Perspiration, or sweating, plays an essential role in maintaining normal health because it helps eliminate toxins, balance salt levels, and regulate body temperature. One might be surprised to learn that sweat, in itself, is actually odorless and does not cause the familiar oniony smell commonly associated with sweat. In fact, it is the human microbiome that is the root of the particularly unsavory smell that we loosely call body odor. The human body is covered with two types of sweat glands: eccrine and apocrine. Eccrine glands excrete salt and water to keep the body cool. Apocrine glands carry protein and fat secretions from within the body. When bacteria feast on these apocrine secretions upon mixing with body sweat, they release trans-3-methyl-2-hexenoic acid in their waste, which is the primary cause of body odor (1). To alleviate this aromatic issue, humans tend to turn to deodorants.
Recognized as the first product ever to prevent odor, the original formula for deodorant was invented in 1888 by Edna Murphey, a U.S. inventor in Philadelphia, Pennsylvania. Murphey marketed his invention under the name of “Mum.” Inspired by the ballpoint pen, roll-on deodorants were invented in the 1940s. This new type of deodorant applicator was tested in the U.S. in 1952 and was marketed under the name of “Ban Roll-On” (8). The first antiperspirant aerosol deodorant launched in 1965, and the biggest nonactive ingredient innovation came in the 1970s, when quick-dry cyclomethicones, or methyl-siloxanes, hit the market (3).
Deodorants vs. Antiperspirants
Before we begin, it is important to differentiate between deodorants and antiperspirants. Deodorants and antiperspirants both help fight body odor, but they do so in different ways. Deodorants fight body odor by targeting and killing the odor-producing bacteria. To do this, they use antibacterial compounds such as triclosan and chlorhexidine. In contrast, antiperspirants fight body odor by cutting down on sweating through the blocking of sweat pores. To do this, they use aluminum or zirconium-based compounds that form a polymeric plug that physically blocks sweat from escaping sweat glands (6). Both deodorants and antiperspirants often use cyclomethicones, which are fast-drying silicone compounds, as solvents.
The Chemistry Behind Deodorants
Deodorants are applied to the body in order to cover the natural odors produced by the bacterial breakdown of proteins present in perspiration. The main product of this bacterial breakdown is E 3-methyl-2-hexenoic acid, or trans 3-methyl-2-hexenoic acid, which is responsible for the unpleasant smell we commonly associate with body odor. Another geometric isomer, Z (or cis) 3-methyl-2-hexenoic acid, also contributes to the unpleasant smell of body odor, but to a lesser extent (4).
Deodorants are mostly alcohol-based, and consist of compounds like sodium stearate, stearyl alcohol, and sodium chloride (1). These ingredients make the target bacterial region too acidic or salty to sustain fermentation by bacteria. Other antimicrobial ingredients like triclosan and ethylenediaminetetraacetic acid (EDTA) impede bacterial growth in the underarm region. In addition to these ingredients are other chemicals that vary from brand to brand. Humectants like propylene glycol and glycerin prevent the product from drying out, additives for pH control like triethanolamine and diethanolamine control pH levels, preservatives like sodium benzoate and parabens extend the shelf life of the product, and scent-specific chemicals cover unpleasant smells and counteract one’s body odor (2).
Furthermore, deodorants also contain cyclomethicone, also known as 2,2,4,4,6,6,8,8,10,10-decamethyl-1,3,5,7,9,2,4,6,8,10-pentaoxapentasilecane or C10H30O5Si5. Cyclomethicone is an odorless and colorless silicone compound that is used in a diverse array of personal care products like deodorants. Unlike aluminum compounds, cyclomethicone is too large to pass through the initial layer of skin, so there is no risk of absorption (5).
Antiperspirants are a subgroup of deodorants that attempt to stop or significantly reduce perspiration with aluminum compounds such as aluminium chloride, aluminium chlorohydrate, aluminium zirconium tetrachlorohydrex gly, and aluminium zirconium trichlorohydrex gly. These aluminum-based compounds react with the sweat directly, as opposed to regular deodorants. The compounds affect the electrolytes present in the sweat to form a gel plug in the duct of the sweat gland, stopping the glands from secreting sweat over the target area. These gel plugs are removed naturally over a period of time through skin peeling (4).
