Glycerin
Glycerin (also known as glycerol) is a polyhydric alcohol and a skin-identical ingredient naturally found in the outermost layer of the skin (stratum corneum). It is a super common, safe, and inexpensive humectant that has been used in cosmetics for more than 50 years. It functions not only as a moisture-binding agent but also plays roles in osmoregulation, maintenance of intracellular lipids in a healthy liquid crystal state, and normalization of skin shedding.
Glycerin helps the skin retain water by acting as a humectant, with effectiveness starting from as low as 3% and increasing up to concentrations of 20–40%. It supports the skin barrier by maintaining healthy intracellular lipid states and protecting against irritation caused by over-washing or environmental stressors. High-glycerin formulations (25–40%) have been shown in a large-scale study to outperform lower-glycerin moisturizers, providing superior hydration and a lasting moisture-reservoir effect even after discontinuation of use.
Pure or very high concentrations of glycerin (e.g., 99.7%) should not be applied undiluted to skin, as excessive humectant activity may draw water from deeper skin layers and increase trans-epidermal water loss (TEWL), potentially leaving skin drier. Optimal use is in formulated products, ideally around 10% for a balance of efficacy and cosmetic elegance.
