Transepidermal water loss (TEWL) is the passive evaporation of water from the skin's deeper layers through the stratum corneum. It is not sweating — sweat is active water secretion from eccrine glands. TEWL is continuous and measurable, and it is the single most reliable clinical indicator of barrier function integrity.
A healthy barrier maintains TEWL at approximately 5-10 g/m²/h on normal skin. When the barrier is compromised — by stripping cleansers, excessive exfoliation, or environmental factors — TEWL can rise to 20-30 g/m²/h or higher. This accelerated water loss triggers a feedback loop: the skin dehydrates, keratinocyte signalling becomes erratic, inflammatory cytokines are released, and the barrier degrades further.
The relationship between TEWL and skin hydration is not linear. A small increase in TEWL (from 5 to 8 g/m²/h) can reduce stratum corneum water content by 30% or more. This is because water moves through skin along a concentration gradient — the gradient collapses quickly when the barrier thins, even by a small amount.
Ingredients that reduce TEWL are classified by mechanism: occlusives (petrolatum, dimethicone, squalane) form a physical film; humectants (glycerin, hyaluronic acid) draw water from the dermis; emollients (ceramides, fatty acids) fill the gaps between corneocytes. The most effective barrier repair formulations combine all three in ratios that match natural skin composition.
Measuring TEWL objectively requires a tewameter or evaporimeter — not a consumer device. However, the symptoms are easy to recognise: persistent tightness after cleansing, shine that is not oil (it is the stratum corneum reflecting light irregularly due to dehydration), and a sensation of sensitivity to products that were previously tolerated.