The skin barrier: what it does and what happens when it fails

The outermost layer of the skin, the stratum corneum, is often described as a brick wall. Corneocytes are the bricks and the intercellular lipid matrix is the mortar holding them together. That mortar is made largely of ceramides, cholesterol and free fatty acids. When it is intact, water stays in and irritants stay out.

When the lipid matrix is depleted, water escapes faster. That loss is measurable as transepidermal water loss, or TEWL, and it is the standard endpoint in barrier studies for exactly that reason. Reduced ceramide levels in the stratum corneum of atopic skin were reported as early as 1991 and remain a central explanation for atopic dry skin.

Moisturizers are generally described across three layers of action. Humectants such as glycerin draw water into the stratum corneum. Emollients supply the lipids that make up the mortar. Occlusives such as petrolatum sit on the surface and slow evaporation. Products differ mainly in which layer they emphasise, which is why two creams with similar claims can behave quite differently.

Barrier disruption is not confined to atopic dermatitis. In rosacea, impaired barrier function with elevated TEWL is documented, which is part of why gentle barrier support is a standard part of routine care alongside prescribed treatment. In seborrheic dermatitis, first-line therapy is topical antifungals and corticosteroids, with moisturizers supporting the barrier during and after treatment rather than replacing it.