Transepidermal Water Loss (TEWL) – Skin Barrier

What is Transepidermal Water Loss?

Transepidermal water loss (abbreviated TEWL) describes the passive diffusion of water vapour through the skin into the surrounding air. This process is distinct from active sweating and occurs continuously without conscious control. In dermatology and cosmetic science, TEWL is a widely used biomarker that provides objective information about the integrity of the skin barrier.

The outermost layer of the skin, the stratum corneum (horny layer), acts as the primary physical and chemical barrier between the body and its environment. It regulates water exchange and protects against environmental stressors. A healthy, intact stratum corneum results in low TEWL values, while a damaged or compromised barrier leads to significantly elevated water loss.

Physiology of the Skin Barrier

The stratum corneum consists of flattened, dead skin cells called corneocytes, embedded in a lipid-rich matrix. This architecture is commonly described using the brick-and-mortar model: corneocytes represent the bricks, and the surrounding lipids -- primarily ceramides, free fatty acids, and cholesterol -- form the mortar. Together, they create an effective waterproofing system.

• Ceramides: The most abundant lipid class in the stratum corneum, essential for barrier integrity and water retention.
• Natural Moisturising Factor (NMF): Hygroscopic compounds within corneocytes that bind water and maintain the flexibility and hydration of the horny layer.
• Skin surface pH: A slightly acidic pH (4.5-5.5) supports enzymatic processes within the stratum corneum and inhibits the growth of pathogenic microorganisms.

Measurement of TEWL

TEWL is measured non-invasively using specialised instruments called tewameters or evaporimeters. These devices detect the water vapour gradient directly above the skin surface and calculate the evaporation rate, expressed in g/(m²·h) (grams per square metre per hour).

Normal Values and Influencing Factors

Under standardised conditions, TEWL values on healthy skin typically range from 2 to 10 g/(m²·h), depending on the body site and individual factors. Key variables that can affect measurements include:

• Ambient temperature and relative humidity
• Body region measured (e.g., face, palms, forearm)
• Age of the subject
• Time of day and acclimatisation period
• Recent application of skincare products

Causes of Elevated TEWL

An elevated TEWL value indicates a disrupted or weakened skin barrier. Common causes include:

• Atopic dermatitis (eczema): A genetically influenced barrier defect, often associated with ceramide deficiency and loss-of-function mutations in the filaggrin gene.
• Psoriasis: Abnormal keratinocyte differentiation leads to structural barrier defects.
• Contact dermatitis: Irritant or allergic inflammatory reactions damage the lipid structure of the stratum corneum.
• Ichthyoses: Inherited keratinisation disorders characterised by massively elevated TEWL.
• Mechanical or chemical damage: Frequent washing, harsh surfactants, UV radiation, or physical abrasion.
• Ageing: Lipid synthesis in the skin declines with age, gradually reducing barrier efficiency.

Clinical Relevance

TEWL is an established biomarker used across dermatological research, the cosmetics industry, and clinical practice. Applications include:

• Objective assessment of the severity of skin diseases.
• Evaluation of the efficacy of moisturisers, emollients, and barrier repair products.
• Early identification of newborns at risk of developing atopic dermatitis.
• Skin compatibility testing during pharmaceutical and cosmetic product development.

Treatment and Prevention of Elevated TEWL

Therapeutic strategies aim to restore and strengthen the skin barrier while minimising water loss. The following approaches are commonly employed:

Topical Treatments

• Emollients and moisturisers: Products containing ceramides, hyaluronic acid, glycerol, and urea help restore barrier function and improve skin hydration.
• Occlusive agents: Substances such as petrolatum or lanolin form a physical film on the skin surface, directly reducing TEWL.
• Corticosteroids and calcineurin inhibitors: Used for inflammatory skin conditions to reduce inflammation and support secondary barrier regeneration.

General Skincare Measures

• Short, gentle bathing or showering with lukewarm water.
• Use of pH-neutral, soap-free cleansers.
• Applying moisturiser immediately after washing (within 3 minutes) to lock in hydration.
• Protection against extreme cold, wind, and low-humidity environments.

TEWL in Newborns and Infants

In newborns, the skin barrier is not yet fully mature. TEWL values are markedly elevated in the first weeks of life and gradually normalise over the first few months. Research indicates that elevated TEWL in the first days of life is a predictive marker for the later development of atopic dermatitis and other allergic conditions. Early and consistent use of barrier-supporting skincare products may potentially reduce this risk.

References

1. Fluhr JW, Darlenski R, Angelova-Fischer I, Tsankov N, Berardesca E. Skin irritation and sensitization: mechanisms and new approaches for risk assessment. Skin Pharmacol Physiol. 2008;21(3):124-135.
2. Elias PM. Stratum corneum defensive functions: an integrated view. J Invest Dermatol. 2005;125(2):183-200.
3. Lowe AJ et al. Transepidermal water loss as a predictor of atopic eczema development. J Allergy Clin Immunol. 2018;141(5):1682-1689.