Stands for “transepidermal water loss,” and refers to the amount of moisture or water that passes through our skin barrier and is lost from evaporation (but unrelated to sweating.) A clinical TEWL test using an instrument called a Tewameter® is used to evaluate how much water is lost over a given time period, and can be a measure of the effectiveness of the skin’s barrier, or how well a product supports the skin barrier.
When a skincare product is designed to support the skin barrier, you want to see a DECREASE in TEWL, meaning less water is evaporating off the surface of the skin. This is because a healthy skin barrier can trap and hold moisture in the skin, thereby reducing transepidermal water loss.
Especially look for this test when shopping for moisturizers. If there are claims of a moisturizer improving the skin barrier and / or a reference to a TEWL test, check if the test was done on the final formula, or if it’s only linked to a certain ingredient in the formula. More often than not, the barrier repair, or TEWL claim is coming from the ingredient supplier and you’re not getting the whole picture.
When Dr. Whitney Bowe Beauty claims a product supports a healthy skin barrier…
A third-party laboratory has conducted a TWEL test with the product’s final formula using a Tewameter®, and test clinically-validated that the product decreases the skin’s water loss (indicating an improvement in skin barrier function).