Humectants are water-soluble materials with high water absorption capabilities. They are hygroscopic and therefore able to attract water from the atmosphere (if atmospheric humidity is greater than 80 percent) and from the underlying epidermis. Although humectants may draw water from the environment to help hydrate the skin, in low-humidity conditions they may take water from the deeper epidermis and dermis, resulting in increased skin dryness.1 For this reason, they work better when combined with occlusives. Humectants are also popular additives to cosmetic moisturizers for many reasons. They prevent product evaporation and thickening, which increases the shelf life of formulations, and some humectants help prevent bacterial growth in products.2 Humectants can cause an almost immediate improvement in skin texture because they draw water into the skin, causing a slight swelling of the stratum corneum (SC) that gives the perception of smoother skin with fewer wrinkles. Humectants, by inducing swelling, can temporarily give the user the perception of increased skin firmness. Humectants have been shown to enhance the penetration of other ingredients by causing swelling of keratinocytes,3 and disruption of the skin barrier by loosening the closely packed SC cells.4 Propylene glycol enhances penetration of minoxidil and steroids,5 while hyaluronic acid increases drug delivery in prescription medications such as Diclofenac.4 Examples of commonly used humectants include glycerin, sorbitol, sodium hyaluronate (hyaluronic acid), urea, propylene glycol, α-hydroxy acids, and sugars.
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