pH — the 'potential of hydrogen' or 'power of hydrogen'. The pH scale is a the numerical scale that measures acidity and alkalinity, ranging from 0 (most acidic) to 14  (most basic). A pH of 7.0 is considered neutral. The importance of a product’s pH is based on its correlation to the skin’s pH. Human skin has a pH in the acidic range, varying from 4.4 to 5.6 depending on the individual and the area of skin tested. The value of the surface pH is due to acids present in the stratum corneum. External factors, such as perspiration, tend to make the skin more acidic. The higher the skin’s pH number the less acidic it is, and the greater its sensitivity reaction to very acidic compounds, often experienced as burning or redness. The lower the skin’s numeric pH the more acidic it is, and thus it will be less sensitive to acid compounds such as peelings and other exfoliants. As a practical example, skin with a pH of 5 will be more sensitive to an AHA product with a pH of 3.8 than will skin with a pH of 4.4.