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Transepidermal water loss (TEWL)

Also known as: TEWL, transepidermal water loss, water loss, evaporimetry

Transepidermal water loss is the rate of water vapour passing outward through a defined area of skin. It is a research measure of the stratum corneum's water-permeability barrier — not a measure of hydration, not a diagnosis, and not a universal 'damaged barrier' score.

Evidence status

Moderate

Well established for controlled comparative research on epidermal water permeability, and its validity as a barrier measure is supported. Absolute readings vary with anatomical site, environment, preparation, instrument and operator, and no validated universal diagnostic threshold exists.

What it is#

Transepidermal water loss is the rate at which water vapour passes outward through a defined area of skin, inferred from vapour flux above the surface and expressed in g/m²/h.[1, 2]

It is not hydration#

The most common error, and it is baked into how devices are marketed. TEWL and stratum-corneum hydration are different measurements. A TEWL instrument does not measure how hydrated skin is.[1, 2]

One is water going out through the barrier; the other is water held in the tissue. They are related, they often move together, and they are not the same number.

It is a valid measure#

Worth stating clearly, because a sceptical entry can easily leave the wrong impression.

The validation study for TEWL concluded that it does reflect permeability barrier status, and described the instruments tested as reliable tools.[3]

Two caveats belong with that, and neither undermines it. The study combined human participants with living and excised hairless-mouse models — so it is not purely human work. And its senior author is a co-inventor of a ceramide-dominant barrier-repair product, a material interest in a field where barrier measurement underpins product claims. Both are disclosed in the reference list.

Under controlled conditions, then, TEWL genuinely detects changes in epidermal water permeability. That is what it is for, and it does it.

What it cannot do#

Being a valid research measure and being a diagnostic test are different things.

There is no context-free ‘normal’ TEWL across body sites, instruments and populations.[5, 6] Healthy-adult values vary substantially by anatomical site, so a forearm number is not directly transferable to a cheek.[5, 6]

And we found no validated universal threshold that diagnoses a damaged skin barrier across sites, devices and populations.[no source found]

So a single reading, from one device, on one site, on one day, cannot tell a client their barrier is damaged. What TEWL is good at is comparison under controlled conditions— same site, same device, same operator, same environment, before and after.

What moves the number#

  • Site. Substantial variation between body sites.
  • Environment. Ambient temperature, relative humidity and air movement all influence a reading.[1, 2]
  • Preparation. Recent washing, product application and exercise all alter results.[1, 2, 7]
  • Device. Open-chamber and closed-chamber instruments give different absolute values while correlating — though one meta-analysis judged the clinical relevance of between-device differences to be minimal.[4, 5, 10]
  • Operator. Calibration, probe handling, contact pressure, angle and stabilisation rule all affect the result.[2, 4, 9]

Which is why a research protocol specifies acclimatisation time, ambient conditions, washout period, site and device — and why a reading taken in a treatment room after a client has walked in from outside has limited comparability, and should not be read against a diagnostic threshold.

The sweat problem#

Perspiration contaminates evaporimetry, because a device measuring water vapour cannot distinguish passive loss from sweat.

The authors of the key study on this went further than most people quoting it realise: they concluded that accurate baseline TEWL measurements may only be made after anticholinergic suppression of the sweat glands.[2, 8]

We take a different practical view — that acclimatisation and controlled conditions are the workable compromise outside a pharmacology lab — and we are stating that as our position rather than implying the paper agrees with it. Almost no cosmetic measurement meets the standard those authors set.

Reading product claims#

“Reduces TEWL by X% in two weeks” is one of the most common claims in barrier skincare. Two questions defuse most of them.

First: compared with what? Against no product, almost any occlusive moisturiser lowers TEWL. That is what occlusion does. Against a matched vehicle is a real comparison.

Second: for how long after stopping? A short-term fall in TEWL after application does not by itself establish durable barrier repair or structural change.[1, 2, 7]i

And it certainly cannot identify what was deficient. TEWL cannot detect a ceramide deficiency or show that ceramides caused a measured change.[1, 2]i

In professional practice#

  • Use it for comparison, not classification. Same client, same site, same conditions, over time.
  • Never quote a number as a diagnosis. There is no threshold to compare it against.
  • Control what you can — acclimatise, avoid product beforehand, use the same site and device.
  • Ask “versus what?” of every TEWL claim a brand makes.
  • Don’t confuse it with hydration. They are different measurements, whatever the device markets itself as.

What remains uncertain#

  • Whether any site-specific reference range could be defined well enough to be clinically useful.
  • How much of a post-product TEWL fall is occlusion versus genuine barrier change.
  • Whether newer measures such as electrical impedance spectroscopy prove more robust to everyday activity.
  • How closely readings taken in salon conditions relate to those taken under research protocols.

Common misconceptions#

“TEWL measures hydration.”

It measures water leaving the skin, not water held in it.[1, 2]

“A high TEWL means a damaged barrier.”

No validated universal threshold exists, and healthy values vary substantially by site.[no source found]

“TEWL is unreliable, so ignore it.”

