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

The water determines both the architecture and the biological functioning of the skin.

Its distribution is, however, highly uneven across the skin layers. The dermis contains about 80% of the skin’s total water. This high content is explained by the richness of its extracellular matrix in hydrophilic macromolecules, including the hyaluronic acid. Owing to its numerous negative charges, the latter attracts and retains water molecules forming a dense semi-fluid gel. This tightly bound water is termed minimally mobile: it contributes to tissue suppleness and the maintenance of tissue volume without circulating freely or evaporating.

Unlike the dermis, the epidermis is not vascularised and relies entirely on passive diffusion of water from the dermis. Its outermost layer, the stratum corneum, contains only 10 to 20% water. Yet it is this fraction that determines the visible and perceived hydration status of the skin. Unlike dermal water, that within the epidermis is mobilisable: it circulates along osmotic and transepidermal gradients. Some of it ultimately reaches the surface and evaporates gradually, even without any visible perspiration.

Beyond its structural role, water is essential to cellular function of the skin. It forms the medium for metabolic reactions, conditions enzymatic activity, particularly that involved in desquamation, and permits the transmission of signals 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 constantly loses a small amount of water.

This physiological phenomenon is known as transepidermal water loss (TEWL). It corresponds to the passive diffusive flux of water migrating from the deeper layers of the skin, which are rich in water, towards the skin surface before evaporating into the environment. This diffusion is continuous and independent of perspiration : it occurs even at rest, at ambient temperature. IWL is an indicator of the effectiveness of the skin barrier.

It is expressed in grams of water lost per square metre of skin per hour (g/m²/h) and is measured using specialised devices employed in dermatology and clinical research, such as evaporimeters or tewameters. An increase in TEWL is generally associated with impaired skin barrier function, whereas a normal or low value is interpreted as indicating an intact or restored barrier. However, this is not a universal absolute value: there is no single “normal TEWL” applicable to all skin. Indeed, transepidermal water loss varies considerably across different areas. Thin regions, those rich in sweat glands or subject to friction, exhibit higher values than other areas.

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

Nevertheless, the clinical significance of this reduction remains debated, as low TEWL does not necessarily reflect better overall skin hydration, particularly since older adults are known to be more prone to xerosis. Indeed, it should be noted that transepidermal water loss is not the only parameter indicative of skin hydration. It measures an outward water flux, but does not directly indicate the amount of water contained in the stratum corneum. A skin might present a reduced superficial water content without TEWL necessarily being high, and vice versa. It is the balance between storage, distribution, retention and evaporation that determines the state of skin hydration.

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

While the skin naturally loses water continuously, it fortunately has several mechanisms to retain it. Within the stratum corneum, corneocytes house hygroscopic compounds grouped under the term natural moisturising factors (NMF). Primarily originating from the breakdown of filaggrin, these molecules have the capacity to capture and retain water. They thus enable the maintenance of a sufficient hydration within the corneocytes and preserve the skin’s suppleness.

The movement of water between the different layers of the epidermis is also regulated by specialised membrane proteins known as aquaporins. Aquaporin-3, particularly abundant in keratinocytes, facilitates the passage of water as well as glycerol, contributing to an even distribution of moisture within the skin. Finally, the main resistance to water loss rests on the lipid organisation of the stratum corneum. Between corneocytes, intercellular lipids, notably ceramides, organise into compact lamellar sheets. They thereby form a cement that limits the outward diffusion of water. Added to this is the hydrolipidic film, a mixture of sebum and sweat, which forms a thin protective layer reducing evaporation at the skin’s surface.

When these mechanisms become impaired, water loss increases and the skin may 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).