What is TEWL?
Transepidermal water loss (TEWL) is the passive evaporation of water from inside the body, through the skin, to the environment. It is the single best objective measure of barrier function.
Healthy vs. compromised
A healthy barrier has low TEWL — water stays in. A compromised barrier has high TEWL — water escapes, leaving skin dehydrated.
How to reduce TEWL
- Humectants (hyaluronic acid, glycerin) draw water into the skin
- Emollients (ceramides, fatty acids) seal the gaps in the barrier
- Occlusives (petrolatum, squalane) form a physical seal
The most effective strategy is a humectant + emollient combination: hydrate first, then seal.