The opposite overcorrection. The validation study found it reflects barrier status and that the instruments are reliable. Its limits are about interpretation, not validity.[3]

“The product lowered TEWL, so it repaired the barrier.”

Occlusion lowers TEWL. A short-term fall does not establish durable repair.[1, 2, 7]i

Frequently asked questions#

Is a TEWL device worth buying for a clinic?

It can produce comparative readings for one client against themselves, if you standardise the conditions. What the evidence reviewed here does not show is that owning one improves diagnosis, treatment selection or outcomes — repeatability and clinical usefulness are different things, and only the first has been demonstrated. For diagnosing anyone, no: there is no threshold to compare against.[5, 6]

What is a normal TEWL reading?

There isn’t one that holds across sites, devices and populations. Healthy-adult values vary substantially by site alone.[5, 6]

Why did the reading change between appointments?

Possibly the skin. Also possibly humidity, temperature, whether they walked in, what they applied this morning, where exactly the probe sat, and how long they acclimatised.[2, 4, 9]

Can I use it to prove a treatment worked?

Only against a fair comparator and beyond the period of occlusion. Otherwise you may be measuring the moisturiser rather than the recovery.[1, 2, 7]i

References#

Each source is graded by evidence tier. Tier 4 material (manufacturer documents, expert consensus, practitioner experience) is useful for protocol and context, and is never presented as equivalent to independent clinical evidence.

  1. Alexander H, Brown S, Danby S, Flohr C. Research Techniques Made Simple: Transepidermal Water Loss Measurement as a Research Tool. Journal of Investigative Dermatology. 2018;138(11):2295–2300.Tier 4Supports: What TEWL is, how it is measured, units of g/m²/h, and the distinction between TEWL and stratum-corneum hydration.
  2. du Plessis JL, Stefaniak AB, Eloff FC, et al. International guidelines for the in vivo assessment of skin properties in non-clinical settings: part 2. transepidermal water loss and skin hydration. Skin Research and Technology. 2013;19(3):265–278.Tier 4Supports: Measurement guidelines: environmental control, acclimatisation, probe handling, site selection and reporting requirements.
  3. Fluhr JW, Feingold KR, Elias PM. Transepidermal water loss reflects permeability barrier status: validation in human and rodent in vivo and ex vivo models. Experimental Dermatology. 2006;15(7):483–492.Animal studyTier 3Supports: The validation study for TEWL as a barrier measure. Its own conclusion is POSITIVE: it verifies the utility of TEWL as a measure of permeability barrier status and describes the tested instruments as reliable tools. It must NOT be cited to argue devices are unreliable or non-interchangeable. Combines human participants with living and excised hairless-mouse models; the full text is paywalled, so participant numbers and strain details are not asserted here.Funding / interest: PubMed lists four NIH grants. Senior author Peter Elias is a co-inventor of EpiCeram, a ceramide-dominant barrier-repair emulsion — a material interest in a field where barrier measurement underpins product claims.
  4. Klotz T, Ibrahim A, Maddern G, Caplash Y, Wagstaff M. Devices measuring transepidermal water loss: a systematic review of measurement properties. Skin Research and Technology. 2022;28(4):499–512.Tier 1Supports: Measurement properties and sources of error across TEWL devices — the appropriate source for device-related variability and comparability.
  5. 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.Tier 1Supports: Healthy-adult TEWL varies substantially by anatomical site. Also concludes that the clinical relevance of differences between measurement devices seems to be minimal.
  6. Green M, Feschuk AM, Kashetsky N, Maibach HI. 'Normal' TEWL — how can it be defined? A systematic review. Experimental Dermatology. 2022;31(12):1618–1631.Tier 1Supports: The absence of one context-free 'normal' TEWL value across body sites, instruments and populations.
  7. Huygen L, Thys PM, Wollenberg A, Gutermuth J, Kortekaas Krohn I. Skin barrier function assessment: electrical impedance spectroscopy is less influenced by daily routine activities than transepidermal water loss. Experimental Dermatology. 2023;32(11):1930–1938.Tier 3Supports: Everyday activities — washing, product application, exercise — alter TEWL readings, so preparation and timing must be specified.
  8. Pinnagoda J, Tupker RA, Coenraads PJ, Nater JP. Transepidermal water loss with and without sweat gland inactivation. Contact Dermatitis. 1989;21(1):16–22.Tier 3Supports: Perspiration contaminates evaporimetry readings. IMPORTANT: the authors conclude that accurate baseline TEWL measurements may only be made after anticholinergic suppression of the sweat glands. This entry takes a different practical view, and says so rather than implying the paper agrees.
  9. Blichmann CW, Serup J. Reproducibility and variability of transepidermal water loss measurement. Acta Dermato-Venereologica. 1987;67(3):206–210.Tier 3Supports: Reproducibility and operator-related variability in TEWL measurement.
  10. Steiner M, Aikman-Green S, Prescott GJ, Dick FD. Side-by-side comparison of an open-chamber (TM 300) and a closed-chamber (Vapometer) transepidermal water loss meter. Skin Research and Technology. 2011;17(3):366–372.Tier 3Supports: Open-chamber and closed-chamber devices produce different absolute values while correlating with each other.