Transepidermal Water Loss (TEWL) Explained

What is Transepidermal Water Loss?

Transepidermal water loss (abbreviated TEWL) describes the passive diffusion and evaporation of water from the body through the outermost layers of the skin into the surrounding environment. This process occurs continuously and is distinct from active sweating (sensible perspiration). TEWL belongs to the category of insensible perspiration – the unnoticed, ongoing loss of water through the skin.

The skin is the largest organ of the human body and serves as a critical protective barrier against fluid loss, pathogens, allergens, and environmental irritants. This protective function is commonly referred to as the skin barrier. An elevated TEWL indicates a compromised or weakened skin barrier.

Physiological Background

The epidermis – the outermost layer of the skin – is composed of several sublayers. The most important of these for barrier function is the stratum corneum (horny layer). It consists of flattened, dead keratinocytes called corneocytes, embedded in a lipid matrix. The key lipids involved are ceramides, cholesterol, and free fatty acids, which together create an effective water-retaining barrier.

When this lipid structure is disrupted, TEWL increases, causing the skin to lose moisture and become dry, rough, or sensitive.

Causes of Elevated TEWL

• Skin diseases: Atopic dermatitis (eczema), psoriasis, ichthyosis, and other chronic skin conditions are typically associated with significantly elevated TEWL.
• Mechanical damage: Excessive washing, scrubbing, or the use of harsh soaps and detergents can disrupt the lipid barrier of the skin.
• Environmental factors: Dry air, extreme cold, and intense UV radiation can impair the skin barrier over time.
• Genetic factors: Mutations in the filaggrin gene are strongly associated with impaired barrier function and elevated TEWL, especially in atopic dermatitis.
• Aging: As the skin ages, lipid production decreases, leading to a naturally higher TEWL in older adults.
• Irritant and allergic contact dermatitis: Inflammatory skin reactions increase the permeability of the barrier and elevate TEWL.

Measurement of TEWL

TEWL is measured non-invasively using specialized devices. The most common methods include:

• Open-chamber method (evaporimeter): Measures the water vapor gradient above the skin surface within an open measurement chamber. Common devices include the Tewameter® and VapoMeter®.
• Closed-chamber method: The skin is covered briefly with a closed chamber, and the pressure increase caused by evaporation is recorded.

Results are expressed in g/(m² · h) (grams per square meter per hour). In healthy skin, normal values typically range from approximately 5 to 15 g/(m² · h) depending on the body site. Damaged skin barriers can show values substantially higher than this range.

Clinical Significance

TEWL is considered the gold standard for assessing skin barrier function in dermatology, cosmetology, and pharmaceutical research. It is used to:

• objectively assess the severity of skin conditions such as atopic dermatitis or psoriasis,
• evaluate the efficacy of skincare products, creams, and ointments,
• study the impact of occupational exposure on skin health (e.g., frequent contact with water or chemicals),
• monitor treatment progress and document therapeutic outcomes.

Treatment and Prevention of Elevated TEWL

The primary therapeutic goal is to restore and strengthen the skin barrier. Relevant approaches include:

• Emollients and moisturizers: Replenishing creams and lotions containing ceramides, glycerin, hyaluronic acid, or urea help reduce TEWL and reinforce the moisture barrier.
• Gentle skin cleansing: pH-neutral, surfactant-free cleansing products minimize damage to the lipid barrier.
• Avoidance of triggers: Irritants, allergens, and extreme environmental conditions should be avoided where possible.
• Pharmacological treatment: For conditions like atopic dermatitis, topical corticosteroids, calcineurin inhibitors, or biologics (e.g., dupilumab) may be used to reduce inflammation and improve barrier function.
• Indoor humidification: Using a humidifier in dry indoor environments can help reduce moisture loss from the skin.

TEWL in Infants and Children

In newborns and infants, the skin barrier is not yet fully developed. As a result, TEWL is physiologically higher in young children than in adults. Children with a genetic filaggrin deficiency are at increased risk of developing atopic dermatitis early in life. Research suggests that early and consistent application of emollients in at-risk infants may delay or even prevent the onset of eczema.

References

1. Proksch E, Brandner JM, Jensen JM. The skin: an indispensable barrier. Experimental Dermatology. 2008;17(12):1063-1072. doi:10.1111/j.1600-0625.2008.00786.x
2. Fluhr JW, Darlenski R, Angelova-Fischer I, Tsankov N, Basketter D. Skin irritation and sensitization: mechanisms and new approaches for risk assessment. Skin Pharmacology and Physiology. 2008;21(3):124-135.
3. Elias PM. Stratum corneum defensive functions: an integrated view. Journal of Investigative Dermatology. 2005;125(2):183-200.