The pH of the skin is closely related to the overall health of the skin. For all the chemical reactions within the layers of the dermis and epidermis to properly progress, several factors, including the pH, water-holding capacity, and correct percentages of lipids must be appropriately maintained. The skin has inbuilt mechanisms to preserve the delicate balance of these factors but can be disrupted or adversely affected by incorrectly formulated skincare products.

The composition of skin:

The stratum corneum (dermis) is composed of flattened corneocyte cells that make up the outermost layer of skin. Sheets of lipids surround these cells. The lipids are composed of 50% ceramides, 25% cholesterol, and 10-20% fatty acids. Stratum corneum integrity is maintained by specialized proteins called corneodsomeosms that anchor neighboring cells together.

The skin provides an effective barrier between the organism and the environment, with the outtermost layer, the stratum corneum acting as a physical barrier against pathogenic (disease-causing) microorganisms, irritants, oxidants, and UV radiation. (1) One of the most important functions is to prevent water loss, known as transepidermal water loss (TEWL). Any disruption to this function has effects on the integrity of the stratum corneum. Natural Moisturizing Factors are a collection of low-molecular-weight components like ceramides, urea and glycerin that absorb atmospheric water and retain it within the skin due to their water-soluble components (1).

Different parts of the body will have skin with differences in the corneocyte shape, size, and the number of corneodesomes. The percentage of lipid components determines the thickness of the stratum corneum and is critical to the proper barrier function and overall skin health. The stratum corneum’s water-holding capacity relates to the softness and smoothness of the skin, and water loss from the epidermis is an indicator of the effectiveness of the barrier function (2).

The facial stratum corneum is thinner and has a lower number of corneocytes than the other areas of the body. The face has unique characteristics that allow for the hydration of the skin surface but has a relatively poor barrier function. For example, the cheek skin has the highest water-holding ability, while the perioral area (around the mouth) has the lowest. These properties are variable within genders and change with age.

In healthy skin, desquamation or shedding of dead skin cells is carefully controlled by hydrolytic enzymes that optimally maintain the thickness and integrity of the stratum corneum. These enzymes are dependent on the water activity and pH of the dermis and can therefore altered depending on what pH  skincare is formulated at.

What is pH?

pH refers to the ‘potential hydrogen’ of a water-based substance and is a scale used to measure how acidic or alkaline a substance is based on the concentration of hydrogen ions. A pH of 0 has the most acidic substances like hydrochloric acid, while sodium hydroxide is one of the most alkaline substances with a value of pH 14. Most substances will fall somewhere in between these two extremes with chemicals at a pH of 7 being considered neutral.

Water has a chemical structure of H₂O, which has an equal number of H⁺ and OH^– ions. On the pH scale, this balances out and gives a neutral pH of 7. When other substances dissolve in water, the ions may no longer be equal. If there are more H⁺ than OH^– ions, the pH is acidic and falls between pH 0 and 7. If the number of OH^– ions is higher than the number of H⁺ ions, the pH is alkaline and falls between pH 7 and 14.

pH is exclusively used for water-based materials, for example, cleansers, toners, and moisturizers. As oils or non-water based products do not contain hydrogen molecules, they do not have a pH value.

Why pH is so important to healthy skin:

Healthy, intact skin has a slightly acidic pH, ranging from 4.0-6.0, with an average value of 4.7 (3). This is an important aspect of the skins barrier function since it regulates the helpful, resident bacterial flora and prevents harmful bacteria from causing unwanted infections. This skin has evolved mechanisms for controlling the pH and as wounds heal the pH of the surrounding area can be seen to become more acidic, to promote a more beneficial environment for healing (4).   Changes in the pH are reported to play a role in the pathogenesis of skin diseases like irritant contact dermatitis, atopic dermatitis, acne vulgaris and Candida albicans infections (6).

It is believed that harmful bacterial colonization may contribute towards a shift to a more alkaline pH as pathogenic (disease-causing) bacteria prefer a more alkaline environment and are therefore more likely to survive and reproduce in these conditions.  Acid skin pH (4–4.5) keeps the resident bacterial flora attached to the skin, whereas an alkaline pH (8–9) promotes the dispersal from the skin (5).

Several components of skin like ceramides and free fatty acids (FFAs) function best in an acidic environment. As pH increases towards the alkaline range, important enzymes are not catalyzed at the same rate, reducing their efficiency. This then has an impact on the skin barrier function.

What pH should the skin be?

The average pH of facial skin has been found to be 4.7. pH readings vary with temperature location and have even been found to follow a circadian (day-night) rhythm (6).

Topical products and skin pH:

This study demonstrated that the topical application of acidic skincare decreased the pathogenic bacterial load on the skin.

