In the last century, deodorant and antiperspirant have made humanity smell a whole lot better, likely improving our relationships with the people surrounding us along the way.

But there’s another relationship these practices may be impacting: the one we have with our microbiome, or the community of tiny organisms such as bacteria, viruses, and fungi, that live in and on us. Research published in the journal PeerJ today (Feb. 2) shows that the more we use deodorants and antiperspirants, the more we alter the microbial community that lives underneath our arms, and not necessarily for the better. As we eliminate some kinds of bacteria that make us smell less than desirable, we may be making room for others instead.

“When you have all these microbes on your skin, most of them are potentially beneficial, or at least benign,” Julie Horvath, an evolutionary geneticist at the North Carolina Central University and co-author of the study, told Quartz. “They don’t do anything, except for maybe create a protective barrier on your skin.” She explained that by taking up residence on our bodies, benign microbes take up resources—namely, the sweat and oils we produce—and limit the ability of pathogenic microbes to survive.

Benevolent bacteria known as Corynebacteria are typically responsible for body odor. When we use antiperspirant or deodorant, we’re eliminating those smells by either preventing our glands from producing sweat, which starves off these bacteria, or killing them directly with other chemicals. But, these products don’t discriminate as they kill off microbes. Horvath and her colleagues wanted to find out how underarm bacteria would be altered by these products, and whether the populations that regrew would be different after the use of these products was discontinued.

The research team sampled 17 men and women who used either deodorant, antiperspirant, or neither on a regular basis for eight days. On the first day of the study, they took samples from participants’ underarms to get a baseline of their typical microbial community. They instructed participants not to use any products for the next five days to sample which kinds of microbes would grow back. On the eighth day, all participants were instructed to use antiperspirant before the researchers collected the final sample.

Over the course of the study, Horvath and her colleagues discovered that different populations of bacteria lived on people with different product habits, including varying amounts of bacteria which the researchers were unable to identify.

On average, people who didn’t use any underarm products at all tended to have larger populations of Corynebacteria bacteria and less Staphylococcaceae bacteria than the other two groups, while less than 10% of their sampled bacteria were unidentifiable species. Deodorant users had a higher number of underarm bacteria on the first day, and over the course of the week tended to harbor more Staphylococcaceae than the other two groups, some Corynebacteria, and only 5% of the bacteria the researchers couldn’t identify in this study. The individuals who regularly used antiperspirant also cultivated almost as much Staphylococcaceae bacteria as the deodorant group, and more than 20% of their microbes were random and unidentifiable.

At the end of the week when participants applied antiperspirant, most of the microbial communities were shown to die off.

Horvath says it’s hard to make a judgment call about what may be harmful or helpful microbial activity based on this one small experiment. The unidentifiable species that tend to regrow in people who wear antiperspirant regularly may not be anything to worry about. Additionally, although some species of Staphylococcaceae are known to be harmful, most are not. The only thing researchers can say definitively is that they grow back the fastest after users who typically wear products under their arms stopped.

What’s important, says Horvath, is to know that our hygiene habits are having an impact on these communities.”It’s a balance,” she says. ”Wearing a product does affect the microbes under your arm, but what those short and long-term consequences are, we don’t really know yet.”