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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.
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).