TEWL
The passive diffusion of water vapour from inside the body through the epidermis to the external environment. TEWL is the single most important measure of skin barrier integrity — a high TEWL reading indicates a compromised or damaged barrier that is failing to retain moisture. UV radiation, harsh surfactants, over-exfoliation, cold weather, and skin conditions like eczema and psoriasis all increase TEWL. Reducing TEWL is the primary goal of barrier-repair skincare.
Measured in g/m²/h with a tewameter or evaporimeter. Normal facial TEWL: 6–10 g/m²/h. A reading above 25 indicates significant barrier compromise.
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