Everything you need to know about the pH of the skin.

Hydrogen potential, better known by the abbreviation pH, is a measure of the acidic or alkaline nature of an aqueous medium.

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

The skin has a slightly acidic physiological pH, between 4.5 and 5.5, often referred to as the acid mantle.

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

The very origin of this acidity lies in a combination of metabolic and cellular processes. Among the principal sources of protons are the hydrolysis of phospholipids into free fatty acids, the activity of the Na+/H+ exchanger (NHE1) in keratinocytes, the degradation of filaggrin into urocanic acid and pyrrolidone carboxylic acid, components of the natural hydration factor (FNH), the persistence and subsequent extrusion of melanin, and certain pathways related to cholesterol sulfate.

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

Skin pH is involved at several levels in ensuring the skin functions properly, from keratinocyte differentiation to the maintenance of the microbiota.

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

The surface acidity also influences the lamellar organisation of epidermal lipids. In an acidic environment, free fatty acids remain non-ionised, which limits electrostatic repulsions and favours the stacking of lipid bilayers. This highly ordered architecture enables the stratum corneum to function as a semi-permeable barrier, regulating transepidermal water loss and the entry of external agents. Conversely, an increase in skin pH towards alkaline values disrupts these lipid interactions, disorganises membrane domains and undermines the integrity of the skin barrier.

The skin’s natural pH also plays a role in the desquamation process. The progressive 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 acid gradient of the stratum corneum, with optimal activity around pH 5.5. This allows the skin’s surface to remain smooth and even, with the pH of the stratum corneum forming a gradient.

Beyond its structural effects, the pH of facial skin, as with that of body skin, actively contributes to the balance of the skin microbiome. The slightly acidic environment limits the proliferation of pathogenic micro-organisms while promoting the survival of commensal bacteria, such as Staphylococcus epidermidis. These micro-organisms contribute to local immune defences 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 alter the homeostasis of the skin barrier and the skin’s sensitivity.

• Age is one of the major determinants. At birth, the skin of infants has a near-neutral pH, which gradually decreases to acidic values during the first weeks of life, in parallel with the maturation of the stratum corneum. In contrast, in older individuals, an increase in skin pH and a reduction in the skin’s buffering capacity have been observed, indicating a more fragile barrier. Between 18 and 60 years, the skin’s physiological pH remains broadly stable.

• The skin areas also influence pH. The body’s skin pH is relatively homogeneous, but some more humid regions, such as the armpits, exhibit slightly higher values (≈ 6.5). This difference is chiefly due to local moisture, perspiration and slight variations in microbiota composition.

• The skin type also plays a role. High sebum production, as found in oily or acne-prone skin, can locally lower pH measurements, especially on the forehead. Conversely, skin barrier alterations associated with dry skin are frequently accompanied by an elevation in pH.

• Variations related to sex and phototype have also been described. Some studies suggest that the skin of men has a slightly more acidic pH than that observed in women's skin. Furthermore, lower pH values have been reported in darker skin compared with lighter skin, although these differences remain poorly explored using objective methods.

• The use of cosmetic products represents one of the most influential external factors on 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 induce more moderate and transient disturbances. Over the long term, repeated use of alkaline products may 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 skin‐care practices.

Many inflammatory dermatoses are accompanied by an increase in skin pH, highlighting 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 presents a higher pH. Moreover, a variation of 0.3 pH units already corresponds to a halving of the H+ ion concentration, which is sufficient to significantly alter enzymatic activity, lipid organisation and barrier function. This alkalinisation is also observed in other inflammatory dermatoses such as contact dermatitis or psoriasis, where the increase sometimes reaches 0.3 to 0.4 units.

An increase in pH is also accompanied by a higher risk of infection. Indeed, studies have shown that certain skin areas in diabetic patients exhibit a higher pH, associated with increased susceptibility to fungal infections, notably to Candida albicans. Experimental observations further demonstrate that fungal lesions develop less readily in a skin environment maintained at low pH, confirming the importance of the protection afforded by 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 re-establish cutaneous homeostasis. Several studies demonstrate 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 with a pH close to the skin's natural pH (approximately 4.0 to 5.0) helps to preserve 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, as well as the lipid content of the stratum corneum, were also observed. When the skin was subsequently subjected to controlled aggression by sodium lauryl sulfate, the increase in pH remained more limited in the areas previously treated at pH 4. These results suggest that appropriate topical reacidification can enhance barrier function and the structure of the stratum corneum for ageing skin.

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

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