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What is pH?

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.

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Where and why is the skin acidic?

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”.


Fotos were kindly provided by M.J. Behne,
Department of Dermatology and Venerology
University Hospital Hamburg-Eppendorf,
Hamburg, Germany

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:

  • An ion-exchanger protein carries hydrogen ions out of the skin cells and sodium ions into the cells. Thus the surroundings of the cells become acidic.
  • Enzymes present in the Stratum corneum produce acids from lipids or proteins when they synthesize special barrier substances, or cleave links between the individual skin cells. Experimental inhibition of any of these sources of acids results in an increased skin pH and impaired barrier function. This proves that the maintenance of pH 5.5 on the skin is crucial for its functionality.

What does the pH 5.5 do for the skin?

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.

The skin’s natural protective acid mantle
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However, many produce substances in their metabolism which smell unpleasant. They are the source of body odour. To minimize body odour and prevent an overgrowth or accumulation of irritant or even toxic microbial metabolites, regular skin cleansing is necessary. It is however not advisable to try to get rid of our lifetime companions completely by using disinfectants, as they form the first line of defense against intruding pathogenic micro-organisms.

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.

The skin has a pH of 5.5

pH neutral is not the pH of healthy skin. pH neutral skin care impairs the skin functions.
Acid mantle and barrier function

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ALL sebamed products have a pH of 5.5.