A nine-week study tracking 11 volunteers found that deodorant, moisturizer, and foot powder alter the skin’s chemistry and bacterial ecosystem in ways that persist for weeks after use stops.
Most people think of their skincare routine as a daily ritual with daily consequences. Apply something in the morning, wash it off at night, start over. A study published in BMC Biology in 2019 complicates that picture considerably.
Researchers tracked 11 healthy volunteers over nine weeks, monitoring how four common personal care products, a facial lotion, an arm moisturizer, a foot powder, and a deodorant antiperspirant, affected the chemistry and microbial composition of their skin. What they found was that the effects did not stop when the products did, and that the microbial changes were more significant in some body areas than others.
How the skin microbiome study worked
Participants followed a structured protocol across three phases. During the first three weeks, they stopped using all personal care products except a standard body wash. In weeks four through six, they applied the four designated products to specific body areas daily. In the final three weeks, they returned to their individual routines.
Researchers collected samples from each volunteer’s face, arms, armpits, and feet at multiple points throughout, using mass spectrometry to track chemical compounds and 16S rRNA analysis to measure bacterial and archaeal diversity. The combination gave researchers an unusually detailed view of how the skin’s surface chemistry and microbial ecosystem shifted in response to changing product use.
Skin care ingredients stay on the skin for weeks
One of the more striking findings was how long product ingredients remained detectable on the skin after use stopped. Compounds from deodorants used before the study began were still measurable during the first three weeks of the washout phase, with half-lives ranging from 0.5 to 1.9 weeks depending on the specific ingredient.
The chemistry behind the variation is straightforward. Water-soluble compounds like polyethylene glycol cleared from the skin faster, within roughly half a week. Oil-soluble compounds like polypropylene glycol and fatty acids from lotions took significantly longer. The skin’s outermost layer turns over every three to four weeks, and lipophilic molecules bind to that surface in ways that resist washing. The practical implication is that switching products does not produce an immediate reset. The chemical environment of the skin carries residue from prior routines for weeks afterward.
The skin microbiome responded differently by body part
Stopping product use reduced both chemical and bacterial diversity across all sites, but the armpit and foot showed the most pronounced changes. When participants stopped using deodorant and foot powder, bacterial and archaeal diversity in those areas dropped measurably. When they resumed use during weeks four through six, diversity in both areas increased significantly.
The face and arms told a different story. Arm and facial lotions increased chemical diversity on the skin but had comparatively little effect on bacterial or archaeal populations. That distinction suggests that not all products exert the same influence on the microbial ecosystem, and that the armpit in particular is a highly reactive environment where even short-term product changes produce noticeable microbial shifts.
Hormones and pheromones were also affected
Beyond bacteria, the study found that stopping and resuming product use altered levels of steroid compounds and pheromones in the skin, including androstenone and androsterone. These are biologically active molecules, not inert surface chemicals, and their fluctuation in response to something as routine as deodorant use represents a dimension of skincare science that has received little public attention.
Skin care effects are highly individual
Despite all participants following the same protocol, the responses were highly individualized. The molecular and bacterial profiles of each volunteer remained distinct throughout the study even as their routines were standardized. Female participants showed higher molecular diversity across most body sites at baseline, while bacterial diversity was higher in women for the face and armpits specifically. The sources of those differences likely include prior product histories, diet, and washing frequency, none of which were uniform across the group.
The researchers concluded that personal care products shape the chemical environment in which skin microbes live, and that environment can influence which microbes thrive. The findings point toward the potential for more targeted skincare formulations designed with the skin microbiome in mind, an area of research that the cosmetics and dermatology industries are only beginning to take seriously.
