Propylene Glycol, USP Propylene Glycol, is a nearly odorless clear liquid with a mild sweet taste. In the skin and hair, propylene glycol works as a humescent, which causes retention of moisture content of skin or cosmetic products by preventing the escape of moisture or water. Propylene glycol is one of the most widely used cosmetic ingredients, as it is a humectant. It is a clear, colorelss, viscous liquid, slightly bitter-tasting. Propylene Glycol USP is the most common moisture-carrying vehicle, other than water itself, in cosmetics. Absorbs moisture, acts as a solvent and a wetting ingredient.