Transepidermal water loss refers to the amount of condensed water that evaporates from a fixed area of the stratum corneum of the surface of the skin over a set period of time. When the skin is healthy, the transepidermal water loss ratio is directly proportional. But in conditions like atopic dermatitis there is an increase in TEWL and a decrease in skin hydration.
It is used widely as a tool to measure skin barrier function in healthy skin as well as to check skin barrier dysfunction in people with skin conditions such as atopic dermatitis. TEWL measure varies greatly at different anatomical skin sites as well as environmental conditions such as humidity, sweat gland activity, skin temperature as well as corneocyte properties.
Stratum corneum: the outermost layer of the epidermis, made up of mostly lipids and keratin.
Corneocyte: dead or dying skin cells(keratinocytes) that form the matrix of the stratum corneum.
Transepidermal water loss is important because it is used as a direct measure of skin hydration and health. Although the process of the diffusion of water across the skin layers is normal, excessive transepidermal water loss can lead to dry, cracked, irritated and possibly infected skin, fine lines and wrinkles.
It’s important to keep the skin hydrated and to help the skin maintain its balance as it pertains to TEWL. Hydration with water as in with short baths and showers, as well as internal hydration as in drinking plenty of water is important in maintaining hydration. The rule of thumb with TEWL is to hydrate with a shower or bath and IMMEDIATELY seal with an occlusive emollient to decrease the amount of water loss.
References: picture: http://www.skinetrate.com/transepidermal-water-loss.html