Most antiperspirants include aluminum chlorohydrate as the active ingredient, which has the chemical formula Al2(OH)5Cl. Perspiration contains several substances, including water. A hydrolysis reaction occurs when aluminum chlorohydrate reacts with water, forming the hydrated aluminum ion Al3+ (aq). This is written as the complex ion [Al(H2O)6]3+ (aq) and is described as being an octahedral complex with six water molecules loosely bonded to the central aluminum (Al3+) ion. These bonds are classified as dative covalent bonds. Dative covalent bonds are bonds in which both of the two shared electrons are provided by the oxygen atom of each water molecule (4).
In the presence of the water in which it is dissolved, this complex undergoes an acid-base equilibrium reaction with the water: [Al(H2O)6]3+ (aq) + H2O(l) ⇋ [Al(H2O)5OH]2+ (aq) + H3O+ (aq). The H3O+ ions that are formed from this reaction are responsible for the acidity (4).
The formation of the hydronium ions has two important effects. First, the presence of hydronium ions causes the pH of the treated area to fall below 7. It should be noted that bacteria prefer to live and thrive in alkaline or basic conditions (when the pH is above 7). In fact, when humans wash their armpits with basic pH soap, the skin loses its acid mantle (pH 4.5-6.0), causing the pH of the skin to rise and disrupting the skin barrier. As more and more bacteria thrive in this elevated pH environment, the skin becomes more susceptible to bacterial colonization, with reverberating effects of sweat and body odor. So when hydronium ions are released, lowering the pH and creating an acidic environment, bacteria cannot thrive in the acidic conditions and essentially die off (4).
Another important effect of the formation hydronium ions has to do with proteins. Sweat contains proteins, which are water-soluble under regular conditions. The formation of hydronium ions causes the structure of the proteins to change, or undergo “denaturation” (4).
During the process of denaturation, the proteins fold themselves differently, causing them to lose their solubility. As a result, the proteins precipitate out as a gel. This gel plugs the sweat glands, blocking or preventing the glands from releasing any more perspiration. This characteristic of aluminum is referred to as “astringency” (4).
Health Risks and Controversies
While deodorants have provided numerous benefits in helping to reduce body odor and attack the human microbiome, there have also been many health risks and controversies associated with deodorants and antiperspirants that are worth mentioning.
As mentioned before, the biggest nonactive ingredient innovation for deodorants came in the 1970s when cyclomethicones hit the market. Why is that? Cyclomethicones are “quick-dry” in that they do not irritate the skin and evaporate quickly, leaving behind the deodorant’s active ingredient but with no greasiness or stickiness. It is for this reason that cyclomethicone solvents are currently found in many antisweat products. However, cyclomethicones are under severe criticism in Europe not for human health effects, but because of some serious concerns that the molecules may bioaccumulate in the environment (3). The cylclomethicone molecules, which are also prominently used in other personal hygiene products like hand conditioners, makeup, and hand creams, does not break down very quickly in water. Because of its slow breakdown, this presents a grave issue regarding the potential bioaccumulation of cyclomethicones in the environment. This bioaccumulation carries with it many ecological ramifications, and it has yet to be fully established whether cyclomethicones pose a long-term toxicity risk to fish and other aquatic organisms (6). As of today, more research is underway to confirm the relationship between the bioaccumulation of cyclomethicones and the risk of toxicity to aquatic organisms.
Additionally, there have also been some concerns regarding the potential breast cancer risk caused by underarm deodorants or antiperspirants. Because deodorants are applied near the breast and contain potentially harmful ingredients, many scientists and others have suggested a possible relationship between the use of deodorants and breast cancer. As mentioned before, antiperspirant deodorants use aluminum-based compounds as the active ingredient to physically block perspiration from being able to escape sweat glands. Some scientific research suggests that the aluminum in these antiperspirant deodorants may be absorbed into the skin and cause estrogen-like effects (7).
