<scp>pH</scp> in nature, humans and skin

<jats:title>Abstract</jats:title><jats:p>The <jats:styled-content style="fixed-case">pH</jats:styled-content> plays an important physiological role in nature and humans. <jats:styled-content style="fixed-case">pH</jats:styled-content> varies from 1 to 8 in human organs with tight regulation in blood and epithelia of barrier organs. The physiological <jats:styled-content style="fixed-case">pH</jats:styled-content> of the stratum corneum is 4.1–5.8 and several mechanisms contribute to its formation: filaggrin degradation, fatty acid content, sodium‐hydrogen exchanger (<jats:styled-content style="fixed-case">NHE</jats:styled-content>1) activation and melanosome release. First, the acidic <jats:styled-content style="fixed-case">pH</jats:styled-content> of the stratum corneum was considered to present an antimicrobial barrier preventing colonization (e.g. by <jats:italic>Staphylococcus aureus</jats:italic> and <jats:italic>Malassezia</jats:italic>). Later on, it was found that the <jats:styled-content style="fixed-case">pH</jats:styled-content> 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 <jats:styled-content style="fixed-case">pH</jats:styled-content>. Experimental disruption of the physical barrier leads to an increase of <jats:styled-content style="fixed-case">pH</jats:styled-content>, 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 <jats:styled-content style="fixed-case">pH</jats:styled-content>. This is known for atopic dermatitis, irritant contact dermatitis, ichthyosis, rosacea and acne, but also for aged and dry skin. Normalizing the <jats:styled-content style="fixed-case">pH</jats:styled-content> by acidification through topical treatment helps to establish a physiological microbiota, to repair skin barrier, to induce epidermal differentiation and to reduce inflammation.</jats:p>