pH (pons hydrogenii) is a parameter to determine acidity and alkalinity of solutions of substances dissolved in water. The exact definition of pH is: the negative decadic logarithm of the concentration of hydrogen ions in an aqueous solution. Important is the fact that pH values range from 0 to 14, with low values standing for acids and high values for lyes (alkaline solutions) and the middle is neutral. The further away the pH is from neutral, the stronger is the acid or lye. Water theoretically has a pH of 7, but in reality, distilled water is slightly acidic (pH approximately 6) due to carbon dioxide from the air dissolving in the water forming carbonic acid, whereas tap water contains salts which make it often slightly alkaline (pH 7.5 to 8). Blood and cells are near to neutral with pH 7.4. Gastric juice has pH 1-2, and bile pH 8 to 8.5. The skin surface has an average pH of 5.5.
The skin consists of different layers. The innermost, namely subcutaneous connective tissue, dermis and epidermis are pH neutral, because they consist of living cells and the connective tissue at pH 7.4. The superficial Stratum corneum, or horny layer, contains dead skin cells embedded in lipids and shows an acidic pH as does the hydrolipid film, a mixture of sweat and sebum which covers the whole skin surface. This phenomenon was therefore termed the “acid mantle”.
A close look at the horny layer with the latest technology of imaging devices (Flurorescence Lifetime Imaging Microscopy) shows that it is not acidic throughout, but only patchwise. The acidic patches are rare in the lower horny layer and become abundant towards the surface. This is decisive for the skin’s protective functions, as the processes maintaining them are located in the acidic patches.
Besides metabolites coming from the skin cells, the sebum and sweat glands, and the micro-organisms present on our skin (namely fatty acids such as lactic acid and amino acids), there exist a variety of mechanisms ensuring the skin surface acidity:
The acid mantle has various protective functions. For a long time it has been known to regulate the composition of the skin’s microflora. The skin is completely populated by many different micro-organisms, even directly after cleansing. These bacteria and fungi feed from the scales on the surface as well as sweat and the sebum. Most of them are completely harmless for our health.
As long as the skin is covered with the these harmless microbial species of the resident flora, there is almost no chance for pathogenic germs to settle and proliferate, because under the physiologically acidic conditions on the skin surface, the normal, harmless skin flora can flourish and suppress pathogenic organisms, which usually prefer neutral environments. The protective eco-system of the skin is supported by skin cleansing and care with pH 5.5, but damaged or even destroyed by neutral or alkaline products. An increased skin surface pH after cleansing or the application of skin care products with pH 6 or more will raise the rate of proliferation of pathogenic germs and hence the risk for skin infections.
pH neutral is not the pH of healthy skin. pH neutral skin care impairs the skin functions.
Acid mantle and barrier function
ALL sebamed products have a pH of 5.5.