Definition of pH
Acidity is a characteristic of aqueous solutions. It is quantitatively determined by measuring the molar concentration of free hydrogen ions, symbolically expressed as [H+]. Since the concentration of free hydrogen ions in a solution can vary over a very large range, it is of practical use to express the acidity of a solution with a number called pH such that
pH = -Log [H+]
We know, therefore, that a solution containing one millimolar free hydrogen ion has a pH=3, a solution with 10 micromolar free hydrogen ion has a pH=5 and a solution containing one molar free hydrogen ion has a pH=0. We call acidic the solutions with pH < 7, and alkaline (or basic) the solutions with pH > 7.
The acidity of non-liquid materials, such as soil, is assessed by mixing a defined quantity of soil in a defined volume of distilled water, allowing the solid material to decant and measuring the supernatant’s pH with an instrument called pH-meter. We know that acidic soil favors the blue color of the flowers of Hortensia (Hydrangea macrophylla) whereas in alkaline soil its flowers turn pink.
So, to say the least, differences in pH do influence our perception of taste and can modify our environment. Does pH play a role in beauty, too? What about acidity and skin?
The pH of Physiological Fluids
Body fluids are known to have different acidity: gastric juices have a pH between 1 and 2, blood pH is 7.4, urine pH is about 6 and sweat pH is between 4.5 and 7. On the surface of the stratum corneum, sweat and sebum mix to form the acid mantle, with a pH between 4.5 and 5.5. All these things are liquid; what about the skin itself?
The pH of the Skin
It is generally understood that, when speaking of “skin’s pH” one means the pH of the stratum corneum. The pH of the horny layer could be properly determined by removing it, grinding it, mixing it with an appropriate volume of water and measuring the supernatant’s pH—all of which is quite unpractical!
The pH of the stratum corneum is generally measured by layering a droplet of distilled water on the surface of the skin, allowing it to rest for a defined time interval and then measuring the acidity of the droplet with a pH-meter equipped with an appropriate probe. In this way, what is really measured is not the pH of the stratum corneum, but the pH of a droplet of water allowed to sit for a determined time interval on the skin’s surface. During this time interval the droplet may well solubilize salts and polymers deposited on the surface by sweat, released by the resident microbiome, left by poorly removed topical treatments or even extracted from the stratum corneum itself. Whatever it is, the pH of that droplet is not the pH of the stratum corneum.
So, what is the pH of the stratum corneum? And, is it really relevant for skin? Not many results are reported on correlations between skin homeostasis and skin pH. A recent paper reports that “experimental disruption of the physical barrier leads to an increase of pH, returning to normal levels only after many hours. Inflammatory skin diseases and diseases with an involvement of the epidermis exhibit a disturbed skin barrier and an increased pH. This is known for atopic dermatitis, irritant contact dermatitis, ichthyosis, rosacea and acne, but also for aged and dry skin.”1
Acid, More or Less?
These results concern skins with experimentally disrupted barrier. What about intact, “healthy” skin? Recent discussions in the field of skin care point out results that seem to indicate that an acidic skin is in better shape than a less acidic one. What is a “more acidic” pH? What does it mean for skin? And, is skin’s pH really acidic?
One could even think that it is not acidic at all since, as reported in reference 1, when the physical barrier is disrupted; that is, when more chemicals can be extracted from the stratum corneum and from the epidermis, the measured pH actually increases! To get a glimpse into the pH of the horny layer, one could check whether a chemical reaction that can only occur within a well-defined pH range does occur or does not occur in the stratum corneum. One such reaction exists: it is the formation of brown compounds by the reaction of free amines with the di-hydroxyacetone (DHA). This reaction occurs in the stratum corneum upon topical application of DHA to achieve a self-tan, and it occurs in vitro only at pH above 7.
This is to say that the pH of the skin is still a matter of debate.
You’re Not What You Eat!
In present day, there is a trend to recommend abolishing the consumption of “acidic” food such as meat, fish and sugar, and to increase the consumption of alkaline foods like fruits, vegetables and tofu, in order to achieve some ill-defined holistic equilibrium. At the same time, a contrarian movement is afoot—the recommendation calling for the consumption of acidic foods because the pH of skin in good shape is acidic; that is, the pH of a droplet of water layered on the surface of a skin in good shape is closer to 4.5 than to 5.5.
What should we make of this? Let us be reassured: whatever we ingest goes into the stomach and encounters a very acidic environment. Upon digestion, nutrients are distributed across the body, a body that spends large amounts of its own energy to maintain every one of its organs at the appropriate pH.
Therefore, eating more Camembert (pH 7.4) will not help one reach a holistic balance within the body; and, if combined with a red wine (acidic) instead of a more “sweet” white wine, will leave a very bad taste in one’s mouth! On the other hand, drinking more sodas (pH 2.5 to 3.5) or biting into lemon slices will not help the skin achieve an acidic pH and will not maintain or improve health and beauty.
- E. Proksch (2018) pH in nature, humans and skin, The Journal of Dermatology 45 : 1538-1546
Paolo Giacomoni, PhD
Insight Analysis Consulting
Paolo Giacomoni acts as an independent consultant to the skin care industry. He served as executive director of research at Estée Lauder and was head of the department of biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents.