The science of the human underarm

A team of researchers from North Carolina Central University are studying how products like deodorants and antiperspirants change the makeup and diversity of the bacteria in the human underarm.


Your armpits carry millions of tiny microbial organisms -- living bacteria that grow and interact to produce particular odors.

A team of researchers from North Carolina Central University is studying how products like deodorants and antiperspirants change the makeup and diversity of the bacteria in the human underarm.

Let's take a look.

This is a story about pits.

No, not the barbecue kind.

There are no construction pits, either.

And it's not even about the pits in fruit.

[ Pop music playing ] Yeah, we're talking those pits.

It is gross to think about, because when it smells, it's really bad.

I don't think about it, but I know the ladies do.

So I got to stay fresh for them.

That's right -- armpits.

Because the reality is, cold day or a hot day, your armpit is a warm, damp place.

And if you're a bacteria, that's a happy place.

Actually, when talking about armpits, it's not the person that has the odor.

It's the bacteria living on the person that produces the odor.

The bacteria metabolize the sweat and, well, give off the smell.

Think about it.

North Carolina summertimes, you know, you're hot, and everything's going on, so it wouldn't surprise me at all.

However, wearing antiperspirants and deodorants doesn't just affect your social life.

It significantly changes the type and quantity of microbial life that lives on you.

More specifically, the chemicals alter the microbiome of the human armpit.

So, some of our research studies started looking at how does your use of antiperspirant or deodorant or maybe no product, how does that influence which microbes live on your skin?

But to understand what's living in your armpits, you first need a refresher of what underarm-hygiene products do.

Take deodorants.

Deodorants contain things that typically kill off microorganisms.

So, things like ethanol or other antimicrobial agents to kill the microbes.

And sometimes they also contain fragrance or other products that have a good odor.

And antiperspirants.

The antiperspirants contain aluminum-based salts.

And the aluminum-based salts will get into your apocrine glands, these sweat glands that produce sweat, and they will block those up so that you're not producing as much sweat.

So, if you don't produce sweat, there's no food for the microbes, so then there are no microbes there.


And we ask them to sample their armpit.

And so they either wore a sleeveless shirt or tucked away to have some privacy, and they [Mimics swabbing sound] you know, sample their armpit.

Researchers took daily armpit swabs from 17 men and women over the course of eight days.

Basically, we have their bacterial cells on here.

Some participants used deodorants, some used antiperspirants, some used no product at all.

The study's subjects then followed their normal hygiene routine on Day 1, quit using all products on Days 2 through 6, then only used antiperspirants on Day 7 and 8.

Bacteria, you know, we can't see them on the swab, we can't see them on your skin because they're individual cells.

Researchers then cultured all of the samples to determine the microbes growing on each participant, and how they changed each day.

While the number of microbes varied widely in the first days of the study, by Day 6, the amount of bacteria for all participants was about the same.

But what we do here is, by giving them this artificial food source and letting them grow, we're letting each particular species, all those cells, multiply to be big enough -- we call them colonies -- that we can see them and observe who's there.

And researchers found dramatic changes in microbial organisms between Day 6, participants' last day of not using any product, and Day 7 and 8, when subjects began using antiperspirants.

Without products, microbes thrived in the armpit microbiome.

Once you stop wearing product, you sort of have this clean slate now, and the microbes that grow back the fastest are the types of staph bacteria.

But once products were applied, there were very few microbes found in the samples.

It was proof how products dramatically change the armpit microbiome environment.

You can just imagine this is like, you know, just had a field, and a fire just went through and wiped out all life there, right.

So, when you wear deodorant or antiperspirant, you're probably wiping out most of the life on your skin.

And so then what grows back first?

Well, it's the weeds in the field, but it's the staph on humans.

So, that's some of what we're looking at.

Although we do see some of the Corynebacteria growing back.

Those are slower growers, typically, but some of those do come back, as well.

Researchers also did genetic sequencing on the samples to identify the specific bacteria and determine how various products affect microbial diversity over time.

Each sequence has been generated from a different sample.

The samples turned up a greater diversity of microbes in the armpits of people who wore antiperspirants.

That might surprise you, because the products are designed to block sweat glands.


You're introducing, you know, a very different environment, right.

You're selecting, for, potentially, different bacteria to grow.

Both tests confirm that using antiperspirants and deodorants completely rearrange the microbial environment on your skin, including what's living on us and in what amounts.

It might smell a little funky the next day, or other days you might be like, 'Huh, I don't smell too bad.'

Until now, nobody knew what microbes were there, or how daily habits changed it.

And a lot of people don't think about it.

And I think our research isn't really telling us all the answers as to whether this is good or bad, but it's getting people to realize, 'I never thought about what this might do to my skin and the microbes on my skin.'