Hot and humid, the armpit populated by bacteria cursed with creating a noxious odor. That smell, however, has proved lucrative. Today more than 90 percent of Americans use some sort of armpit cosmetic, creating a worldwide deodorant bonanza worth $18 billion.

But what if part of that industry is predicated on a notion that smells fishy?

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New research published in the Archives of Dermatological Research suggests antiperspirants actually increase the levels of the odorous bacteria populating the armpit, which “could lead toward an altered, more unpleasant, underarm odor,” lead author Chris Callewaert of Belgium’s Ghent University told The Washington Post in an e-mail. “Deodorants were generally not really a problem. … The deodorant-antiperspirant industry should investigate what their products do to the underarm microbiome,” or community of microscopic organisms living on our skin. Antiperspirants “should not enhance the odor-causing bacteria, but rather ‘steer’ towards a non odor-causing microbiome,” Callewaert said.

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Observers, however, have already highlighted one problem with Callewaert’s research: the small sample size. In all, only nine people — eight of whom stopped using deodorant or antiperspirant for one month — were tested. “The sample size is rather small,” Callewaert conceded in an interview with Real Clear Science. “However, we see consistent outcomes.”

Even more consistent is Callewaert’s unmitigated passion for the armpit and its many mysteries. The PhD student did a Ted Talk on smelly armpits, co-authored several studies on armpits and directs the uninitiated to his Web site, “Dr. Armpit.” At DrArmpit.com, there’s an image of a youthful, wavy-haired researcher — presumably Dr. Armpit himself — swabbing the right armpit of a man who looks like he just wants to go home.

Dr. Armpit preaches the gospel of underarm science at nearly every chance. “The major determining parts that determine your body odor are the specific bacterial species in your armpits,” Callewaert said in his Ted talk. “A lot of bacteria reside there. In fact, there are more bacteria in your armpits than there are humans on this planet. So you should never feel alone.”

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But not all bacteria are created equal. As Callewaert puts it, armpits have two types of bacteria. The good kind, known as staphylococci, don’t make much of an odor. The bad kind, known as corynebacterium, do, transforming lipids and amino acids in our sweat “into compounds which have a particular pungent odor,” Callewaert wrote in his e-mail.

This smell can be masked with deodorant. Or the sweat can be stopped with antiperspirants that contain an aluminum-based compound, which temporarily plugs sweat ducts. But what effect, if any, do such techniques have on the bacteria itself?

Callewaert and a team of three other researchers were determined to find out. They tested nine people in 28 days and collected armpit samples before, during and after the trial period. They found that the diversity of bacteria in the armpit increased but that their abundance decreased. “The armpit microbiome changed considerably when starting or stopping the use of deodorants and antiperspirants,” Callewaert said. “In the case of the antiperspirants, the dominance changed toward the ‘smelly’ actinobacteria. The aluminum salts [of antiperspirants] have a much stronger effect on the ‘good’ than on the ‘bad’ bacteria.”

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The study says the “long-term use” of antiperspirants “can lead toward altered odor production of the armpit.”

The findings surprised scientists.”I had an idea that diversity [of bacteria] would increase,” Callewaert said. “But I did not expect the changes would be that big. Also, I did not expect a change toward actinobacteria” — the smelly stuff — “when using antiperspirants.”

Still, it’s not time for sweeping conclusions yet — and it’s unlikely the study will bring about the collapse of the antiperspirant trade. Dr. Armpit wants to conduct more studies.