What Does Transepidermal Water Loss Actually Measure?

Transepidermal water loss (TEWL) measures the rate at which water vapour leaves the skin, in grams per square metre per hour. It is a measure of outward flux, not of how much of an applied substance gets in. Pooled across 212 studies, values run from 2.3 g·m⁻²·h⁻¹ on breast skin to 44.0 in the axilla — so the site chosen determines the number far more than the product applied does.

Published 13 October 2026. Last reviewed 13 October 2026.

Definitions, each standing on its own

• Transepidermal water loss (TEWL). The flux of water vapour from inside the body outwards across the epidermis, measured at the skin surface and expressed in g·m⁻²·h⁻¹. It excludes water produced by active sweating, which instruments attempt to avoid rather than subtract.
• Evaporimetry. The measurement technique. Open-chamber devices infer flux from a vapour-pressure gradient in a cylinder held against the skin; closed-chamber devices measure the rate of humidity increase in a sealed cell.
• Permeability barrier function. The skin's resistance to the passage of substances in either direction. TEWL is used as a proxy for it, which is precisely the point in dispute.
• Stratum corneum hydration. The water content of the horny layer, usually measured by capacitance (corneometry). It is a different quantity from TEWL, and the two can move in the same direction or in opposite directions.

The reference values, and the size of the site effect

The most complete synthesis available is a systematic review and meta-analysis update by Akdeniz and colleagues, which identified TEWL estimates for 86 skin areas across 212 studies, with per-site sample sizes from 4 (a forehead site) to 4,013 (a mid-volar right forearm site) (British Journal of Dermatology, 2018). It updates the authors' earlier meta-analysis of 167 studies and 50 skin areas (Archives of Dermatological Research, 2013).

Two anchors define the range. The lowest pooled value was breast skin at 2.3 g·m⁻²·h⁻¹ (95% CI 1.9–2.7); the highest was the axilla at 44.0 g·m⁻²·h⁻¹ (95% CI 39.8–48.2). That is a nineteen-fold spread between two sites on the same healthy body. Any TEWL figure reported without its anatomical site is uninterpretable.

Two further findings of that review matter operationally. Differences between measuring devices were judged to be of minimal clinical relevance, which means device choice is not the main source of disagreement between studies. And right- and left-side measurements at the same site gave similar values, which supports the within-subject, contralateral-control design used in most instrumental testing.

The finding that contradicts the intuitive story

The commercial account of ageing skin predicts rising water loss. The pooled evidence does not show it. In the 2013 meta-analysis, TEWL in participants aged 65 and above was consistently lower than in the 18-to-64 group; the 2018 update reports TEWL in elderly participants as similar to or lower than in younger groups, while noting that the evidence base for the comparison is limited.

This is not a minor caveat to be buried. Any communication that treats "increased transepidermal water loss with age" as established fact is contradicted by the two largest syntheses of the measurement. The physiologically coherent reading is that aged stratum corneum can be simultaneously drier and less permeable to water vapour — which is exactly the reason TEWL and corneometry must be reported separately rather than merged into a single "hydration" claim.

Does TEWL predict how much gets in?

This is where the literature divides, and the division should be stated rather than resolved by preference. Chilcott and colleagues tested the proposition directly in vitro and found no correlation between basal TEWL and permeability: none for tritiated water across human epidermal membranes (p = 0.72), none for a lipophilic agent across the same membranes (p = 0.74), none for tritiated water across full-thickness pig skin (p = 0.68), and none after graded physical damage — up to 15 tape strips (p = 0.64) or up to four needle-stick punctures (p = 0.13). Their conclusion was that TEWL cannot be treated unconditionally as a measure of barrier function (Journal of Investigative Dermatology 2002;118(5):871–875; PMID 11982767).

Other work, principally in vivo and ex vivo in human and rodent models, has argued the opposite and underpins the routine clinical use of the measurement. The honest summary is that TEWL is a well-characterised descriptor of water flux whose relationship to the penetration of applied substances is model-dependent and unsettled. It is not a surrogate for delivery, and treating it as one is the single most common overreach in the technical literature of the category.

