What Is TEWL?
TEWL — transepidermal water loss — is the measurement of water that passively evaporates through the skin's stratum corneum to the environment. It is a gold-standard clinical measurement of skin barrier integrity: a high TEWL value indicates a compromised or permeable barrier, while a low TEWL indicates an intact, functional barrier. TEWL is measured in g/m²/h using an evaporimeter and is a primary endpoint in clinical dermatology studies evaluating barrier function, moisturiser efficacy, and treatment outcomes.
Normal vs. Elevated TEWL
Healthy facial skin typically shows TEWL values of 4–8 g/m²/h at rest. Forearm skin is lower; palms and soles are typically higher due to thick stratum corneum. In atopic dermatitis (eczema), TEWL on lesional skin can exceed 20–40 g/m²/h. Post-procedure skin (laser, microneedling, peels) shows dramatically elevated TEWL during healing. Elevated TEWL is both a symptom of barrier damage and a driver of further inflammation — a vicious cycle in inflammatory skin conditions.
What Increases TEWL
Barrier disruption (any cause) immediately elevates TEWL. Dry, low-humidity environments pull water out faster. Harsh cleansers strip the lipid matrix, raising TEWL for hours. Inflammation (from any cause: sun, irritants, eczema) compromises barrier architecture. Age reduces ceramide synthesis, gradually elevating baseline TEWL. Over-exfoliation physically disrupts the SC.
The occlusive solution: Occlusives (petrolatum, dimethicone, lanolin) physically reduce TEWL by forming a film on the skin surface that slows water evaporation. Petrolatum (Vaseline) reduces TEWL by approximately 99% — making it the most effective occlusive available, and a cornerstone of barrier-repair dermatology.
See TEWL Science and Lipid Barrier.