[noun] The PH (potential of Hydrogen) is measured on a scale of 1 (very acidic) to 14 (very basic, alkaline). The outer surface of the epidermis is protected by a hydrolipidic film with a typical average PH of 5.5, meaning slightly acidic. This acidity improves epidermal protection from the bacteria present on the skin as well as harmful external environmental elements (UVA, UVB, pollution, air conditioning, etc.). These external elements can nevertheless upset the skin’s PH balance and naturally acidic protection, leaving the skin vulnerable to reactions, such as redness and irritation.