What TEWL does discriminate

Where the barrier is clinically disrupted, TEWL separates groups cleanly. In a single-centre study of 92 patients with psoriasis and 65 with atopic dermatitis against matched controls, mean values were as follows (Journal of Clinical Medicine, 2021).

The gradient is unambiguous: lesional skin loses roughly 1.5 times (psoriasis) to 2.5 times (atopic dermatitis) the water of healthy skin at a comparable site. But the quantitative association with disease severity is weak. Correlation with the SCORAD index was r = 0.27 (p = 0.036) at eczematous lesions and r = 0.27 (p = 0.038) at uninvolved skin. The diagnostic cut-offs the authors derive perform modestly: TEWL above 23.19 g·m⁻²·h⁻¹ for moderate-to-severe atopic dermatitis gave sensitivity 73.5%, specificity 53.8% and an area under the curve of 0.633; above 13.85 g·m⁻²·h⁻¹ for moderate-to-severe psoriasis gave sensitivity 81.8%, specificity 50.8% and an area under the curve of 0.636.

An area under the curve near 0.63 describes a test that is better than chance and well short of decisive. TEWL distinguishes a disrupted barrier from an intact one; it does not grade how disrupted.

Consequences for instrumental testing of cosmetic products

Four consequences follow directly, and none of them is a product claim.

1. Report the site, always, and keep it fixed. With a nineteen-fold spread between healthy sites, a change of measurement site between visits can manufacture or erase any effect a study might report.
2. Report TEWL and stratum corneum hydration separately. They are different physical quantities and, in ageing skin, they appear to move in opposite directions.
3. Do not read a TEWL reduction as evidence of penetration. The in vitro evidence against that inference is explicit and quantified.
4. Treat the clinical reference values as reference values. The psoriasis and atopic dermatitis figures above are drawn from dermatological research to locate the scale. They are not a context in which a cosmetic product has any role: under Regulation (EC) No 1223/2009 a cosmetic is defined by its purpose, and a product presented as acting on a disease falls outside that definition and into the medicinal-product regime.

Where a claim is made from an instrumental endpoint such as TEWL, Regulation (EU) No 655/2013 establishes the common criteria for cosmetic product claims in the European Union, among them that claims be supported by adequate and verifiable evidence; in the United States, the cosmetic–drug boundary under the Federal Food, Drug, and Cosmetic Act turns on the claim made rather than on the ingredient used. The practical reading of both is the same: the measurement supports a statement about the measurement, and nothing wider.

References

1. Akdeniz M, Gabriel S, Lichterfeld-Kottner A, Blume-Peytavi U, Kottner J. Transepidermal water loss in healthy adults: a systematic review and meta-analysis update. British Journal of Dermatology 2018;179(5):1049–1055. DOI 10.1111/bjd.17025. PMID 30022486.
2. Kottner J, Lichterfeld A, Blume-Peytavi U. Transepidermal water loss in young and aged healthy humans: a systematic review and meta-analysis. Archives of Dermatological Research 2013;305(4):315–323. DOI 10.1007/s00403-012-1313-6. PMID 23341028.
3. Chilcott RP, Dalton CH, Emmanuel AJ, Allen CE, Bradley ST. Transepidermal water loss does not correlate with skin barrier function in vitro. Journal of Investigative Dermatology 2002;118(5):871–875. PMID 11982767.
4. Montero-Vilchez T, Segura-Fernández-Nogueras MV, Pérez-Rodríguez I, Soler-Gongora M, Martinez-Lopez A, Fernández-González A, Molina-Leyva A, Arias-Santiago S. Skin Barrier Function in Psoriasis and Atopic Dermatitis: Transepidermal Water Loss and Temperature as Useful Tools to Assess Disease Severity. Journal of Clinical Medicine 2021;10(2):359. DOI 10.3390/jcm10020359. PMID 33477944.

Scope

This article describes findings published in the peer-reviewed literature and the conditions under which they were obtained. It does not constitute a claim about any product, is not a statement of cosmetic efficacy, and is not medical advice. Cosmetic products are not intended to diagnose, treat, cure or prevent any disease.

Research Team, TTS Clinical. 13 October 2026.