Transepidermal Water Loss (TEWL)

TEWL is reported in grams per square metre per hour and measured by a probe held against the skin. Open-chamber instruments infer flux from the vapour gradient between two humidity sensors set at known heights in an open cylinder; closed-chamber and condenser instruments measure humidity rise or condensation in a sealed head, which makes them less sensitive to air movement but requires recovery time between readings. Because the quantity is a flux, it responds to barrier damage within minutes.

Typical intact forearm values fall around five to fifteen units, with higher baselines on the face and much higher on palms and soles, while acutely disrupted or eczematous skin can read several times normal. The measurement is famously confounder-sensitive: ambient humidity and temperature, air currents, sweating, recent washing or product application, and insufficient acclimatisation all move it, which is why guidelines specify a controlled room and twenty to thirty minutes of equilibration.

Its value is that it tracks a function rather than a structure, so it can follow barrier recovery kinetics after a standardised insult such as tape stripping, and it correlates with susceptibility to irritants. It remains a surrogate, and a good one only for permeability; it says nothing directly about symptoms, appearance, or how much of an active crossed.

Two errors are constant. The first conflates TEWL with hydration, which corneometry measures, when the two can move in opposite directions. The second reads an immediate post-application drop as barrier repair, since any occlusive film produces one within minutes and it disappears with washing. A repair claim needs measurement at a later visit with the product withheld.