Transepidermal water loss (TEWL) measurement is today indispensable in evaluating skin barrier function. A non-invasive, quantitative, and reproducible method, it constitutes the reference indicator for barrier claim substantiation studies, whether in the context of strengthening, repair, or protection. This guide details the scientific foundations of TEWL, measurement protocols, available equipment, and key points to consider when designing a valid study.
What is TEWL and why is it the reference measurement?
TEWL measures the flux of water vapor that passively escapes through the skin by diffusion, independent of active sweating. This flux is directly conditioned by the integrity of the stratum corneum: a healthy barrier, with an intact intercellular lipid matrix and functional tight junctions, naturally limits this loss. An impaired barrier, whether through chemical irritation, mechanical trauma, pathology (atopic dermatitis, psoriasis), or environmental factors, allows more water to escape, resulting in a measurable increase in TEWL.
It is this direct relationship between the structural integrity of the stratum corneum and water vapor flux that makes TEWL a functional biomarker, not merely a hydration indicator. It does not measure the quantity of water in the skin, but the skin’s capacity to retain it.
TEWL and hydration: two complementary, non-interchangeable parameters
A common confusion is equating TEWL with a skin hydration measurement. The two parameters are related but distinct:
| Parameter | What it measures | Reference equipment |
|---|---|---|
| TEWL | Passive water vapor flux (barrier permeability) | Tewameter, Aquaflux, Vapometer |
| SC hydration | Water content of corneal layers (capacitance) | Corneometer®, Skicon-200, DPM 9003 |
| Epidermal hydration | Water content of the epidermis | MoistureMeterEpiD |
| Dermal hydration | Water content of the dermis | MoistureMeterD |
A formula can improve stratum corneum hydration (occlusive or humectant effect) without modifying TEWL, and conversely, a repairing formula can reduce TEWL without an immediate increase in capacitance. Combining both parameters in the same protocol considerably strengthens the robustness of the claims file.
TEWL measurement equipment: characteristics and differences
Several devices are available on the market, based on different measurement principles. The choice of equipment must be consistent with the literature and the practices of the selected CRO, to ensure comparability of results.
Open principle (open chamber)
Open-chamber devices measure the water vapor gradient between two sensors placed at different distances from the skin surface. This is the principle used by the Tewameter 300 and Nano (Courage+Khazaka), which remains the most widely used historical reference in clinical research and in the international literature. The Evaporimeter (ServoMed) is based on the same principle.
Advantage: large normative database available in the literature. Disadvantage: sensitivity to ambient air currents, requiring a perfectly controlled room.
Closed principle (closed chamber)
Closed-chamber devices, such as the Aquaflux (Biox Systems) and Vapometer (Delfin Technologies), measure the accumulation of water vapor in a closed chamber placed on the skin. This approach eliminates the influence of air currents and offers better reproducibility under less controlled conditions.
| Equipment | Manufacturer | Principle | Feature |
|---|---|---|---|
| Tewameter TM 300 | Courage+Khazaka | Open chamber | Historical reference, large normative base |
| Tewameter TM Nano | Courage+Khazaka | Open chamber | Compact, suitable for hard-to-reach areas |
| Aquaflux AF200 | Biox Systems | Closed chamber | Very good reproducibility, insensitive to air currents |
| Vapometer | Delfin Technologies | Closed chamber | Robust, usable under imperfectly controlled conditions |
| Dermalab | Cortex Technology | Open chamber | Multiparametric, combinable with other measurements |
| Evaporimeter EP3 | ServoMed | Open chamber | Older standard, still in use in some CROs |
Measurement protocols: mandatory standardization conditions
TEWL is an extremely sensitive measurement to environmental conditions. A variation of a few points in temperature or humidity can cause measured values to vary significantly, compromising any interpretation. Standardization conditions are not recommendations: they are absolute prerequisites for data validity.
Environmental conditions
The measurement room must be maintained at a temperature of 20°C (±1°C) and a relative humidity of 50% (±10%). These values must be recorded continuously during the measurement session and attached to the raw data.
Subject acclimatization
Before any measurement, subjects must have rested for at least 20 to 30 minutes in the controlled room, in a seated position, without physical activity. Acclimatization allows skin temperature and sweat flux to stabilize. A measurement taken on a subject who has not had time to acclimatize will produce artificially elevated values.
