Transepidermal Water Loss (TEWL)
Your skin leaks water 24/7. TEWL is the leak rate, and it's the realest skin metric.
Grounded in the research on Transepidermal water loss (Wikipedia)
What even is TEWL?
Transepidermal water loss is the amount of water that passively evaporates out through your skin into the air. It happens constantly, even when you're not sweating. It's measured in grams of water per square meter per hour (g/m2/h). Low number = your barrier is sealed and holding water in. High number = your barrier is leaking. It's basically the single most objective read on how good your skin barrier actually is.
What's actually doing the sealing?
The stratum corneum, the outermost ~15-20 micron layer of dead, flattened skin cells (corneocytes). Think bricks and mortar: corneocytes are the bricks, and the mortar is intercellular lipids, ceramides, cholesterol, and free fatty acids in a roughly 1:1:1 ratio. That lipid mortar is the actual waterproofing. Damage it and water escapes. This is the layer your whole 'barrier' depends on, and it's the layer cheap harsh products wreck first.