Conventional soap is alkaline, with a pH of around 9 to 10. After washing, the pH of the skin surface rises, and it takes several hours for it to return to its usual acidic value.
This delay is normal. The skin knows how to restore its acidity. It is simply useful to be aware that this delay exists, and what happens during that time.
The starting point: an acidic surface
Measured without recent washing or applied products, the skin surface is slightly acidic. A study of 330 volunteers estimated its “natural” pH at 4.7 on average (Lambers et al., 2006). Usual values range between 4 and 6 depending on the body area, age and habits.
This acidity originates in the stratum corneum, the outermost layer of the epidermis. Several mechanisms contribute to it, including the production of free fatty acids and the breakdown of certain proteins (Proksch, 2018).
What soap does
Soap is obtained by reacting a fatty substance with a base. The final product is alkaline. An Indian study measured the pH of commercial toilet soaps: most were between 9 and 10 (Tarun et al., 2014).
On contact with the skin, this alkaline solution neutralises part of the surface acidity. The skin pH rises immediately. The extent of the increase depends on the product, the duration of washing and rinsing.
Synthetic cleansers, known as syndets, can be formulated at a neutral or slightly acidic pH. In a comparative trial, they raised the skin pH less than soap (Korting et al., 1987).
The return to acidity takes time
After washing with soap, the skin pH gradually falls again. A Japanese research team summarises the state of knowledge as follows: the pH rises after cleansing, then returns to a more acidic value “within a few hours” (Takagi et al., 2015).
Water alone already has an effect. In Europe, tap water often has a pH close to 8. According to Lambers and his colleagues, simple rinsing can raise the skin pH for up to six hours before it returns to its baseline value (Lambers et al., 2006).
The same Japanese study adds a reassuring nuance. It compared people who had used soap for more than five years with people loyal to a slightly acidic cleanser. The speed of return to the initial pH was the same in both groups. Long-term use of soap did not appear to impair the skin's ability to restore its acidity (Takagi et al., 2015).
Why these few hours matter
During this period, the skin surface operates at a higher pH than usual. Yet several processes in the stratum corneum are tuned to an acidic environment. This is the case for the enzymes that assemble the lipids of the skin barrier, whose activity is optimal at acidic pH (Proksch, 2018).
A single wash remains a brief episode. The question arises especially when washing is repeated. In a cross-over trial, ten healthy volunteers washed their forehead and forearm twice daily for four weeks, with soap and then with a syndet, or in the reverse order. During the soap period, the skin pH was on average 0.3 units higher (Korting et al., 1987). The authors conclude that the skin pH can change durably according to washing habits.
A few pointers for cleansing
- Lukewarm water is sufficient for daily cleansing.
- A brief wash, limited to the areas that need it, reduces contact time.
- Thorough rinsing removes product residues left on the surface.
- The pH of the cleanser is sometimes shown on the packaging. A pH close to that of the skin alters its surface acidity less.
- Frequency matters as much as the product. Two full washes a day place more demands on the skin than one.
If your skin remains uncomfortable despite gentle cleansing, talk to a dermatologist.
This article is part of the PHYTO 5 Library.
Sources
- Korting HC, Kober M, Mueller M, Braun-Falco O. Influence of repeated washings with soap and synthetic detergents on pH and resident flora of the skin of forehead and forearm. Results of a cross-over trial in health probationers. Acta Dermato-Venereologica. 1987;67(1):41-47. DOI : 10.2340/00015555674147
- 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
- Takagi Y, Kaneda K, Miyaki M, Matsuo K, Kawada H, Hosokawa H. The long-term use of soap does not affect the pH-maintenance mechanism of human skin. Skin Research and Technology. 2015;21(2):144-148. DOI : 10.1111/srt.12170
- Tarun J, Susan J, Suria J, Susan VJ, Criton S. Evaluation of pH of bathing soaps and shampoos for skin and hair care. Indian Journal of Dermatology. 2014;59(5):442-444. DOI : 10.4103/0019-5154.139861