"Deodorant" and "antiperspirant" are often used interchangeably. This is for good reason, as they're often offered in the same product. But there are a few key differences between them.
Deodorants are used to suppress the often pungent smell associated with sweaty armpits.
The smell is produced by bacteria that live on the skin and feed on fats and proteins secreted by apocrine glands, a type of sweat gland.
These fats and proteins don't smell.
But as bacteria feed, they produce various by-products including propionic acid and butyric acid. It's these that lead to the unpleasant smell.
To combat these bacteria, deodorants contain two primary chemicals.
The first is ethanol. Just like alcohol-based hand sanitiser, this kills many bacteria, so reducing noxious acids produced.
Backing this up are fragrances that mask what scent is left over - including that of the ethanol.
While deodorants are good for suppressing the smell associated with sweaty armpits, they do not deal with the sweat itself. This is where antiperspirants come into play.
Sweat is around 98 per cent water, with the remaining fraction comprising salts including sodium, potassium and calcium. It's pumped out by another type of sweat gland called eccrine glands.
These long, twisting structures channel water from below to the skin's surface when the body is under stress or is too hot.
Like the fats and proteins that ooze from apocrine glands, sweat itself doesn't smell.
But many choose to suppress the production of sweat, despite body odour having little to do with the liquid itself.
Antiperspirants contain aluminium salts, most commonly aluminium chloride hexahydrate. These can act in a couple of ways to keep sweatiness at bay.
They precipitate and form a gel-like substance that "plugs" the end of eccrine glands for a period.
The salts can also travel down the glands and enter cells lining the duct.
Thanks to osmosis, which is the movement of water to equilibrate salt concentrations, the cells swell with liquid and block the eccrine passage.
Eventually, the aluminium salts are transported out of the cells and they shrink back to normal, opening the duct once again.
It's for this reason that antiperspirants need to be applied daily - and also why they cause yellow stains.
Contrary to popular belief, the yellow stains come from aluminium salts, not the sweat itself.
By combining deodorant and antiperspirant together, as many modern products do, we can fight both body odour and sticky feelings at the same time.
For some of us, these daily grooming rituals can feel like a bit of a grind - is there a better way?
According to recent research, those same bacteria that cause sweaty smells - your skin microbiome - may also hold the key to stopping those odours before they begin.
We each have our own unique composition of skin bacteria, which impact how bad we smell without deodorant or antiperspirant. This makes sense, because it's the bacteria who break down the molecules in sweat to create bad smells.
In general, armpit microbomes dominated by corynebacteria are associated with worse body odour.
Chris Callewaert, a researcher at Ghent University in Belgium, has been working to understand the skin microbiome for several years.
In particular, he's been testing whether we can treat bad body odour by transplanting bacteria from a less smelly person's armpit to a smellier person's armpit.
First, the transplant recipient has to use antibacterial and antibiotic products to kill off as many of their current underarm bacteria as possible and make room for the transplanted bacteria to move in.
Chris Callewaert, a researcher at Ghent University in Belgium, [has been] testing whether we can treat bad body odour by transplanting bacteria from a less smelly person's armpit to a smellier person's armpit.
Then, a sample of sloughed-off skin and bacteria is collected from the donor's armpit and applied to the recipient.
Although the trials have been small so far - about 18 people - Callewaert's website says the improvements in body odour are promising.
However, even if you can get over the ick factor, there are some downsides to the microbiome transplant approach.
For example, there's a risk of transferring microbes that cause disease.
Another strategy could be to create an artificial healthy skin bacterial community that can be grown in the lab and standardised.
Or we could look into using bacteriophages - viruses that kill bacteria - to selectively remove bacterial species that cause the worst odour.
Perhaps one day we'll be able to stop using deodorant and antiperspirant, and apply our ideal non-smelly skin microbiome to our armpits instead.
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