Everything you need to know about skin pH.

The hydrogen potential, better known by the abbreviation pH, is a measure of the acidic or alkaline nature of an aqueous solution.

Chemically, it reflects the concentration of hydronium ions (H3O+) according to the relationship: pH = –log [H3O+]. The pH scale ranges from 0 to 14: a value below 7 indicates an acidic solution, 7 corresponds to a neutral solution, while a value above 7 indicates a basic solution. This scale is logarithmic, which means that a one-unit change corresponds to a tenfold change in acidity.

The skin has a slightly acidic physiological pH, ranging from 4.5 to 5.5, often referred to as the “acid mantle.”

pH can be measured experimentally using colorimetric test strips or, more precisely, with electrodes connected to a pH meter—tools employed in dermatological research to investigate variations in the pH of facial skin, dry skin, oily skin, or acne-prone skin. These measurements confirm that maintaining a physiological skin pH is a central parameter of skin homeostasis, in both women and men.

The very origin of this acidity lies in a combination of metabolic and cellular processes. Among the primary proton sources are the hydrolysis of phospholipids into free fatty acids, the activity of the Na+/H+ exchanger (NHE1) in keratinocytes, and the breakdown of filaggrin into urocanic acid and pyrrolidone-carboxylic acid, components of the natural moisturizing factor (NMF), the persistence and subsequent extrusion of melanin, as well as certain pathways linked to cholesterol sulfate.

Skin pH is not a passive parameter, but the result of a complex biological regulation aimed at preserving epidermal homeostasis.

Skin pH contributes at multiple levels to proper skin function, from keratinocyte differentiation to maintaining the microbiome.

The formation of a functional epidermal barrier relies in particular on the activity of acid-dependent hydrolase enzymes, such as β-glucocerebrosidase and acid sphingomyelinase, which reach maximal efficacy at around pH 5 to 5.5, values close to the skin’s physiological pH. These enzymes catalyze the final steps in the synthesis of ceramides, key structural lipids accounting for over 50% of the lipids in the stratum corneum and essential for the cohesion of the skin barrier.

Surface acidity also governs the lamellar organization of epidermal lipids. In an acidic environment, free fatty acids remain non-ionized, which limits electrostatic repulsion and promotes stacking into lipid bilayers. This highly ordered architecture allows the stratum corneum to function as a semi-permeable barrier, regulating transepidermal water loss and the ingress of external agents. Conversely, an increase in skin pH toward alkaline values disrupts these lipid interactions, disorganizes membrane domains, and compromises the integrity of the skin barrier.

The skin’s natural pH also plays a role in the desquamation process. The gradual degradation of corneodesmosomes, the adhesive structures linking corneocytes together, depends on proteolytic enzymes such as kallikreins, serine proteases, and certain cathepsins. Their activation is finely regulated by the acidic gradient of the stratum corneum, with optimal activity around pH 5.5. This enables the skin’s surface to remain smooth and uniform, as the stratum corneum pH forms a gradient.

Beyond its structural roles, the pH of facial skin, like that of body skin, actively contributes to the balance of the cutaneous microbiome. This mildly acidic environment limits the growth of pathogenic microorganisms while supporting the survival of commensal bacteria, such asStaphylococcus epidermidis. These microorganisms aid in local immune defenses by producing antimicrobial peptides and modulating cytokine expression.

Skin pH is not a fixed value and is influenced by numerous physiological and environmental factors. These sometimes subtle variations can disrupt the homeostasis of the skin barrier and affect the skin’s sensitivity.

• The age constitutes one of the major determinants. At birth, the infant skin exhibits a pH close to neutrality, which gradually shifts toward acidic values during the first weeks of life in parallel with stratum corneum maturation. Conversely, in older adults, an increase in skin pH and a reduction in the skin’s buffering capacity have been observed, reflecting a more fragile barrier. Between 18 and 60 years of age, the skin’s physiological pH remains generally stable.

• The skin areas influence pH as well. Body skin pH is relatively uniform, but certain more humid regions, such as the underarms, exhibit slightly higher values (≈ 6.5). This difference can be attributed mainly to local humidity, perspiration, and slight variations in microbiota composition.

• The skin type also plays a role. A significant sebum production, as seen in oily or acne-prone skin, can locally lower the pH measurement, particularly on the forehead. Conversely, skin barrier impairments associated with the dry skin are frequently accompanied by an increase in pH.

• Variations related to sex and phototype have also been described. Some studies suggest that the skin of men exhibits a slightly more acidic pH than that observed for female skin. Moreover, lower pH values have been reported in dark skin compared with light skin, although these differences remain underexplored by objective methods.

• The use of cosmetic products represents one of the most influential external factors affecting skin pH. Cleansing with alkaline soaps can raise skin pH for several hours, whereas synthetic cleansers formulated at a pH close to the skin’s natural pH cause more moderate and transient disturbances. Over the long term, repeated use of alkaline products can alter the skin’s microbial flora, while slightly acidic formulations tend to maintain a more stable physiological skin pH.

The skin’s pH reflects the ongoing interplay between individual characteristics and skincare routines.

Many inflammatory dermatoses are accompanied by an increase in skin pH, underscoring the central role of skin acidity in epidermal homeostasis. In atopic dermatitis, for example, the pH of lesional areas is on average 0.2 to 0.3 units higher than that of healthy skin, and even clinically unaffected skin shows an elevated pH. Furthermore, a 0.3-unit change in pH already corresponds to a twofold decrease in H⁺ ion concentration, which is enough to significantly alter enzymatic activity, lipid organization, and barrier function. This alkalinization is also observed in other inflammatory dermatoses such as contact dermatitis or psoriasis, where the increase sometimes reaches 0.3 to 0.4 units.

A rise in pH is also accompanied by a heightened risk of infection. Indeed, studies have shown that certain skin areas in diabetic patients exhibit a higher pH, linked to increased susceptibility to fungal infections, notably toCandida albicans. Experimental observations further show that fungal lesions develop less readily in a skin environment maintained at low pH, underscoring the critical protective role of the acid mantle.

Maintaining a slightly acidic skin pH is essential for the proper functioning of the epidermal barrier, the balance of the microbiome, and the limitation of inflammatory phenomena. When the physiological pH of the skin increases, it is important to restore it to reestablish cutaneous homeostasis. Several studies show that a controlled topical acidification using treatments formulated with a slightly acidic pH (≤ 4.5) can lower an excessively high skin pH while improving hydration and the integrity of the skin barrier, particularly for mature or weakened skin. Additionally, the daily use of cleansers and emollients whose pH is close to the skin's natural pH (approximately 4.0 to 5.0) helps maintain the skin's acid mantle.

A randomized clinical study conducted with 20 elderly subjects concretely illustrates these effects. After four weeks of applying water-in-oil emulsions formulated at either pH 4 or pH 5.8, the areas treated with the more acidic formulation exhibited a significantly lower skin pH and were associated with better hydration. More pronounced improvements in the organization of intercellular lipid lamellae, their length, and the lipid content of the stratum corneum were also observed. When the skin was then subjected to a controlled challenge with sodium lauryl sulfate, the rise in pH remained more limited in the areas previously treated at pH 4. These results suggest that appropriate topical reacidification can enhance the barrier function and structure of the stratum corneum for aging skin.

Favoring mild cleansers, avoiding alkaline formulations, and supporting the skin barrier by moisturizing daily helps maintain or restore its physiological pH.

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