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The body skin pH can usually vary from 4.0 to 7.0 depending on location. The presence of natural acidic compounds on the skin surface helps maintain the skin’s physicochemical properties as well as its protective functions. Since the slightly acidic pH of the skin is extremely important for the skin’s protective function, the skin is widely known as “acid mantel.” Factors such as age, race, gender, body sites, biochemical differences, and even washing affect the pH of the stratum corneum. Recent clinical studies using an emollient-base finish product using the traditional way of measuring skin pH produced results that indicated an apparent increase in skin pH. The apparent pH increase with these products is most probably an artifact of the skin pH measurement technique. Our findings show that certain petrolatum-based emollients and components could create a protective barrier and help maintain the healthy acidity of the skin. Our work provides new evidence of emollients helping to stabilize skin pH in its natural balanced state rather than affecting it. This new learning should be taken into consideration by other researchers in the area of skin pH as well as in clinical studies to avoid misleading results.

© 2018 S. Karger AG, Basel

Skin, the largest organ that covers the exterior of the body, forms a protective barrier against the environment, and its overall physiology is maintained by its physicochemical properties such as structure, hydration, temperature, pH, oxygen, and carbon dioxide gradients. Skin surface pH influences the stratum corneum’s lipids composition and hydration, the skin’s microbiota, and barrier function.

When a pH electrode is brought into contact with normal skin surface, the pH of the liquid on the electrode becomes more acidic, with an average pH reading of 5–6. Schade and Marchonini underlined acidity and the skin’s protective feature and called it the “acid mantel” in 1928 [1]. Endogenous and exogenous factors such as eccrine and sebaceous secretions, anatomic sites, moisture, proton pumps, genetic predispositions, and age influence the skin pH [2, 3]. Active proton pumps (e.g., NHE1 – sodium/hydrogen anion exchange proteins) acidify the intracellular space in the lower stratum corneum [3, 4]. Lactic acid produced by passive processes acidifies the superficial layers of the skin [2]. Other important components of passive metabolic processes such as free fatty acids generated by lipases, cholesterol sulfate, urocanic acid, and pyrrolidone carboxylic acid contribute to the acid mantel [2, 3]. Oil and sweat excreted by sebaceous and sudoriferous glands on the pores of the skin also have a pH of about 5.5 [5]. The dry and densely packed top layer of the skin is the first line of defense against many bacteria. Salty secretions from sweat glands create a hyperosmotic environment that is unfavorable for bacteria to grow. Acidic mantel also protects against the colonization of pathogenic bacteria, such as Staphylococcus aureus, that grow best at neutral pH [6]. Factors, such as age, race, gender, body sites (Table 1), and biochemical differences affect the overall pH of the skin. Newborn babies’ body skin pH is usually around 7 (neutral) and, within a month, becomes acidic similar to adult skin. Higher skin pH in infants may be due to the different chemical composition of the skin lipids [7]. Normal skin pH remains unaltered between 18 and 60 years of age. In the older age group, skin pH increases in both men and women [8, 9].

Cutaneous pH plays an important role in maintaining the normal bacterial flora of the skin and preventing pathogenic invasions [10]. The wettability of stratum corneum, which influences the spreading of chemicals and adhesion of pathogens, is also influenced by the pH. Bromberg et al. [11] observed changes in the wettability of human skin as a result of alterations in the pH of an anionic surfactant. Under normal skin pH of 5.5, the growth of Propionibacterium acnes is at its minimum; however, a slight increase in the pH results in increased growth of P. acnes [12]. Changes in skin pH from acidic to alkaline are also related to the development of candidal infections [13], atopic dermatitis, and increased colonization of S. aureus [14, 15]. The higher physiological pH in the axilla region promotes the growth of local flora, which in turn creates underarm odor [16, 17]. Application of a deodorant product reduces the axillary pH leading to the inhibition of the growth of indigenous bacteria [16].

Table 1. Measured pH of human skin at various body locations

Skin pH also plays an important role in wound healing [18–20]. A pH imbalance caused by the overproduction of reactive oxygen species and mitogen-activated protein kinase can lead to collagen breakdown, thereby impacting wound healing [18]. Recent in vivo experiments show that prolonged acidic condition prevents wound closure and re-epithelialization [21].