Measured on skin that has been neither washed nor cared for in 24 hours, the pH of the skin surface averages 4.7. This figure, from a study of 330 volunteers, is lower than the often-cited value of 5.5.
The skin is therefore clearly acidic at the surface. Here is where this figure comes from, what makes it vary, and what produces this acidity.
pH in a nutshell
pH measures the acidity of a medium on a scale from 0 to 14. The number 7 corresponds to neutrality, that of pure water. Below it, the medium is acidic; above it, it is alkaline.
The scale is logarithmic. Each unit corresponds to a factor of ten. A surface at pH 4.7 is thus about two hundred times more acidic than neutral water.
Where the figure of 4.7 comes from
In 2006, a Dutch team measured the pH of the forearm of 330 people in several centres. A first measurement was taken under usual living conditions. A second followed after 24 hours without a shower or cosmetic product. The average pH dropped from 5.12 to 4.93 (Lambers et al., 2006).
The authors took this drop into account to estimate the “natural” pH of the skin, free from the influence of washing and products. They put it at an average of 4.7. They also note that about half of the measurements published until then were already below 5.
The same study recalls an often-forgotten point. Tap water in Europe generally has a pH close to 8. A simple rinse can raise the pH of the skin for nearly six hours before it returns to its usual value (Lambers et al., 2006).
Why 5.5 is so often mentioned
The value of 5.5 has become established in everyday language and on packaging. It partly reflects measurements taken on skin subjected to daily routines: shower, soap, cream. Measurements taken after a period without washing give lower figures. Both values describe the same skin under different conditions.
A value that varies
The figure of 4.7 is an average. The literature places the physiological pH of the stratum corneum between about 4.1 and 5.8 (Proksch, 2018). It depends on many factors (Schmid-Wendtner and Korting, 2006):
| Factor | What is observed |
|---|---|
| Body area | Some areas, such as the armpits, are often less acidic than the forearm. |
| Age | pH changes over the course of life, from birth to old age. |
| Sweat and sebum | Their quantity and composition alter surface acidity. |
| Routines and products | Washing, cosmetics and covering clothing make pH vary. |
pH also changes with depth. It follows a marked gradient through the stratum corneum: acidic at the surface, it approaches neutrality towards the living layers of the epidermis (Schmid-Wendtner and Korting, 2006). Inside the body, blood is maintained at around 7.4.
Where this acidity comes from
The acidity of the surface results from several mechanisms that act together (Proksch, 2018):
- the breakdown of filaggrin, a protein of the stratum corneum, which releases amino acids and their derivatives;
- free fatty acids, part of which comes from the breakdown of phospholipids in cells as they transform into corneocytes;
- an ion exchanger in the cells of the epidermis, which releases protons outwards;
- the secretions of the sweat and sebaceous glands, whose exact share remains debated (Schmid-Wendtner and Korting, 2006).
The role of fatty acids has been demonstrated in mice. When the enzyme that releases them from phospholipids is blocked, the pH of the surface rises. Adding fatty acids brings it back down (Fluhr et al., 2001).
What this figure helps us understand
Knowing the average value of 4.7 helps to read labels with more perspective. A product described as “neutral” in principle has a pH of 7, that is, an acidity about two hundred times lower than that of the skin surface. A product labelled pH 5 or 5.5, by contrast, remains in the acidic zone where the skin naturally sits.
This article is part of the PHYTO 5 Library.
Sources
- Fluhr JW, Kao J, Jain M, Ahn SK, Feingold KR, Elias PM. Generation of free fatty acids from phospholipids regulates stratum corneum acidification and integrity. Journal of Investigative Dermatology. 2001;117(1):44-51. doi:10.1046/j.0022-202x.2001.01399.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
- Schmid-Wendtner MH, Korting HC. The pH of the skin surface and its impact on the barrier function. Skin Pharmacology and Physiology. 2006;19(6):296-302. doi:10.1159/000094670