If you've been feeling a bit sweatier than usual lately, it's no wonder — Australia just copped its hottest month on record.
But perspiration does more than keep us cool. The watery stuff that oozes out of our skin can give us the lowdown on what's going on inside our body.
Most notably, sweat is used to test for cystic fibrosis, a genetic condition that thickens mucous in the lungs and digestive system.
The cystic fibrosis sweat test turns 60 next month. And the concept behind it has remained largely unchanged since.
So how can sweat diagnose cystic fibrosis — and what else can it be used for?
First, we need to know what sweat is and what's in it.
Sweat comes from sweat glands. They comprise a bunch of coiled tubes, called the bulb, which is where sweat is made, and a duct that carries the liquid to the skin's surface.
There are two types of sweat gland: apocrine and eccrine.
Apocrine sweat glands are concentrated in our armpits, groin and genitals — anywhere that's particularly hairy.
The sweat that seeps from apocrine ducts is odourless and oily, but eventually takes on a bit of a stink thanks to the microbes that live on our skin, which munch on proteins and fats in apocrine sweat and excrete smelly compounds.
Eccrine sweat glands, on the other hand, are all over the body. Their main role is to regulate temperature by leaking watery, slightly salty solution onto the skin, which cools us as it evaporates.
In hot weather or during exercise, we can secrete more than a litre of sweat from our eccrine glands every day.
Salty sweat in cystic fibrosis
We make what's called "primary sweat" in the bulb of eccrine sweat glands, said John Massie, a paediatric respiratory physician at Melbourne's Royal Children's Hospital.
At this point, sweat is somewhat salty, with sodium and chloride concentrations similar to those found in blood.
"In healthy people, as the sweat travels from the bulb to the surface of the skin, most of that sodium and chloride is reabsorbed by cells that line the duct," Dr Massie said.
But people with cystic fibrosis have mutations in the gene that makes the main chloride-removing protein.
Usually, when chloride is pulled out of sweat in the duct, sodium follows, said Ronda Greaves, a paediatric clinical biochemist at the Murdoch Children's Research Institute in Melbourne.
"Sodium's positive and chloride is negative — sodium chloride is table salt — so when you have high chloride, you'll often have high sodium," Professor Greaves said.
So in the case of a cystic fibrosis patient, what oozes from their sweat ducts out isn't quite as salty as primary sweat — because other proteins can remove some chloride — but it's much saltier than usual, a difference you can literally taste.
"Back in the Middle Ages, there was an old German proverb which said, 'beware the child who tastes salty when kissed, for they will soon die'," Dr Massie said.
"Very few of our patients' parents, before the diagnosis of cystic fibrosis is made, detect the salty taste of their kids, but afterwards, they often will say this."
In March 1959, Lewis Gibson and Robert Cooke, paediatricians at Johns Hopkins University in the US, unveiled their cystic fibrosis test.
It calculated sweat saltiness using sodium levels as a proxy for chloride, because sodium was easier to measure.
As the decades passed, technology improved and made detecting chloride levels easier, but the general procedure is pretty much the same: collect sweat and calculate its chloride concentration.
Today, cystic fibrosis is initially diagnosed by measuring levels of a peptide in blood, then genetic testing.
Around 3,400 people in Australia have cystic fibrosis.
But the sweat test is still used to confirm how well a patient's chloride proteins function. Some people may have mutations in genes which gives them a milder form of the condition, Professor Greaves said.
Beyond salt in sweat
Molecules the body makes in response to inflammation can make their way into eccrine sweat, said Nicolas Voelcker, a nanotechnologist who develops biosensors at Monash University.
"Inflammatory markers could tell you if someone has an autoimmune disease or an infectious disease," he said.
Lactate and glucose, too, could be incredibly valuable to monitor metabolic diseases, pre-diabetes and diabetes.
"There's a relationship between the concentration of glucose in sweat and the blood — it's not the same, but if blood glucose goes up, so does sweat glucose," Professor Voelcker said.
Minuscule amounts of drugs and the compounds they break down into, called metabolites, could also be detected in sweat.
Illicit drug sensors have the potential to be used in, for instance, roadside drug-testing.
It wouldn't be as convenient as a handheld breath test, but a sample could be analysed in a couple of seconds.
"You could detect multiple drugs at a time," Professor Voelcker added.
Monitoring legal drugs, too, could ensure a patient is sticking to his or her treatment plan.
How the population of skin microbes — called the skin microbiome — interacts with sweat might also be harnessed. Bacteria eat fats we sweat out, but they also make their own.
With the microbiome implicated in diseases, including neurodegenerative diseases such as Parkinson's disease, a person's "lipid fingerprint" might tell a clinician about their overall health.
Not there yet, but getting closer
There are clear benefits to using sweat as a clinical diagnostic tool — it's not as invasive as, say, taking a blood prick from your fingertip.
The problem is understanding how sweat compounds correspond with different diseases.
"With cystic fibrosis, we understand that very well," Professor Greaves said. "Others, not so much."
There's also the issue of collecting enough sweat to analyse. Athletes monitoring their sweat for salt might be able to churn out enough, but not your average person sitting in a doctor's clinic.
The original cystic fibrosis sweat test used a mild electric current combined with a topical medication called pilocarpine to stimulate sweat glands on a person's forearm.
The other issue is the issue that some people sweat a lot, others don't sweat much, and people smear cosmetics on their skin, which could affect what's detected.
"There is still a lot that needs to be known about how that influences molecules in sweat," Professor Voelcker said.