TEWL stands for transepidermal water loss. It is the amount of water that evaporates from the outermost layer of the skin (the stratum corneum) to the surrounding environment. TEWL is a natural process, but it can be increased by certain factors, such as dry air, hot weather, and harsh soaps.

TEWL is measured in grams per square meter per hour (g/m2/h). Normal TEWL rates range from 2.3 to 44 g/m2/h. However, TEWL can be higher in people with dry skin, eczema, or other skin conditions.

From the classic experiment by Pinson in 1942, comparing insensible perspiration from the skin on contralateral body sites with and without sweat glands inactivated by formaldehyde to our modern-day evaluations of skin-surface water evaporation, TEWL is taken as a true reflection of SC barrier function only when there is no sweat gland activity and the skin surface is dry.

This is achieved by conducting measurements in controlled temperature and humidity environments, typically 21°C, 50% relative humidity (RH) with subjects at rest. Basal or baseline TEWL is the resting rate of evaporative loss of water through normal nonperturbed skin. Although variation is observed from one body site to another, basal TEWL is low in normal healthy intact human skin. Basal TEWL is a primary end point used to dimension the variability in skin barrier function across age, body site, ethnicity, and other factors. Although in vitro evidence has challenged the validity of the assumption that TEWL is predictive of the skin’s permeability to topical penetrants more recent validation studies support the generally agreed conclusion that TEWL is the current best objective measure of the skin’s barrier to evaporative water loss.

The three instruments most commonly used to noninvasively measure TEWL are the Tewameter® evaporimeter (Courage & Khazaka, Cologne, Germany), the Dermalab TEWL module (Cortex Technologies, Hadsund, Denmark), and the ServoMed® evaporimeter (Servomed, Varberg, Sweden).

Source: Dermatologic, Cosmeceutic and Cosmeticdevelopment - Kenneth A. Walters, Michael S. Roberts

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