Activated carbon is known for adsorbing odor compounds. However, applying the black powder directly to underarms presents a visual problem. A 2020 study developed a material that combined odor capture with a more suitable appearance on skin by coating activated carbon white with titanium dioxide and a polymer.
In a trial applying a spray containing the material to one underarm of each of eight people, odor after six hours was lower than on the untreated side. The research connected material development, adsorption tests for individual compounds, and a small human study.
Does one adsorbent capture all body-odor compounds equally?
The researchers selected five compounds known to be associated with body odor: diacetyl, 2-nonenal, acetic acid, 3-methyl-2-hexenoic acid, and isovaleric acid. These include molecules with differing chemical properties, such as acids, ketones, and aldehydes.
They first examined activated carbon and the comparison material zinc oxide. Each odor compound and material were placed on a small piece of cotton, kept in a sealed container at 37°C for 30 minutes, and measured by GC–MS in the air above the sample. The reduction in signal relative to a control without the material served as an adsorption measure. The in vitro measurements used three independent replicates.
Activated carbon showed higher adsorption of diacetyl and 2-nonenal than zinc oxide. Differences between the two materials varied by compound. The authors chose activated carbon as the starting point for development because it could capture compounds with properties different from those for which zinc oxide performs well.
Whiter was not necessarily better
Activated carbon has numerous small pores and a large surface area. Molecules adhering to that surface are the basis of adsorption. The researchers compared four candidate activated carbons and considered density, pore structure, and surface properties relevant to adsorption. However, the screening figure is inconsistent: its text and caption identify acetic acid, while the vertical axis identifies diacetyl. This makes detailed compound-specific comparisons in that figure difficult to establish.
Next, they added titanium dioxide, which strongly scatters light, to make the selected activated carbon look white. Simply mixing the two did not provide sufficient coverage, and the material remained gray. An acrylic copolymer used as a binder produced a whiter material, with titanium-dioxide coverage also confirmed by electron microscopy. The paper calls this white activated carbon, or WAC.
Increasing titanium dioxide improved whiteness but reduced diacetyl adsorption. The authors attributed this to blockage of the carbon’s pores. Considering appearance and adsorption together, they selected an activated-carbon-to-titanium-dioxide ratio of 1:6 within the tested range. Carbon with a mean particle diameter of 40.5 micrometers also performed better in whiteness and adsorption than carbon with a diameter of 9.4 micrometers.
Photographs of application to the forearm showed an appearance that blended better with skin color than a sample containing black activated carbon alone. This was an appearance demonstration, not a long-term skin-safety evaluation.
Adsorption by the finished material depended on the compound
When WAC was tested against the five odor compounds, adsorption was higher than zinc oxide for diacetyl and 2-nonenal. WAC adsorption was lower for acetic acid, while there was no significant difference for 3-methyl-2-hexenoic acid or isovaleric acid. The important finding was a difference in which compounds the materials handled well, not uniform superiority for every body-odor compound.
In a diacetyl comparison using equal amounts of activated carbon, WAC and black activated carbon did not differ significantly in adsorption. The authors interpreted this as evidence that the white coating did not completely block the pores, leaving adsorption capacity intact.
Titanium dioxide alone also captured some acids. This material was designed to use whiteness and adsorption, rather than primarily to decompose molecules through titanium dioxide’s photocatalytic activity.
Lower underarm odor after six hours in eight people
In the human study, participants washed their underarms with unscented soap and had a spray containing 1.5% WAC applied to one side. The test product also contained isopropyl myristate and propellant. The other underarm received no product, and five assessors rated odor after six hours on a 0–5 scale.
Odor intensity after six hours was significantly lower on the treated side than on the untreated side. The increase in scores from baseline to six hours was also smaller on the treated side. Here, the increase refers to a difference in rating scores, not a percentage reduction in odor molecules.
The researchers also measured ATP in underarm samples collected at six hours. An indicator of bacterial quantity, calculated by converting ATP signals using a Staphylococcus epidermidis standard, did not differ significantly between treated and untreated sides. The authors considered deodorization and adsorption, rather than bacterial killing, to explain the lower odor.
However, this was not a comparison of bacterial counts obtained directly by culture, nor did it examine bacterial species or their metabolic activities. There was also no comparison with an otherwise identical formulation lacking WAC, so the human-study difference cannot be attributed entirely to WAC alone.
What the material development achieved, and what remains
The study found a way to address the practical problem of activated carbon’s black appearance while retaining adsorption. Its materials-science contribution lies in adjusting particle size, coating, and mixing ratios, rather than simply adding more white powder.
The human study, however, was a small evaluation in eight people over six hours, without detailed information on allocation or assessor blinding. Longer use, heavy-sweating conditions, and acceptability and safety across different skin types need further study. Applications such as wall materials and mass production mentioned by the authors also remain prospects beyond the scope tested here.
Summary
Coating activated carbon with titanium dioxide and a polymer produced a material combining a white appearance with adsorption of odor compounds. Compared with zinc oxide, the finished material captured diacetyl and 2-nonenal better, adsorbed less acetic acid, and showed no significant difference for the other two acids.
In eight people using a spray containing the material, underarm odor after six hours and its increase over that period were smaller than on the untreated side. The study offers an example of designing both odor-capture ability and an appearance suitable for use on skin.
Research funding and conflicts of interest: Mandom funded the research, and all three authors were company employees at the time. The paper’s conflict-of-interest statement also declares that the company had no role in study design, data collection, or the decision to publish.
Reference
Hara T, Nabei H, Kyuka A. Activated carbon/titanium dioxide composite to adsorb volatile organic compounds associated with human body odor. Heliyon. 2020;6:e05455. doi:10.1016/j.heliyon.2020.e05455
