Remember: TEWL (pronounced like tool) stands for Transepidermal water loss. As the name suggests, it’s how much water evaporates out of your skin. The higher the TEWL rating, the worse the skin barrier. This is a really useful way to test several different claims. For example, the tester may purposely damage a patch of skin to create an area with a high TEWL, then apply a particular cream to bring the value back down. Neat, eh?