Anatomical site and reproducibility
Reference TEWL values vary significantly by anatomical site. The lowest values (most intact skin) are measured on the forearm. The face shows higher values, particularly on the nasal wings and forehead. Hands and feet have specific profiles related to stratum corneum thickness. For a longitudinal study, the same anatomical site must be used at each measurement time point, with precise marking (dermograph or standardized photography).
Number of measurements and statistical processing
For each site, a minimum of three consecutive measurements must be taken, and the mean is calculated after exclusion of outliers. In an intra-individual design (same subject, treated site vs. control site), statistical analysis is generally based on a Wilcoxon test or paired t-test depending on data distribution. Statistical power must be calculated upstream to define the required sample size.
TEWL and cosmetic claims: how to integrate it into a substantiation file
Barrier strengthening claim (healthy skin)
The protocol measures TEWL at baseline (D0) then after product application according to a defined schedule (D14, D28, optionally D56). A statistically significant decrease in TEWL compared to baseline, confirmed on the treated site vs. the control site, substantiates the barrier strengthening claim.
Barrier repair claim (impaired skin)
TEWL is measured before impairment, after impairment (impaired baseline), then at each recovery time point under treatment. The recovery kinetics (rate of return toward healthy baseline values) constitutes proof of the repairing effect. A placebo group or an untreated site is essential to distinguish spontaneous recovery from the active effect.
Barrier protection claim
The product is applied before exposure to an aggressor (SLS, cold, wind). TEWL measured after aggression on the pre-treated area is compared to that measured on the non-pre-treated area. A lower TEWL on the pre-treated area demonstrates a protective effect.
| Claim | TEWL timing | Design | Comparator |
|---|---|---|---|
| Strengthens the barrier | D0, D14, D28 (±D56) | Parallel or intra-individual | Healthy baseline + untreated control |
| Repairs the barrier | Before impairment, after impairment, D3, D7, D14, D28 | Intra-individual (split-body) | Impaired baseline + spontaneous recovery |
| Protects the barrier | Before aggression (pre-treated vs. non-pre-treated), after aggression | Intra-individual | Non-pre-treated area |
Selecting the right laboratory for a TEWL study
Not all biometrological studies are equal. The validity of a TEWL measurement depends as much on the equipment as on the conditions in which it is used and the experience of the operator. When selecting a CRO for a TEWL study, the criteria to evaluate are:
Certification and accreditation: a CRO accredited to ISO 22716 (cosmetic Good Manufacturing Practices) or working according to GCP principles (Good Clinical Practices, ICH E6) offers a higher level of assurance on the rigor of the data produced.
Documented equipment: the CRO must be able to specify the equipment used, its serial number, its last calibration date, and the reference values of the measurement room. These elements must appear in the final report.
Experience with the study type: a CRO that has published or conducted barrier repair studies with SLS or tape stripping models has documented expertise. Requesting references or examples of similar protocols is a common and legitimate practice.
Multiparametric capacity: for studies combining TEWL, Corneometer, imaging, and biomarkers, the CRO must have the complementary equipment or a network of qualified subcontractors.
On the Skinobs platform, brands can filter laboratories by method (TEWL, Corneometer, Raman, tape stripping), by geographic area, and by study type. The platform references 138 laboratories in 38 countries for skin barrier substantiation.
Reference values and interpretation of TEWL data
Normal TEWL values vary by anatomical site, age, phototype, and measurement conditions. As a guide, on the forearm in a controlled room with a Tewameter:
| Anatomical site | Typical baseline TEWL (g/m²/h) |
|---|---|
| Forearm (flexor area) | 5 to 10 |
| Forehead | 15 to 25 |
| Cheek | 10 to 18 |
| Hand (dorsum) | 8 to 15 |
| Leg (shin) | 6 to 12 |
These values are given as a guide. Each CRO has its own reference values, established on its population and with its equipment. Data interpretation must always be relative (variation compared to the same subject’s baseline), never absolute, except in the context of a normalization study or inter-population comparison.
Conclusion
TEWL is far more than a simple hydration parameter: it is a functional biomarker of barrier integrity, whose probative value in cosmetic claim substantiation is today universally recognized. Its rigorous implementation, under strict standardization conditions, with calibrated equipment and protocols adapted to each claim type, is the condition for a solid and defensible body of evidence.
Looking for an equipped and experienced laboratory to perform TEWL measurements as part of your barrier studies? Skinobs allows you to identify the CROs adapted to your protocol, anywhere in the world. [Access the platform]
