Humectants are agents that work by being incorporated in the stratum corneum and by attracting the water that is in the lower layers of the epidermis. These agents include urea, sodium lactate, lactic acid, propylene glycol and work even if applied in between baths. Lactic acid has hygroscopic activity at low concentrations and keratolytic activity at higher concentrations. Propylene glycol is a very effective humectant because it is hygroscopic. Urea is a humectant at low concentrations and a keratolytic agent at higher concentrations.
Emollients are agents that soften the skin because they decrease transepidermal water loss.
They are usually oils and work if applied after a bath. They fill the spaces between dry flakes with droplets of oil. Used alone they provide only temporary relief. Keratolytic agents cause cellular damage of the cells in the stratum corneum with the result of cell shedding. Keratoplastic agents cause a normalization of the epidermal cell kinetics by having a cytostatic effect on the basal layer and therefore, slowing down epidermal proliferation.