a whole lot more complex than the simple question of whether you remembered to use deodorant this morning! And as with just about everything nowadays, there seems to be an important microbial component in all of this. So work is focussing on our microbiome - the bacteria living on us and in us - to explain our problem with body odour, and even to offer solutions for it.
Chris Callewaert of the University of California San Diego calls himself Dr Armpit, he's sniffed thousands of other people's 'pits, and according to Chris there are more bacteria in your armpits than there are humans on this planet. And all of these billions of bacteria are not the same.
"The higher the diversity in the armpit is, the more chance of having body odour." Chris Callewaert (aka Dr Armpit) of the University of California San Diego
Our armpit ecosystem has up to 200 different types of bacteria but two dominant types. According to Chris, it is the relationship and the balance between the two that can determine whether we have smelly armpits or not. In one corner staphylococci, a genus of gram positive bacteria with about 40 different species; these don't make much of an odour. The more problematic ones are called corynebacterium, these are the beasties that feast on the fats and amino acids in our sweat excreting these volatile compounds that have the unmistakable and pungent tang of body odour.
"If you have corynebacterium, so the bad one, he doesn't come alone and he brings along a whole bunch of friends which cause more body odour." Chris Callewaert
Basing his work on the study of a pair of identical male twins who smelt very different and had very different armpit microbiomes. He's now trying out microbial transplants between one person and another. So he swabs and cultures bacteria from a 'pleasant' armpit and then transfers this bacteria to a not so fragrant armpit, so far with promising results.
"We immediately saw an improvement in body odour that lasted for a long time, even up to a year after the transplant we still had the good bacteria so that was the first successful case of an armpit transplant and the basis for further investigation". Chris Callewaert
How fabrics hold onto smells... and release them
Do you have a shirt that seems to smell a little bit worse than everything else in your wardrobe? Perhaps it's an artificial fabric, like polypro or polyester whereas other wool or cotton clothes you wear just never seem to smell as bad.
Another front on the war against sweat - and the odours it can bring - is being fought on our fabrics. A huge market in gym and outdoor wear has sprung up offering us specially designed fabrics that claim to wick sweat away from the skin, lock in smells, or incorporate anti-microbials like silver particles, Triclosan and quaternary ammonium compounds to help neutralise nasty niffs.
At the University of Otago in Dunedin, Tobi Richter is studying the absorption and release of volatile odour compounds in three different fabrics - wool, cotton and polyester - to better understand how smells build up on our clothes and how to reduce them.
He's discovering that cotton tends to absorb few of these compounds in the first place. But although wool does absorb them, it is then far better at holding onto these smells than polyester. He speculates that this can be explained by the fact that wool is a complex structure with more binding mechanisms than man-made fibres.
"For polyester there was an increase in volatile compounds so that explains why, if you leave your sports gear in a bag for a couple of days, it starts to smell quite strongly" Tobi Richter, University of Otago
Can you really sweat like a pig?
Adults can sweat at rates of several litres an hour, and up to 14 litres per day. Can our porcine cousins really beat us in the sweat stakes?
According to Nick Cave, Associate Professor in Small Animal Medicine and Nutrition at Massey University, almost all mammals sweat to cool down. For example, dogs pant to lose body heat via their mouths. Pigs, on the other hand tend to sweat relatively little, and mainly wallow in mud to regulate their body temperature.
Chris Rogers of Massey University has a special interest in equine biomechanics and says that in fact it's horses that are champion sweaters, producing up to 10 litres of sweat in a matter of hours. So a better and more accurate insult would be to tell someone they're sweating like a horse, not a pig.
A sweaty future
In a world where one day we may be swapping armpit bacteria in a bid to stave off smells, and where new synthetic fibres could be designed to mimic wool's ability to hold onto volatile odour compounds rather than release them, sweat could also be a useful tool in monitoring our state of health and even our state of mind.
With rapid improvements in low cost, wearable technology, and the emergence of accurate ways to collect and analyse our sweat in real time, attention is focussing on the field of wearable diagnostics; using the body's chemical signals to tell how we are feeling. Sweat is of particular interest because it is so easy to collect (no needles required!) and because it contains biomarkers or chemical signatures for a whole host of electrolytes, metabolites, proteins, and amino acids.
Already people are using sweat to diagnose the disease cystic fibrosis, and using sensors to measure a surgeon's stress levels to assess their readiness to perform surgery. A team in San Diego has also just created a skin-patch that uses lactic acid and glucose found in your sweat to power your wearables and other digital devices.
Some of the most interesting work is being done at The University of Texas at Dallas where researchers are using a tiny patch to measure biomarkers in sweat that show the fluctuating blood sugar levels of Type 2 diabetes patients in real time.
Professor Shalini Prasad was inspired by the weave patterns of saris in her native India to design sensors that could collect and apply sweat in sufficient volumes to take a reading. She and her team can now measure chemicals associated with alcohol consumption, the stress hormone cortisol, and glucose-related compounds in microscopic amounts of sweat.
"This will tell you whether you are hypoglycaemic, hyperglycaemic or if you are taking insulin medication where exactly in the spectrum you are at that point in a given day." Professor Shalini Prasad
Presenters: Alison Ballance and Simon Morton
Producer: Richard Scott
Executive producer: Tim Watkin
Music: Bevan Smith