Water loss through the skin, a key indicator of a healthy barrier
By Jeremy Fain, founder of Restelys. Updated 17 October 2026. 2 sources cited.
Skin with a strong barrier lets little water escape. That loss can be measured, and it says a lot about what a care product does.
Dermatologists speak of transepidermal water loss, or TEWL: the amount of water that crosses the skin and evaporates. The more damaged the barrier, the higher this loss.
It is measured with devices that capture water vapour above the skin, following precise European guidance. It is one of the reference methods for assessing the mildness of a cleanser or the repairing effect of a care.
It also explains our choices: gentle cleansing bases, which increase this loss less than a harsh soap, and emollients that limit it.
Sources
- Rogiers V. “EEMCO guidance for the assessment of transepidermal water loss in cosmetic sciences”. Skin Pharmacology and Applied Skin Physiology, 2001.
- Proksch E., Brandner J.M., Jensen J.M. “The skin: an indispensable barrier”. Experimental Dermatology, 2008.