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The pH plays an important physiological role in nature and humans. pH varies from 1 to 8 in human organs with tight regulation in blood and epithelia of barrier organs. The physiological pH of the stratum corneum is 4.1-5.8 and several mechanisms contribute to its formation: filaggrin degradation, fatty acid content, sodium-hydrogen exchanger (NHE1) activation and melanosome release. First, the acidic pH of the stratum corneum was considered to present an antimicrobial barrier preventing colonization (e.g. by Staphylococcus aureus and Malassezia). Later on, it was found that the pH influences skin barrier function, lipid synthesis and aggregation, epidermal differentiation and desquamation. Enzymes of ceramide metabolism (e.g. β-glucocerebrosidase or acid sphingomyelinase) as well as proteases (e.g. chymotryptic enzyme or cathepsin D linked to epidermal differentiation and desquamation) are regulated by the pH. 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. Normalizing the pH by acidification through topical treatment helps to establish a physiological microbiota, to repair skin barrier, to induce epidermal differentiation and to reduce inflammation., (© 2018 Japanese Dermatological Association.)