Urea

Urea is a component of the skin's natural moisturising factor. Below about 10 % it acts as a humectant and also upregulates barrier genes; above 20 % it becomes keratolytic, dissolving the bonds between corneocytes[1][2].

The evidence

Trials in xerosis, ichthyosis and atopic dermatitis consistently show improved hydration, reduced TEWL and softer skin[1]. For thick heel or elbow skin, 20–40 % urea is the standard.

Pairs with lactic acidLactic acidModerateLactic Acid · Alpha-hydroxy acidGentler, larger AHA that also hydrates; good for texture in drier or more reactive skin.Open in the Index → in the classic "keratolytic-humectant" formulas, and with glycerinGlycerinStrongGlycerin · Polyol humectantThe workhorse humectant: cheap, endlessly studied, and better than the fashionable alternatives at holding water.Open in the Index → and ceramide NPCeramide NPModerateCeramide NP · Sphingolipid (non-hydroxy fatty acid, phytosphingosine)The most abundant ceramide class in the stratum corneum; topical replacement improves barrier in controlled trials.Open in the Index → for straightforward dry skin.