pH in nature, humans and skin

Abstract

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.

Skin acidification with a water‐in‐oil emulsion (pH 4) restores disrupted epidermal barrier and improves structure of lipid lamellae in the elderly

“…In line with results of this study, our previous study with the same cosmetic products utilizing two different models (a short‐term chemical damage and mechanical damage of epidermal barrier after long‐term treatment) could clearly show that pH is important and an independent factor influencing epidermal barrier . Finally, it is also important to note that not only aging skin, but also diseased skin (atopic dermatitis, psoriasis or ichthyosis vulgaris) have been linked to increased pH …”

“…46,47 Finally, it is also important to note that not only aging skin, but also diseased skin (atopic dermatitis, psoriasis or ichthyosis vulgaris) have been linked to increased pH. 48 Normalization of skin pH following exogenous acidification is improving epidermal permeability barrier homeostasis, SC integrity/cohesion and anti-inflammatory function 12,13 and reduces damage to exogenous stress (SDS) as shown in this study. Moreover, normalization of an increased pH improves the activity of pH-dependent enzymes involved in epidermal differentiation 2,49 differentiation.…”

Barrier damaging effects of n‐propanol in occlusion‐modified tandem repeated irritation test: Modulation by exposure factors and atopic skin disease

“…42 Beyond the impaired barrier function and increased skin diffusivity, the findings of earlier studies showing that increased skin surface pH, as found in AD, results in the activation of proteases involved in the processing of the pro-forms of the IL-1 cytokines in the epidermis, provide another possible explanation for the enhanced barrier responses in atopic compared to healthy skin, observed in the present om-TRIT. 35,[43][44][45] As the volunteers in the study were not genotyped, our results do now allow for conclusions on the influence of FLG mutation carrier status on the outcomes of repeated exposure to npropanol in atopic or healthy skin.…”

Noninvasive objective skin measurement methods for rosacea assessment: a systematic review

“…Biophysical parameters are strongly influenced by intrinsic and extrinsic factors like age, sex, race, circadian rhythm, season, exercise and acclimatization period; 22,95,99,100,[124][125][126][127][128][129][130][131] therefore data collection is inherently difficult to standardize.…”