Water constantly evaporates from the deeper layers of the skin, an effect known as transepidermal water loss (TEWL). By regulating its water content, skin maintains a dry, easily shed surface as a barrier against pathogens, dirt, or damage, while protecting itself from drying out and becoming brittle and rigid. The ability to retain moisture depends on the lipid bilayer between the corneocytes.
Emollients prevent evaporation of water from the skin by forming an occlusive coating on the surface of the stratum corneum. TEWL is normally about 4–8 g/(m²⋅h). A layer of petrolatum applied to normal skin can reduce the TEWL by 50–75% for several hours.
emollient also have an emollient effect, but they act differently, by drawing water into the stratum corneum.
The more lipid in the formulation, the greater the emollient effect. ointments are more emollient than oily creams which are more so than aqueous ccreams , while most lotions have no emollient effect.
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