Transepidermal water loss, universally known as TEWL, is one of the most informative and widely used measurements in cosmetic and dermatological research. Simple in principle yet demanding in execution, it provides a direct window into the integrity of the skin barrier — and by extension, into the capacity of a formula to protect, repair or reinforce it.
For any brand working on barrier repair, dry skin, sensitive skin or long-lasting hydration revendications, understanding TEWL is not optional. It is a scientific cornerstone.
What TEWL Actually Measures
The skin barrier is not simply a physical wall. It is a dynamic, multilayered system whose primary function is to regulate water exchanges between the body and its environment. Under healthy conditions, a small but constant amount of water diffuses passively from the deeper skin layers toward the surface, where it evaporates. This passive, invisible water loss is what TEWL captures.
When the barrier is compromised — whether by genetic factors, environmental aggressors, irritation or product misuse — this water loss increases significantly. A high TEWL reading is therefore a reliable signal of barrier dysfunction. Conversely, a formula that reduces TEWL over time demonstrates that it has reinforced the barrier and improved the skin’s capacity to retain water.
This is why TEWL is described as an indirect hydration marker: it does not measure water content directly, but it reflects the skin’s ability to hold onto it.
The Biological Context: Why TEWL Matters for Revendications
The practical significance of TEWL extends well beyond the laboratory. Regulatory agencies and scientific reviewers increasingly expect quantitative, reproducible data to support barrier-related revendications. A well-conducted TEWL study can substantiate revendications such as:
| Revendication type | Expected TEWL result |
|---|---|
| Barrier repair | Significant decrease in TEWL vs. baseline |
| Dry skin relief | TEWL reduction combined with cornéométrie increase |
| Long-lasting hydration | TEWL stability over 8 to 24 hours post-application |
| Sensitive skin tolerance | TEWL within normal range after repeated application |
| Anti-pollution protection | Attenuated TEWL increase after controlled exposure |
Selecting TEWL as a primary or secondary endpoint must therefore be driven by the revendication you intend to support, not by habit or convention.
Standard Measurement Devices
Several validated devices are available for TEWL measurement. They differ in their underlying physical principle, their measurement chamber design and their suitability for different study contexts.
| Device | Manufacturer | Principle | Key feature |
|---|---|---|---|
| Tewameter TM 300 | Courage + Khazaka | Open chamber, diffusion gradient | Most widely published reference |
| Tewameter TM Nano | Courage + Khazaka | Open chamber, compact | Suitable for facial measurements |
| Aquaflux AF200 | Biox Systems | Closed chamber with condensation | Less sensitive to air currents |
| Vapometer | Delfin Technologies | Closed chamber | Robust in variable environments |
| Dermalab | Cortex Technology | Open chamber | Combined with other skin parameters |
| Evaporimeter EP1 | ServoMed | Open chamber | Historical reference, less common today |
The open-chamber devices measure water vapour diffusion between two sensors placed at different heights above the skin surface. They are the most widely used in clinical research and are generally preferred when study conditions can be well controlled.
Closed-chamber devices work differently: they trap a small volume of air above the skin and measure the rate of humidity increase. They are less sensitive to ambient air movements, which makes them more robust in field conditions or poorly controlled environments.
The choice between the two should be discussed with your CRO at the protocol design stage, particularly if your study involves body sites where air circulation may vary.
Protocol Requirements: What Makes or Breaks a TEWL Study
TEWL is highly sensitive to environmental and procedural conditions. A poorly controlled protocol will generate noisy, unreliable data regardless of the quality of the device. The following parameters must be precisely defined and monitored throughout the study.
Environmental conditions. Temperature and relative humidity are the two most critical variables. Standard conditions are typically 20 to 22 degrees Celsius and 40 to 60 percent relative humidity, maintained throughout the acclimatisation period and all measurements. Any deviation must be recorded and reported.
Acclimatisation. Subjects must rest in the controlled environment for a minimum of 20 to 30 minutes before measurements are taken. Physical activity, emotional stress and recent product application all elevate TEWL temporarily and must be excluded.
Measurement site. TEWL values vary considerably across body sites. The volar forearm is the most common reference site for standardisation purposes, but studies on facial, body or scalp products require adapted sites. The exact anatomical location must be defined and reproducibly marked across time points.
Baseline selection. In vehicle-controlled studies, the contra-lateral site is used as the internal control. In open studies, a pre-treatment baseline measurement is taken under identical conditions.
Number of measurements. A minimum of three consecutive readings per site is recommended, with the mean value retained. Some protocols require five readings if variability is high.
Interpreting TEWL Data
Raw TEWL values are expressed in grams per square metre per hour (g/m²/h). Reference ranges for healthy skin on the volar forearm typically fall between 5 and 15 g/m²/h, though these vary by age, skin phototype and body site.
Statistical interpretation should account for natural variability between subjects and across time points. For revendication support, a statistically significant reduction in TEWL versus baseline or vehicle control, maintained over the relevant time window, is the expected standard.
It is also important to contextualise TEWL data alongside other measurements. A decrease in TEWL without a corresponding increase in stratum corneum hydration (as measured by cornéométrie) may indicate occlusion rather than genuine barrier repair. Reviewers will look for biological coherence across the measurement battery.
Selecting the Right Laboratory
Not all clinical laboratories are equally equipped or experienced for TEWL studies. Key criteria to consider when selecting a CRO include the availability of validated TEWL devices, experience with environmental control and acclimatisation protocols, and familiarity with the specific body site and population relevant to your formula.
Geographic location also matters. TEWL studies conducted across multiple sites or countries require strict harmonisation of equipment and procedures to ensure data comparability — a factor that becomes critical in regulatory dossiers.
Skinobs: your shortcut to TEWL-specialist laboratories
Finding a laboratory with genuine TEWL expertise and controlled environmental facilities takes time and multiple contacts. Skinobs removes that friction entirely.
On the Skinobs platform, you can search for laboratories by method, filter by the body site studied, the skin type of the panel and the geographic zone, and directly compare the capabilities of dozens of specialists. Whether you need a single-site study or a multi-country programme, the right partner is searchable in minutes.
Browse TEWL-specialist labs on Skinobs and get your study started at skinobs.com/register
