Refers to something with a pH (potential of hydrogen) value lower than 7. 7 is neutral, eg water. Skin has an average pH level of 5.5, often referred to as the acid mantle. This is a protective mechanism that wards of infection and bacteria. Bacteria prefer an alkaline environment, and increased skin alkalinity exposes the skin to various skin diseases. Studies have shown that bacteria responsible for acne proliferate in skin that is alkaline.

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