Transepidermal Water Loss (TEWL)
Definition
Transepidermal water loss (TEWL) is the process by which water passes from the dermis through the epidermis and evaporates from the skin's surface into the surrounding atmosphere. A certain amount of TEWL is normal and unavoidable, but when the skin barrier is compromised — through damage, dryness, skin conditions, or environmental factors — TEWL increases significantly, leading to dehydrated, dry, tight, and irritated skin. TEWL is measured in dermatological research using devices called evaporimeters and is used as an objective indicator of skin barrier integrity. Factors that increase TEWL include low humidity, wind, cold weather, harsh cleansers, over-exfoliation, and inflammatory skin conditions. Reducing excessive TEWL involves strengthening the skin barrier with ceramides and other barrier-repair ingredients, using humectants to attract moisture, and applying occlusives to physically seal water in. Measuring TEWL is also used to evaluate the effectiveness of moisturizing products.
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