Because it has been known that estrogen can induce the growth of breast cancer cells, some scientists have suggested that the aluminum compounds in antiperspirant deodorants may have a significant contribution to the development of breast cancer. Moreover, it has also been suggested that aluminum may have direct activity in breast tissue. However, no scientific evidence or studies to date have confirmed a clear correlation that links the use of deodorants with the development of breast cancer. As a matter of fact, a 2014 review on the National Center for Biotechnology Information website established that there was no clear evidence showing that the use of aluminum-containing antiperspirant deodorants or cosmetics led to an increase in the risk of breast cancer (7).
Furthermore, some research has suggested that parabens may be related to the risk of breast cancer. Parabens are preservatives that are used in some deodorants, and have been shown to mimic estrogen activity in the body’s cells (see fig. 8). While it has been shown that parabens are found in breast tumors, there is no clear evidence that parabens have any kind of a role in causing breast cancer. It is also important to note that most deodorants and antiperspirants in the U.S. do not currently contain parabens (7).
Despite all the claims that link the use of deodorants to the development of breast cancer, there has been no consistent scientific evidence to date that support these claims about such a relationship. One study published in 2006 found no association between antiperspirant use and breast cancer risk. Another study published in 2003 reported younger age at breast cancer diagnosis for women who used deodorants frequently, but its results have been deemed inconclusive for its retrospective nature (7). Thus, because such studies of deodorants and breast cancer have often yielded conflicting results, there exists no conclusive relationship between deodorants and breast cancer, and additional research would be needed to clearly determine whether such a relationship even exists.
In conclusion, there is a lot of chemistry at work in the deodorants and their application to the human body, and that chemistry may yield both benefits and risks. From antibacterial compounds in alcohol-based deodorants to aluminum-based compounds in antiperspirant deodorants, deodorants use a wide variety of chemicals in many different ways to essentially kill off odor-producing bacteria and block sweat glands to prevent the release of sweat. While some potential risks of cancer and harmful bioaccumulation have been associated with deodorants, there have been no clear and consistent studies that conclusively support these claims. In the end, deodorants have a fascinating chemistry behind them, and additional research into their structure and use will certainly have significant chemical and health ramifications in the near future.
References and Footnotes
1) Ashish. “How Do Deodorants and Antiperspirants Work?” Science ABC, Science ABC, 18 Feb. 2017, http://www.scienceabc.com/humans/how-do-deodorants-and-antiperspirants-work-do-they-pose-any-danger-to-the-body.html.
2) Boyd, Christopher. “Which Chemicals Make Deodorants and Antiperspirants Work?” Chem Service, Chem Service Inc., 22 Aug. 2014, http://www.chemservice.com/news/2014/08/which-chemicals-make-deodorants-and-antiperspirants-work/.
3) Everts, Sarah. “What are Deodorants and Antiperspirants, and How Do They Fight Sweat?” CEN RSS, American Chemical Society, 2 July 2012, cen.acs.org/articles/90/i27/Deodorants-Antiperspirants.html.
4) Gros, Leo. “Deodorants and Antiperspirants.” Chemistry and Industry for Teachers in European Schools, 16 Mar. 2009.
5) Hock, Solveig. “The Chemistry of Deodorants.” ChemistryViews, ChemViews Magazine, 1 May 2018, http://www.chemistryviews.org/details/ezine/11012500/The_Chemistry_of_Deodorants.html.
6) Interest, Compound. “The Chemistry of Deodorants vs. Antiperspirants.” Compound Interest, Compound Interest, 21 Sept. 2015, http://www.compoundchem.com/2015/09/21/deodorants-antiperspirants/.
7) National Cancer Institute. “Antiperspirants/Deodorants and Breast Cancer.” National Cancer Institute, National Cancer Institute, 9 Aug. 2016, http://www.cancer.gov/about-cancer/causes-prevention/risk/myths/antiperspirants-fact-sheet.
8) “Chemistry of Deodorant.” Chemistry of Deodorant, Gwinnett K12, 2004, http://www.gwinnett.k12.ga.us/LilburnES/PromoteGA/beauty/deodorant_intro.html .
9) “Deodorant.” Wikipedia, Wikimedia Foundation, 28 May 2018, en.wikipedia.org/wiki/Deodorant.