pH is a measure of the acidity of a solution. Pure water is said to be neutral, with a pH close to 7.0 at 25 °C. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are alkaline.
Adult skin is naturally mildly acidic having what is often referred to as an acid mantle. This acid mantle is formed by the skin’s natural secretions sebum and sweat, which together create a protective layer on the skin surface. This layer has a pH of about 4 to 5.5 and helps to protect the skin both physically and chemically. Physically it is a barrier that screens the skin from the environment (e.g wind and pollutants). Chemically, its natural acidity inhibits the growth of harmful bacteria and fungi.
The skin’s acid mantle develops at puberty, which is why children are more susceptible to disease, viruses and fungal infections such as ringworm. The pH of children’s skin is closer to neutral (pH 7). In adult skin, if the acid mantle is disrupted or loses its acidity, the skin becomes more prone to damage and infection. For this reason highly alkaline products such as soap and detergents are poorly tolerated by sensitive or problem skin.
The role of pH in Acne
Changes in the pH are reported to play a role in the pathogenesis of skin diseases like irritant contact dermatitis, atopic dermatitis, ichthyosis, acne vulgaris and Candida albicans infections.  When such diseases are present skin pH frequently measures above 6, suggesting that disruption of the skin’s acid mantle is a significant factor in skin disease.
In the case of acne, the loss of the acid mantle encourages the growth of Propionibacterium acnes bacteria. P. acnes bacteria are found naturally in human skin, regardless of the presence or absence of acne. However, the number of P. acnes is greatly increased when a person suffers from acne. It has been shown that P. acnes thrives when the skin becomes more alkaline. On the other hand, the growth of P. acnes is inhibited when the skin pH is slightly acidic. Therefore, maintaining the skin’s acid mantle may help to control acne.
 H. Lambers, S. Piessens, A. Bloem, H. Pronk, P. Finke: Natural skin surface pH is on average below 5, which is beneficial for its resident flora, International Journal of Cosmetic Science 2006 28:5 359-370
 Schmid-Wendtner M-H, Korting HC:The pH of the Skin Surface and Its Impact on the Barrier Function. Skin Pharmacol Physiol 2006;19:296-302
 C. Selander, A. Zargari, R. Möllby, O. Rasool, A. Scheynius: Higher pH level, corresponding to that on the skin of patients with atopic eczema, stimulates the release of Malassezia sympodialis allergens, Allergy 2006; 61:8 1002–1008