The “acid mantle” refers to the slight acidity of the skin's outermost layer. Research ranks it among its natural defences, alongside the lipid barrier, the low water content of the surface and the resident flora.
These elements do not work in isolation. They support one another, and pH plays a role in each of them.
A term almost a century old
The expression “acid mantle”, Säuremantel in German, dates back to 1928 and the work of two German physicians, Heinrich Schade and Alfred Marchionini. For a long time it was regarded as a historical curiosity. Over the past twenty years or so, research has restored it to a central place. It links the surface pH to the maintenance of the barrier, the cohesion of the stratum corneum and the balance of the resident flora (Ali and Yosipovitch, 2013).
The acid mantle refers to the acidity of the stratum corneum and of the film that covers it, without forming a layer that could be isolated. On average, the pH of the skin surface is around 4.7 (Lambers et al., 2006).
A multi-layered defence
The skin performs several barrier functions at once. It retains the body's water and limits the penetration of external substances. It also hosts, on its surface, a flora that is specific to it.
The American dermatologist Peter Elias, one of the leading researchers in the field, describes these functions as closely linked. Most take place in the stratum corneum. He cites four components: its low water content, its acidic pH, its resident flora and certain lipids deposited on its surface, such as free fatty acids (Elias, 2007).
The same author stresses one point. The organisation of lipids into regular layers, between the cells of the stratum corneum, serves both to retain water and to maintain the cohesion of the whole. A well-organised barrier therefore fulfils all of its functions better.
pH and the resident flora
The skin permanently hosts micro-organisms that are specific to it: the so-called “resident” flora. Its balance depends in part on pH.
The study by Lambers and his colleagues observed this directly. At an acidic pH, between 4 and 4.5, the bacteria of the resident flora remain attached to the skin. At an alkaline pH, between 8 and 9, they detach more readily (Lambers et al., 2006). The very title of the article summarises the authors' conclusion: a surface pH below 5 is favourable to the resident flora.
pH and the cohesion of the stratum corneum
The stratum corneum is made of flattened cells, the corneocytes, linked together by attachment points called corneodesmosomes. These attachments gradually come undone, allowing the oldest cells to detach discreetly.
This detachment is driven by enzymes whose activity depends on pH: moderate in an acidic environment, it accelerates when the stratum corneum is brought towards neutrality, and cohesion decreases (Hachem et al., 2003). This work was carried out in mice; the mechanism is detailed in the article “What happens at the skin surface when its pH rises”.
A defence that rebuilds itself
The acid mantle varies throughout the day. Washing, sweating or applying a product alter it every day. When the barrier is experimentally disrupted, the surface pH rises. It only returns to its usual value after several hours (Proksch, 2018).
This return relies on the mechanisms that produce acidity on a daily basis: release of fatty acids, breakdown of filaggrin, ion exchanges in the epidermis (Proksch, 2018).
In summary
- The acid mantle refers to the natural acidity of the stratum corneum, around 4.7 on average.
- Together with the lipid barrier, the low water content and the resident flora, it is part of the natural defences described by research.
- An acidic pH favours the attachment of the resident flora and the cohesion of the stratum corneum.
- After a disruption, the skin restores its acidity within a few hours.
These elements describe the physiology of the skin. They do not relate to any particular product.
This article is part of the PHYTO 5 Library.
Sources
- Ali SM, Yosipovitch G. Skin pH: from basic science to basic skin care. Acta Dermato-Venereologica. 2013;93(3):261-267. doi:10.2340/00015555-1531
- Elias PM. The skin barrier as an innate immune element. Seminars in Immunopathology. 2007;29(1):3-14. doi:10.1007/s00281-007-0060-9
- Hachem JP, Crumrine D, Fluhr J, Brown BE, Feingold KR, Elias PM. pH directly regulates epidermal permeability barrier homeostasis, and stratum corneum integrity/cohesion. Journal of Investigative Dermatology. 2003;121(2):345-353. doi:10.1046/j.1523-1747.2003.12365.x
- Lambers H, Piessens S, Bloem A, Pronk H, Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science. 2006;28(5):359-370. doi:10.1111/j.1467-2494.2006.00344.x
- Proksch E. pH in nature, humans and skin. The Journal of Dermatology. 2018;45(9):1044-1052. doi:10.1111/1346-8138.14489