Skincare products, especially cleansers have a significant influence on the skin’s pH. The use of tap water (its pH of 7.6 at my house in NZ), has the capability to increase the skin’s pH for up to 6 hours, before slowly returning to its baseline value. By measuring parameters of the skins barrier function, it can be shown that skin with a pH value of <5.0 is in better health and condition than skin with a pH above 6.0.

The role of pH in acne:

Resident flora that contributes to the health of the skin has evolved to live in an acidic environment. Conversely, the bacteria that cause acne, predominantly Propionibacterium acnes require a more alkaline environment. Without intervention (the change of pH of the skin caused by pH imbalanced skincare and cosmetic products) the bacteria responsible, P. acnes would not be able to compete with the resident, healthy bacteria, and would be prone to developing acne.

This has been shown experimentally where cutaneous isolated strains of P. acnes were cultured at varying pH ranges from 5.0-8.5. P. acnes grew more optimally in the pH range of 6.0-7.0. With less growth observed in the acidic environment below 6.0. This study supports the hypothesis that even minor changes to the pH of the skin, caused by neutral (pH 7.0) or alkaline (>7.0) skincare products can increase the number of P. acnes bacteria present on the skin surface.

What should I look for in skincare?

Skin care products formulated with an effective buffer system at a more acidic pH, may be the best option to improve the acid mantle and skin barrier function and thus keep the skin in "good condition." (9).  For this reason, Eunoia skincare products are formulated at pH 4.7, the average pH of healthy skin for gentle, effective biocompatible skincare that works in harmony with the skins internal processes to ensure skin is in its best health, and damaged skin is able to heal and become healthy and beautiful. 

References:

1. Jensen JM, Proksch E. The skin's barrier. G Ital Dermatol Venereol. 2009 Dec;144(6):689-700. PMID: 19907407.

2. Ali SM, Yosipovitch G. Skin pH: from basic science to basic skin care. Acta Derm Venereol. 2013 May;93(3):261-7. doi: 10.2340/00015555-1531. PMID: 23322028.

3. Tagami H. Location-related differences in structure and function of the stratum corneum with special emphasis on those of the facial skin. Int J Cosmet Sci. 2008 Dec;30(6):413-34. doi: 10.1111/j.1468-2494.2008.00459.x. PMID: 19099543.

4. Lambers, H., Piessens, S., Bloem, A., Pronk, H. and Finkel, P. (2006), Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science, 28: 359-370. https://doi.org/10.1111/j.1467-2494.2006.00344.x

5. Eleri M. Jones, Christine A. Cochrane, and Steven L. Percival.Advances in Wound Care.Jul 2015.431-439.http://doi.org/10.1089/wound.2014.0538

6. Schmid-Wendtner MH, Korting HC. The pH of the skin surface and its impact on the barrier function. Skin Pharmacol Physiol. 2006;19(6):296-302. doi: 10.1159/000094670. Epub 2006 Jul 19. PMID: 16864974.

7.  Yosipovitch G, Xiong GL, Haus E, Sackett-Lundeen L, Ashkenazi I, Maibach HI. Time-dependent variations of the skin barrier function in humans: transepidermal water loss, stratum corneum hydration, skin surface pH, and skin temperature. J Invest Dermatol. 1998 Jan;110(1):20-3. doi: 10.1046/j.1523-1747.1998.00069.x. PMID: 9424081.

 8. Korting HC, Lukacs A, Vogt N, Urban J, Ehret W, Ruckdeschel G. Influence of the pH-value on the growth of Staphylococcus epidermidis, Staphylococcus aureus and Propionibacterium acnes in continuous culture. Zentralbl Hyg Umweltmed. 1992 Jun;193(1):78-90. PMID: 1503605.

9. Abels C, Angelova-Fischer I. Skin Care Products: Age-Appropriate Cosmetics. Curr Probl Dermatol. 2018;54:173-182. doi: 10.1159/000489531. Epub 2018 Aug 21. PMID: 30130785.

Mary-Margaret Kober, Whitney P. Bowe, The effect of probiotics on immune regulation, acne, and photoaging, International Journal of Women's Dermatology, Volume 1, Issue 2, 2015, Pages 85-89, ISSN 2352-6475, https://doi.org/10.1016/j.ijwd.2015.02.001.

Clapp M, Aurora N, Herrera L, Bhatia M, Wilen E, Wakefield S. Gut microbiota's effect on mental health: The gut-brain axis. Clin Pract. 2017;7(4):987. Published 2017 Sep 15. doi:10.4081/cp.2017.987

Kuo SH, Shen CJ, Shen CF, Cheng CM. Role of pH Value in Clinically Relevant Diagnosis. Diagnostics (Basel). 2020;10(2):107. Published 2020 Feb 16. doi:10.3390/diagnostics10020107