Transepidermal Water Loss (TEWL)

Transepidermal water loss, or TEWL, is the amount of water that passively evaporates from inside the skin out through the epidermis into the air. A low, steady level of TEWL is normal and constant; the concept matters because the rate reflects how well the skin barrier is holding water in.

The stratum corneum and its lipid matrix are what slow this evaporation. When the barrier is healthy, TEWL stays low. When it is disrupted, by over-exfoliation, harsh surfactants, dry air, or damage, TEWL rises and the skin loses water faster than it can be replaced, which contributes to tightness, roughness and dehydrated skin. TEWL is often used in research as an indirect measure of barrier integrity.

Skincare strategies address TEWL from two directions. Humectants pull and hold water in the upper layers, while occlusives form a surface film that physically slows evaporation; emollients help smooth and reinforce the lipid structure in between. Layering a humectant under an occlusive is a common way to both add water and reduce its loss. Because environmental factors such as low humidity and wind increase evaporation, barrier support becomes especially relevant in cold or dry conditions.