Transepidermal Water Loss (TEWL): Definition, Measurement and Skin Barrier Function

What Is Transepidermal Water Loss?

Transepidermal Water Loss, commonly abbreviated as TEWL, describes the amount of water that evaporates from the outermost layer of the skin, known as the stratum corneum, into the surrounding environment.

TEWL is a natural physiological process. Even healthy skin continuously loses a certain amount of water to the environment. However, the rate of water loss can increase when the skin barrier is compromised or when environmental and external factors place additional stress on the skin.

Factors such as dry air, hot weather and harsh cleansing products can increase TEWL. Elevated TEWL is also commonly associated with dry skin and certain skin conditions.

TEWL and the Skin Barrier

The Role of the Stratum Corneum

The stratum corneum forms the outermost layer of the epidermis and plays an important role in limiting the loss of water from the skin.

A healthy and intact skin barrier helps regulate the movement of water between the skin and the surrounding environment. When the barrier is disrupted, water can evaporate more readily from the skin surface, resulting in increased TEWL.

TEWL as an Indicator of Barrier Function

TEWL is widely used as an objective, non-invasive measurement of the skin's barrier to evaporative water loss.

The original research history of TEWL measurements dates back to the classic experiment by Pinson in 1942, which compared insensible perspiration from corresponding skin sites with and without active sweat glands.

Today, TEWL measurements are used in dermatological and cosmetic research to evaluate changes in skin barrier function and to assess how different products or treatments affect the skin.

What Can Increase TEWL?

Dry Environmental Conditions

Dry air can increase the rate at which water evaporates from the skin. Low environmental humidity can therefore contribute to increased TEWL and a drier skin feel.

Heat

Higher temperatures can influence water evaporation from the skin and may contribute to changes in TEWL measurements.

Harsh Cleansing

Aggressive cleansing products or excessive cleansing can disturb the skin surface and its barrier properties. This may increase water loss and contribute to dry or uncomfortable skin.

Skin Barrier Disruption

Any factor that compromises the integrity of the stratum corneum can potentially increase TEWL. This is why TEWL is frequently used when evaluating the condition and recovery of the skin barrier.

How Is TEWL Measured?

TEWL is measured by assessing the amount of water vapour leaving the skin surface. The result is commonly expressed in grams per square metre per hour (g/m²/h).

Reported TEWL values can vary considerably depending on factors such as body site, environmental conditions, age and individual skin characteristics.

For TEWL to provide a meaningful indication of skin barrier function, measurements need to be performed under controlled conditions and without active sweating.

Controlled Conditions for TEWL Measurement

Temperature and Relative Humidity

TEWL measurements are typically performed in controlled temperature and humidity conditions.

Subjects are generally allowed to rest before measurement so that external factors such as physical activity and increased sweating do not interfere with the results.

Dry Skin Surface

The skin surface should be dry when TEWL is measured. Active sweat gland activity can affect water evaporation independently of the passive water loss associated with the skin barrier.

For this reason, TEWL is considered a true reflection of stratum corneum barrier function only when the skin surface is dry and sweat gland activity is absent or adequately controlled.

Basal TEWL

Basal or baseline TEWL refers to the resting rate of evaporative water loss through normal, non-perturbed skin.

Basal TEWL is generally low in healthy, intact skin, although significant differences can occur between different anatomical sites.

It is used as an important endpoint when studying natural variation in skin barrier function across factors such as age, body site and other individual characteristics.

TEWL in Cosmetic Research

Evaluating Skin Barrier Products

TEWL can be used to evaluate whether a cosmetic product supports or affects the skin barrier. Changes in TEWL before and after product application can provide objective information about changes in evaporative water loss.

Assessing Moisturising Formulations

Moisturising and barrier-supporting formulations can be evaluated using TEWL alongside other measurements of skin hydration and skin condition.

A reduction in TEWL may indicate improved resistance to evaporative water loss, although TEWL should be interpreted together with other relevant parameters rather than used as the sole measure of product performance.

Measuring Skin Stress and Recovery

TEWL can also be used in cosmetic research to assess how the skin responds to controlled stress or irritation and how quickly barrier function returns towards baseline.

TEWL Measurement Instruments

Several non-invasive instruments are commonly used to measure TEWL.

Tewameter® Evaporimeter

The Tewameter® evaporimeter, manufactured by Courage + Khazaka in Cologne, Germany, is one of the established instruments used for measuring transepidermal water loss.

Dermalab TEWL Module

The Dermalab TEWL module, developed by Cortex Technologies in Hadsund, Denmark, is another system used for non-invasive TEWL measurements.

ServoMed® Evaporimeter

The ServoMed® evaporimeter, manufactured by Servomed in Varberg, Sweden, is also used for evaluating evaporative water loss from the skin.

TEWL and Skin Permeability

TEWL is primarily a measurement of evaporative water loss and skin barrier function. The relationship between TEWL and the skin's permeability to topical substances has been investigated extensively.

While some in vitro evidence has questioned whether TEWL can directly predict the permeability of skin to topical substances, subsequent validation studies have continued to support TEWL as an important objective measure of the skin barrier to evaporative water loss.

For cosmetic formulation and research, TEWL is therefore best understood as a specific measure of water loss and barrier function rather than a universal measurement of skin permeability.

Why Is TEWL Important in Cosmetic Formulation?

TEWL provides formulators and cosmetic researchers with an objective parameter for evaluating the condition of the skin barrier.

When developing moisturisers, barrier-supporting products and other skincare formulations, TEWL measurements can complement subjective assessments and other instrumental measurements. This can help provide a more complete picture of how a product influences skin hydration and barrier function.

References

Source: Dermatologic, Cosmeceutic and Cosmetic Development – Kenneth A. Walters, Michael S. Roberts.