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The science of stinky sweat and earwax

Which bacteria give you body odour? Why do some people sweat more than others? And what's that got to do with earwax? Dr Karl has the answers.

I love wearing bright shirts (after all, why should women have all the fun?). But early on, I learnt an embarrassing lesson — polyester clothes stink after sweating, no matter how colourful the print.

It turns out that fresh sweat has little odour. (The long-chain fatty acids in sweat are too large to trigger your olfactory epithelium - the tricky bit in your nose that does the smelling thing.)

A bacterium called Corynebacterium causes stinky armpit on your skin. But while Cornynebacterium is abundant in the armpit, it won't grow well on textiles.

Then there's Staphylococci bacteria that live happily on both armpit skin and textiles. But they're not very stinky — they tend to generate only a 'normal' non-stinky body odour.

Bacteria called Micrococci are the 'real' stink-masters. They can break down the long-chain fatty acids, the hormones and the amino acids that are naturally present in sweat. In doing that, they create smaller and more volatile chemicals - which generate the typical stinky body odour.

Micrococci grow really well on polyester (dunno why, yet). They don't grow well on cotton (also dunno why, yet).

So what are the solutions to smelly armpits?

First, wear cotton - and put up with sometimes wearing colours and patterns that are less vibrant.

Second, avoid polyester. Or, wear it if you love it (nice colours, patterns, etc), but add deodorant. And no, deodorants do not cause cancer. But deodorants can encourage Corynebacteria to grow more in your armpits (leading to smelly armpit skin).

And finally, Dr Chris Callewaert of Ghent University in Belgium is exploring a different solution. With the best of intentions, he wants to transplant bacteria from non-stinky people to their stinky relatives — hopefully to crowd out the Micrococci.

So remember this rhyme: polyester pongs stronger, and you can wear cotton longer.

Now there's a strange link between sweat and the colour of your earwax.

Some people (such as Europeans and Africans) sweat a reasonable amount, and they tend to have wet earwax. And some people (such as Koreans and Chinese) don't sweat much at all, and they tend to have dry earwax.

Earwax is manufactured inside your ear canal. It's made from dead skin cells, as well as the secretions from two types of glands - sebaceous glands and sweat glands. These glands are mostly in the outer one third of your ear canal. When you analyse earwax, you will find triglycerides, free fatty acids, cholesterol, a whole bunch of other fats and waxes, amino acids and minerals, and dry flaky skin. Earwax both cleans and protects the ear canal.

But ear doctors have long known that there are two quite different types of earwax — wet and dry.

Wet earwax is sticky, and light-to-golden brown in colour — although it can darken over time. Dry earwax appears "ashen and flaky", is brittle, and ranges in colour between brownish grey and light grey.

The genetics behind wet versus dry earwax was discovered in 2006. Dry earwax is very common (80-95 per cent) among East Asians, but is lower (30-50 per cent) in southern Asia, Pacific Islands, Central Asia and Asia Minor, and in Native North Americans and Inuit peoples of Asian ancestry. It's very uncommon among Europeans and Africans (0-3 per cent). They overwhelmingly have wet earwax.

The type of earwax you have (wet or dry) is caused by a single variation in your DNA, on a very specific location on your 16th chromosome. This variation in the DNA turns out to have effects on the liver, pancreas, kidneys, placenta, breast tissue, gut, glands in your ear canal, and wait for it, sweat glands in your skin — and that includes your armpits.

And yes, the genetics tells us that humans with wet earwax will sweat more from their armpits.

Now the annual US market for deodorants and anti-perspirants is worth about $2 billion, while the UK market runs to over half-a-billion pounds sterling. In a study of over 18,000 people in the UK, about 93 per cent of women and 83 per cent of men used an armpit deodorant nearly every day.

And what about that microscopic minority (about 2 per cent) of Britons who have dry earwax — and so do not sweat much from the armpits? They surely don't need to use a deodorant. And yet, according to a study in 2013, about three-quarters of them do use a deodorant daily. This shows the power of social conditioning and the effect of relentless marketing.

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