Skin and hydration: what is transepidermal water loss (TEWL)?

The water conditions both the architecture and the biological function of the skin.

Its distribution is, however, very uneven across the skin’s layers. The dermis concentrates about 80% of the skin’s total water. This high content is explained by the richness of its extracellular matrix in hydrophilic macromolecules, including hyaluronic acid. Thanks to its numerous negative charges, the latter attracts and binds water molecules by forming a dense, semi-fluid gel. This strongly bound water is considered poorly mobilizable: it contributes to tissue suppleness and volume maintenance, yet does not circulate freely or evaporate.

The epidermis, by contrast, is not vascularized and depends entirely on passive diffusion of water from the dermis. Its most superficial layer, the stratum corneum, contains only 10 to 20% water. Yet it is this fraction that determines the visible and perceived hydration state of the skin. Unlike dermal water, that of the epidermis is mobilizable: it moves along osmotic and transepidermal gradients. Some of it eventually reaches the surface and gradually evaporates, even in the absence of visible perspiration.

Beyond its structural role, water is essential to cellular function of the skin. It serves as the medium for metabolic reactions, modulates enzymatic activity—especially that involved in desquamation—and facilitates signal diffusion between cells. It also promotes wound healing by creating an environment conducive to cell migration and collagen synthesis, while contributing to thermoregulation through sweat evaporation.

Even when the skin is not perspiring, it continuously loses a small amount of water.

This physiological phenomenon is called transepidermal water loss (TEWL). It corresponds to the passive diffusive flux of water that migrates from the deep layers of the skin, which are rich in water, toward the skin surface, before evaporating into the environment. This diffusion is continuous and independent of sweating : it occurs even at rest, at room temperature. TEWL is an indicator of the effectiveness of the skin barrier.

It is expressed in grams of water lost per square meter of skin per hour (g/m²/h) and measured using specialized devices used in dermatology and clinical research, such as evaporimeters or tewameters. An increase in TEWL is generally associated with a compromised skin barrier, while a normal or low value is interpreted as a sign of an intact or restored barrier. However, this is not an absolute universal value: there is no single “normal TEWL” applicable to all skin. In fact, transepidermal water loss varies considerably depending on the area. Thin regions, those rich in sweat glands, or areas subjected to friction show higher values than other zones.

Transepidermal water loss is also influenced by numerous external factors : ambient temperature, relative humidity, season, pollution... For this reason, IWL is more of a relative parameter than an absolute one. Moreover, the skin aging alters transepidermal water loss. Contrary to the intuitive idea that aged skin would be more permeable, several analyses suggest that TEWL in older individuals is comparable to, or even slightly lower than, that observed in younger adults. Several hypotheses have been proposed to explain this trend: increased corneocyte size, changes in lipid organization, or increased water diffusion resistance in the stratum corneum.

Nevertheless, the clinical significance of this decrease remains debated, since a low TEWL does not necessarily reflect better overall skin hydration, especially given that older adults are often more prone to xerosis. Indeed, transepidermal water loss is not the only parameter indicative of skin hydration. It measures an outgoing water flux, but does not directly inform on the amount of water contained within the stratum corneum. Skin may exhibit a reduced superficial water content without TEWL necessarily being elevated, and vice versa. It is the balance between storage, distribution, retention, and evaporation that determines the state of skin hydration.

Key point : Transepidermal water loss reflects the dynamic equilibrium between water supplied from the dermis and the epidermal barrier’s capacity to limit its diffusion.

If the skin naturally loses water continuously, it fortunately relies on various mechanisms to retain it. Within the stratum corneum, corneocytes contain hygroscopic compounds collectively known as natural moisturizing factors (NMF). Primarily derived from the breakdown of filaggrin, these molecules can attract and hold water. They thus help maintain adequate hydration inside the corneocytes and preserve the skin’s suppleness.

The movement of water between the different layers of the epidermis is also regulated by specialized membrane proteins known as aquaporins. Aquaporin-3, particularly abundant in keratinocytes, facilitates the transport of water and glycerol, contributing to an even distribution of moisture throughout the skin. Finally, the main barrier against water loss relies on the lipid organization of the stratum corneum. Between corneocytes, intercellular lipids, particularly ceramides, organize into compact lamellar sheets. They form a mortar-like barrier that limits water diffusion to the outside. Added to this is the hydrolipid film, a mixture of sebum and sweat, which forms a thin protective layer that reduces evaporation at the skin’s surface.

When these mechanisms become impaired, water loss increases and the skin can become drier and rougher.

• CELLENO L. & al. The dynamics of transepidermal water loss (TEWL) from hydrated skin. Skin Research and Technology (2002).

• BONTÉ F. & al. Skin hydration: a review on its molecular mechanisms. Journal of Cosmetic Dermatology (2007).

• BLUME-PEYTAVI U. & al. Transepidermal water loss in young and aged healthy humans: A systematic review and meta-analysis. Archives of Dermatological Research (2013).

• KOTTNER J. & al. Transepidermal water loss in healthy adults: A systematic review and meta‐analysis update. British Journal of Dermatology (2018).