Applying deodorant or antiperspirant clearly alters your body’s smell, but it may also alter your body's bacteria. A PeerJ study finds the organisms that live in and on your skin are drastically changed by what you put under your arms. Evolutionary genomicist Julie Horvath recruited 17 participants—antiperspirant users, deodorant users, and those who went au naturel—and swabbed their armpits twice a day for eight days, report Quartz and the Washington Post. On the first day, researchers found deodorant users had more microbes than those who didn’t use any products. Participants who used antiperspirant, which blocks sweat glands, had hardly any. Most surprising, however, was that the armpits of the three groups remained strikingly different after all participants went product-free for five days.
The control group’s armpits contained 62% Corynebacteria (body odor bacteria), 21% Staphylococcaceae bacteria, and 17% "other" bacteria. The armpits of antiperspirant users contained 60% Staphylococcaceae, 14% Corynebacteria, and 20% other bacteria, while deodorant users armpits held 61% Staphylococcaceae, 29% Corynebacterium, and 10% other microbes. "Using antiperspirant and deodorant completely rearranges the microbial ecosystem of your skin—what's living on us and in what amounts," Horvath tells Medical Daily, adding "we have no idea what effect, if any, that has on our skin and on our health." More work is needed to understand the differences, but experts speculate that antiperspirant kills off typical armpit microbes, allowing less common bacteria to propagate. Horvath now plans to compare the results to a study of microbes in earwax. (Here's why pits are so stinky.)