First, the basics: Why do armpits smell in the first place? Believe it or not, sweat — which is mostly water — isn't inherently smelly. It's the bacteria it catches on its way out that makes us wrinkle our noses.
Most (natural) smells that we think of as being nasty can be traced to certain species of bacteria. And it just so happens that your armpits play host to an estimated one million bacteria per square centimeter.
"The bacteria in my underarm are more similar to those in your underarm than they are to those on my forearm," Julia Segre of the National Human Genome Research Institute told National Geographic back in 2009. The armpit is so lush and full of unique life compared to the rest of our skin that Segre called it a rain forest in a world of arid desert.
In any case, there's a lot going on up there — and it gets pretty funky. Here's a little more about the chemical compounds you can find in armpit sweat, courtesy of local bacteria, from the site Compound Interest:
Underarm odour actually introduces two organic compounds that don’t contain sulfur into the mix. 3-methyl-2-hexenoic acid is widely considered to be one of the main contributors to the human ‘sweaty’ aroma, which itself has an aroma delightfully described as ‘goat-like’. 3-hydroxy-3-methylhexanoic acid contributes a cumin-like scent, whilst 3-methyl-3-sulfanylhexan-1-ol (along with other sulfur-containing compounds) provides an onion note.
Goat, cumin and onion? Kinda sounds like we're making stew in there.
To keep that stench under control, you've got basically two options: killing the bacteria that are making you stink or stopping the flow of sweat that's giving them their power.
Antiperspirants do the latter, using aluminum chloride compounds to block up sweat ducts. Because these compounds work better on a dry surface, you actually want to use them before you shower, giving them ample time to work before you get wet. So if you shower in the morning, you actually want to put them on at night.
You may have read that aluminum chloride compounds are associated with Alzheimer's or cancer, but most scientific studies suggest otherwise. It's up to you, but most scientists think these compounds are safe.
Since deodorants work by simply killing bacteria, they're often thought to be more safe than antiperspirants. But they might not always be innocuous: The antibacterial chemical triclosan, found in many hand soaps and toothpastes, is often used as the bacteria-fighting agent in deodorant. The FDA has put this chemical on notice, since some research suggests it may be carcinogenic. It's probably safe, according to most studies, but we need more studies to be sure.
If you're nervous about any of the above mentioned ingredients, you don't have to throw down tons of money on "natural" alternatives that play on your fears — just take the opportunity to cook up a little science in your kitchen! There are lots of recipes for homemade deodorant available online, and most of them use baking soda and essential oils as their antibacterial agents. Of course, they won't work as well as their store-bought counterparts — so you might want to wait until the weather is a bit chillier to give it your first go.