Ever since Johnson & Johnson introduced its “pH 5.5” product range, which is in fact formulated at a pH of 5.5, consumers have come to know that the pH of the skin is 5.5—or at least they believe it should be. However, reports in the literature suggest that the pH of skin is lower, more on the order of 4.7. This review discusses the meaning of pH and how it is measured on human skin; which processes regulate the natural skin surface pH; and what processes are regulated by natural skin pH. It then describes the importance of this acidic pH for normal skin homeostasis, what factors cause deviations from natural skin pH, and finally discusses the implications of a pH lower than the generally assumed level of 5.5 for the formulator of cosmetic products.
pH, Natural Skin pH and How to Measure It
Most individuals will have come across pH for the first time during their secondary education, learning that it was the -10log of the concentration of hydrogen ions in an aqueous solution. Water was described as a molecule that could dissociate into H+ ions and OH- ions according to the reaction:
