Power of hydrogen, a term more commonly known as pH, is the measure of how concentrated hydrogen ions are in a solution. Neutral pH, like that of distilled water measures a 7. Anything below 7 is considered acidic; anything above is alkaline (or basic). For context, lemon juice is highly acidic with a pH of 2.2 while baking soda is alkaline with a pH of 9. (Two ‘natural’ products not meant for your face by the way, no matter what Pinterest tells you).
In 1928, scientists A. Marchionini and H. Schade developed a method to prove the skin naturally secretes a thin film, they termed the acid mantle, to protect against external threats. This film is left over after sweat evaporates and consists of oils, amino acids and other components of the skin’s own natural moisturising factor. The pH of the acid mantle is generally close to 4.7 but varies from person to person and increases as we age.
Maintaining a gently acidic pH is critical to proper skin barrier function. Skin barrier formation relies heavily on pH dependent enzymes and a very close relationship exists between skin surface pH and the maintenance of healthy flora. When judging by the parameters of barrier function and moisturization, skin with pH values below 5 consistently demonstrates better overall condition. Abnormally high acid mantle pH is present in skin conditions like atopic dermatitis, acne, rosacea and malassezia.
Washing with water (pH 7 – 8.5) and soap (pH 9 – 10) causes a temporary increase in the surface pH of skin. For most, this returns to normal within a few hours. However, increased skin surface pH and a weakened ability to buffer against alkalizing agents is associated with aging and individuals with sensitive skin. In these cases, the application of gently acidic products (pH ≃ 4) is suggested to improve skin barrier integrity and moisturization.
Despite this, many commercial skincare products are formulated at pH levels of 6 and higher to support the use of synthetic thickeners (carbomer and crosslinked acrylate copolymers). What's worse, the closer formulations are to neutral pH, the higher the preservative level required to maintain product safety; another key issue for consumers with skin sensitivities.
In support of informed skincare purchasing decisions, Aurum lists the pH of each product along with full ingredients here.