Transepidermal Water Loss (TEWL)
Transepidermal Water Loss (TEWL) — the natural process by which water moves from the deeper layers of the skin to the surface and evaporates into the air. A certain amount is normal, but elevated levels can indicate a compromised skin barrier and contribute to dryness.
Key points
- TEWL refers to the passive evaporation of water through the outermost skin layer, the stratum corneum.
- A healthy, intact barrier keeps TEWL at a low, balanced level.
- Increased TEWL is often a sign of a weakened or damaged skin barrier.
- Factors like low humidity, over-exfoliation, and harsh cleansing can raise TEWL.
- Moisturizers with occlusives and emollients help reduce excess water loss.
What moves the reading
| Situation | Effect on TEWL |
|---|---|
| Intact barrier at rest | Low and steady — a normal baseline, not zero |
| Frequent or harsh cleansing | Rises for hours as surface lipids and NMF are stripped |
| Low ambient humidity | Steepens the vapour gradient, so more water leaves per hour |
| Hot water | Raises it more than warm water, and the rise outlasts the wash |
| Occlusion | Falls sharply while covered; readings just after removal are unreliable |
| Damaged or inflamed barrier | Often several times baseline, and tracks with severity |
| Body site | Palms, soles and forehead read far higher than forearm or back |
How the measurement is taken
TEWL is not something you can see; it is a number produced by an instrument. A probe placed against the skin measures the water vapour gradient in the air just above it, using paired humidity and temperature sensors at known heights, and reports the diffusion rate in grams per square metre per hour.
Because it measures evaporation, the reading is easily contaminated. Draughts, sunlight, a warm room, recent exercise, sweating or any product left on the site will shift it, so comparable measurements need fifteen to twenty minutes of acclimatisation in a controlled room. Open-chamber, closed-chamber and condenser designs give related but not interchangeable numbers.
Why some loss is meant to happen
A common misreading is that TEWL is a fault to be eliminated. Water is continually supplied from the well-hydrated tissue below and the outer layer is not waterproof, so a steady low flux is unavoidable in living skin. The stratum corneum is built to slow that flux, not to stop it.
The resulting moisture gradient is itself functional: the enzymes that release NMF from filaggrin and those that let corneocytes shed both depend on local water content. Experimental work also indicates that increased flux is one of the signals prompting barrier repair, and that sealing damaged skin under a fully impermeable film delays it.
What raises it, and in whom
Anything that removes or disorganises the lipid matrix raises TEWL: surfactants, solvents, alcohol, abrasion, tape stripping, prolonged water contact and repeated exfoliation. Low humidity and moving air act differently — they do not damage the barrier, they steepen the gradient it works against, so the rate rises only while conditions last.
Baseline values differ by person and site. Palms, soles, forehead and skin folds read considerably higher than forearm or back. Newborn skin, especially preterm, loses water far faster until the barrier matures; in older skin the resting value is often unremarkable but recovery after damage is slower. Persistent elevation is characteristic of conditions such as eczema and psoriasis, which a clinician should assess.
Frequently asked
How can I reduce transepidermal water loss?
Supporting the skin barrier with gentle cleansing and moisturizers containing occlusives, emollients, and humectants can help limit excess water loss.
Is some transepidermal water loss normal?
Yes; a baseline level of water loss is completely normal, and only elevated TEWL tends to signal a compromised barrier.
Is high TEWL the same as dry skin?
No. TEWL measures how fast water leaves the surface, while hydration measurements estimate how much water the outer layer currently holds. They often move together, but skin can feel dry with a normal reading.
Can TEWL be measured at home?
Not reliably. Consumer devices generally estimate surface hydration by electrical capacitance rather than measuring evaporation, and genuine evaporimetry needs a controlled room and a rested subject with bare skin.
Does a lower number always mean healthier skin?
No. Applying an occlusive lowers the reading within minutes without changing the barrier beneath it, and the value rises again once the film is gone. A low number means something only on bare, settled skin.
Does sweating affect a TEWL reading?
Considerably. Sweat evaporating from the surface is counted as water leaving the skin, so a warm room, exertion or nervousness can inflate the result. Protocols control for this with rest and a cool room.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
