--- title: "Transepidermal water loss in cosmetics" description: "Learn what transepidermal water loss measures, how it differs from skin hydration and why controlled conditions matter in cosmetic testing." --- # Transepidermal water loss in cosmetics > Learn what transepidermal water loss measures, how it differs from skin hydration and why controlled conditions matter in cosmetic testing. Source: https://cosmetic-labs.com/glossary/transepidermal-water-loss Created: 2026-11-10T10:23:45+00:00 Modified: 2026-11-10T10:23:45+00:00 Author: Emilie SCHOL ## Definition of Transepidermal water loss in cosmetics Transepidermal water loss (TEWL) is the passive outward movement of water through the outer skin layer into the surrounding air. It is measured as a water-loss rate and used to assess barrier-related changes, but a single reading is not a diagnosis or a direct measure of skin hydration. ### Learn more ## What TEWL measures Transepidermal water loss describes water passing outward through the stratum corneum, the outermost skin layer, by diffusion. It excludes water loss through perspiration, although sweating can interfere with measurement. Results are commonly expressed as grams of water per square meter per hour. The number describes a rate of water movement over an area, not the amount of water stored in that skin area. This distinction is important when interpreting a moisturizer or cleanser development study. ## How it differs from skin hydration Skin hydration concerns the water content of the outer skin layer. TEWL concerns outward water movement. These endpoints are related to skin condition but are not interchangeable. Hydration instruments use electrical properties such as capacitance or conductance, whereas TEWL instruments estimate water-vapor movement. A hydration result should not be relabeled as a TEWL result, and a change in one endpoint does not automatically establish the size or direction of change in the other. ## Why the test conditions matter Ambient temperature, relative humidity, air movement, anatomical site and instrument characteristics can influence the measurement. Recent washing, topical products and occlusion can also affect results. Participants need time to acclimatize to the measurement environment, and sweating should be avoided. For development work, define and report the test conditions instead of treating the reading as an isolated fact about the skin. Comparisons are more useful when the same site, timing and measurement approach are maintained. ## Using TEWL in a cosmetic study TEWL can contribute to evaluating barrier-related changes during product testing. A useful protocol states the baseline, comparison condition, application schedule and measurement times in advance. Record the device and the handling of repeated measurements, and distinguish immediate effects after application from later assessments. These are practical study-design considerations, not a universal protocol for every product. The interpretation must stay within the conditions actually tested and should not turn an instrumental difference into a broad promise of skin repair. ## What one number cannot tell you A single TEWL value cannot identify why a result is high or establish a skin diagnosis. Values depend on the person, the body site and measurement conditions as well as skin condition. There is no sound basis for using one unqualified cutoff across every cosmetic study and every skin location. Consider the pattern of change and the relevant comparison, together with other observations, rather than using one reading as a complete assessment of barrier health. ## Writing proportionate product conclusions Keep a water-loss result separate from claims about hydration, comfort, irritation or treatment of disease unless those outcomes were also evaluated appropriately. If a study measures a short-term change under controlled conditions, the conclusion should preserve that timing and context. TEWL is useful because it provides an objective instrumental endpoint, but its usefulness depends on disciplined measurement and careful interpretation. It is one part of the evidence for a finished cosmetic, not a replacement for the rest of the development assessment. Sources ## Sources and references References supporting this page are listed below. Source access dates and supported claims are recorded in the evidence notes where available. [International guidelines for skin assessment: TEWL and skin hydration](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) URL [https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) [International guidelines: hydration measurement](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) URL [https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) [International guidelines: interpretation limitations](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) URL [https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) [International guidelines: measurement preparation](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) URL [https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4522909/?pdf=1) --- ## Site identity - Organization: Cosmetic Labs - Description: Cosmetic Labs is a directory and a platform to meet all your cosmetic product development needs. From sourcing raw material to finding the best CRO for your needs. - Website: https://cosmetic-labs.com