In our quest to look and smell good, we apply all kinds of chemicals to our bodies every day without giving it a second thought. Some of those products, like antiperspirant and deodorant, are specifically designed to kill bacteria. But what kind of effect do they have on your bacterial ecosystem as a whole? It depends on the product, scientists say in a new paper published in PeerJ.

By now, you probably know that our bodies are full of—and covered with—bacteria and other microorganisms. The sum total of all of these microbes is known as the microbiome. The microbiome is really hot right now; it seems like every week we’re learning more about what’s in there, what it’s doing, and how it can change. Recent studies have shown that health of your microbiome is related to all kinds of surprising things, from socializing to obesity. But there’s still a lot we don’t know. 

To find out how hygiene habits can affect the armpit microbiome, a team of researchers recruited 17 participants and divided them into three groups: people who use deodorant, people who use antiperspirant (not the same thing), and people who used neither. The volunteers were then given an eight-day schedule. On the first day, participants would use whatever product they usually used. From days two through six, they wouldn’t use anything at all. On days seven and eight, everybody used antiperspirant. And twice a day, the researchers swabbed the subjects’ armpits to collect bacteria.

The scientists were right to divide the antiperspirant and deodorant users into two camps. Analysis of the study participants’ underarm swabs revealed some pretty big differences in the products’ microbial consequences. 

"We found that, on the first day, people using antiperspirant had fewer microbes in their samples than people who didn't use product at all—but there was a lot of variability, making it hard to draw firm conclusions," author Julie Horvath said in a press statement. "In addition, people who used deodorant actually often had more microbes—on average—than those who didn't use product."

The effects of antiperspirant and deodorant use or non-use were surprisingly speedy. By the third day of the study, day two without product, the armpit ecosystems of regular antiperspirant users had already begun to rebound. By the fifth day without deodorant or antiperspirant, the participants’ “armpit communities” (as the researchers called them) were all equally lively.

But things took another quick turn when all the study participants wore antiperspirant: “… we found very few microbes on any of the participants,” Horvath said in the press statement, “verifying that these products dramatically reduce microbial growth." 

This doesn't necessarily mean that you should chuck your deodorant or stock up on antiperspirant. Having more bacteria is not intrinsically good or bad. It depends on what those bacteria are, and their role in the overall ecosystem. 

The makeup of the study subjects’ microbiomes varied depending on their choice of armpit products. The researchers found not only different amounts of bacteria on the skin of each group, but also different species. 

"Using antiperspirant and deodorant completely rearranges the microbial ecosystem of your skin—what's living on us and in what amounts," Horvath continued in the press release. "And we have no idea what effect, if any, that has on our skin and on our health. Is it beneficial? Is it detrimental? We really don't know at this point. Those are questions that we're potentially interested in exploring."