Sweat smells bad, stains clothes and embarrasses us in public. But it’s vital to health, keeping our bodies cool during exercise, on a sunny day or when we eat certain foods such as chilli. We also produce sweat when we feel under pressure, as part of our stress response. Here, the experts reveal the latest understanding about sweat’s role in health — and how to keep it under control. We have nearly four million sweat glands in our skin which produce up to 25ml of sweat an hour to regulate our body temperature — this can rise to two to four litres an hour during exercise, says George Havenith, a professor of environmental physiology and ergonomics at Loughborough University. Almost two million people in the UK suffer from excessive sweating or hyperhidrosis. “They can’t function normally: they have to change their shirt every hour or can’t shake hands with people,” says Dr Anton Alexandroff, a consultant dermatologist at the University Hospitals of Leicester NHS Trust. We’re losing moisture through our skin all the time, but don’t notice because the air makes most of the liquid evaporate quickly, says Professor Havenith. “People who say they don’t sweat are wrong,” he adds. If we didn’t sweat we would collapse and die half an hour into a run as our body temperature would rise too much. “The difference in sweat levels lies in how sweat is distributed between glands: some people feel it extensively as it all comes out of one area, for example the forehead. In people who say they don’t sweat, the perspiration is just more evenly distributed across glands, so doesn’t build up a layer of liquid that they notice,” he explains. We produce two types of sweat — the watery sweat that cools us down, and an oily liquid that may be linked to sexual attraction. Cooling sweat is produced by the eccrine glands, found just under the skin all over the body. When our body heats up, the hypothalamus, the temperature centre of the brain, instructs these glands to produce sweat: this evaporates on the skin, taking heat from our bodies. We produce seven times more watery sweat than the oily kind because of its role in protecting us from overheating, says Dr Justine Hextall, a consultant dermatologist at Western Sussex Hospitals. This sweat is produced by filtering fluid in the eccrine glands. “Salts are extracted back into the blood if they are needed and the remaining salty liquid passes out as a fluid onto the skin,” explains Professor Havenith. The second type of sweat is produced by the apocrine glands in the armpits, genitals and nipples. They produce an oily liquid full of fat and protein — animal studies suggest this sweat contributes to sexual attraction. Though it’s odourless when released, once this sweat is on the skin it reacts with bacteria such as Staphylococcus hominis, producing malodorous by-products. “A hairy armpit has a big surface for debris and bacteria to adhere to so tends to be more smelly,” says Professor Havenith. People from east Asia typically don’t have body odour because they have a gene that means they don’t produce certain proteins that would be converted by bacteria into odours, he adds. This type of sweat can be triggered by hormones such as cortisol, and is released at times of stress or extreme emotion. It’s part of our fight or flight response. “A small amount of sweat on our hands and feet improves the friction in our skin and helps us grip,” says Professor Havenith. “Teenagers’ sweat tends to smell because their fluctuating sex hormones stimulate the apocrine glands to release oily sweat,” says Dr Hextall. “Because of the habits teens tend to have — such as not washing — odour-causing bacteria builds up.” Some sweat glands are more active than others. One theory is that heat exposure before the age of four determines how well your glands cool you down, says Professor Havenith. “That’s why people who grow up in the tropics sweat more than those who holiday there, because more glands are active and these activated glands are also more efficient at cooling the body down, producing more sweat.” Having a tattoo might also make you sweat less, as the inking damages sweat glands, according to a study published earlier this year in the journal Medicine & Science in Sports & Exercise. To reduce excessive sweating on holiday, prepare your body weeks before as athletes do, says Professor Havenith. Before an event in a hot climate, many train their sweat glands to work efficiently in high temperatures by working out in hot rooms.