The routine of slicking on antiperspirant or deodorant may be good for your social life, but bad for your microbial life. According to new research published in the journal PeerJ, the type and quantity of the thousands of bacteria species that live on your skin— in particular the armpit— is determined by the use of deodorant and/or antiperspirant, raising questions about the benefit or harm of the products.

Researchers from  North Carolina State University, the North Carolina Museum of Natural Sciences, North Carolina Central University, Rutgers University and Duke University collaborated on the work, which involved 17 study participants. Three men and four women used antiperspirant products, three men and two women used deodorant, and three men and two women used neither product.

For eight days, participants had swabs taken of their armpits. On the first day, participants followed their normal hygiene routine. On days two through six, they did not use any deodorant or antiperspirant. For the last two days, all participants used antiperspirant.

With the first day’s samples, researchers observed that antiperspirant users had fewer microbes and that deodorant users had more microbes, compared to those who did not use any product. As the study period progressed, the amount of bacteria in all participants was comparable.

"However, once all participants began using antiperspirant on days seven and eight, we found very few microbes on any of the participants, verifying that these products dramatically reduce microbial growth," study author Julie Horvath, head of the genomics and microbiology research laboratory at the NC Museum of Natural Sciences, an associate research professor at NC Central, said in a news release. 

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Researchers also studied the composition and variety of types of bacteria. Those who did not use any underarm product had cultures that contained 62 percent of the bacteria partially responsible for body odor smells— which are also thought to defend against pathogens and 21 percent Staphylococcaceae bacteria. Regular antiperspirant users’ microbes were 60 percent Staphylococcaceae bacteria, the most common microbes found on human skin, most of which are considered beneficial and 14 percent odor-causing bacteria.

"Using antiperspirant and deodorant completely rearranges the microbial ecosystem of your skin - what's living on us and in what amounts," Horvath said in the news 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."

The study follows another paper that observed that armpit microbes in primates—which are vastly different than those of humans— have evolved over time in conjunction with the primates they live on.

"Over evolutionary time, we would expect our microbes to co-evolve with us," Horvath said in the news release. "But we appear to have altered that process considerably through our habits, from bathing to taking steps to change the way we look or smell."