“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 leftover – 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% 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 skins 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.
Jake Port contributes to the Cosmos explainer series.
Read science facts, not fiction...
There’s never been a more important time to explain the facts, cherish evidence-based knowledge and to showcase the latest scientific, technological and engineering breakthroughs. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science. Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today.