TEWL stands for transepidermal water loss. It refers to the water that passively evaporates through your skin into the surrounding air, driven by the difference in moisture levels between the inside of your body and the environment. TEWL is the standard measurement dermatologists and skincare scientists use to assess how well your skin barrier is functioning.
What TEWL Actually Measures
Your skin constantly loses small amounts of water through evaporation, even when you’re not sweating. This invisible process happens because water vapor naturally moves from where there’s more moisture (inside your body) to where there’s less (the air around you). TEWL captures the rate of that water movement, measured in grams of water lost per square meter of skin per hour (g/m²/h).
The outermost layer of skin, called the stratum corneum, acts as the main barrier controlling this water loss. It’s made up of tightly packed dead skin cells held together by a mix of ceramides, cholesterol, and fatty acids that form a water-resistant seal. When that seal is intact, TEWL stays low. When it’s damaged or compromised, water escapes faster.
What Normal TEWL Looks Like
Technical guidelines break TEWL values into ranges that correspond to skin health:
- 0 to 10 g/m²/h: very healthy skin
- 10 to 15: healthy skin
- 15 to 25: normal skin
- 25 to 30: affected skin
- Above 30: critical condition
These numbers vary by body site. Thinner skin on the face and hands typically loses more water than thicker skin on the torso. Age, genetics, and daily habits all shift your baseline as well.
Why Skincare Products Target TEWL
Moisturizers work, in large part, by reducing TEWL. Occlusive ingredients like petrolatum, dimethicone, and lanolin form a thin hydrophobic film over the skin surface that physically blocks water from evaporating. In clinical testing, a well-formulated moisturizer reduced TEWL by about 25% within two hours of application, and that reduction held at roughly 22% even 24 hours later.
This is why applying moisturizer to damp skin is a common recommendation. You’re not just adding moisture from the outside; you’re trapping the moisture already present in the upper layers of skin before it escapes. Products containing ceramides or fatty acids go a step further by actually repairing the lipid structure of the skin barrier itself, addressing the root cause of elevated TEWL rather than just covering it up.
What Raises TEWL
Anything that disrupts the skin’s lipid barrier increases water loss. Harsh cleansers, over-exfoliation, and prolonged water exposure (like long hot showers) break down the fatty layers between skin cells, causing corneocytes to swell and the protective structure to loosen. The result is more water escaping and skin that feels tight, dry, or irritated.
Environmental conditions play a role too, though in a less intuitive way. Research published in the Journal of Investigative Dermatology found that TEWL actually increased two to threefold when humidity rose from extremely dry (2 to 3%) to moderate levels (30 to 50%). This happens because added moisture in the air increases the water content of the stratum corneum, which makes it more permeable. At higher humidity (around 70 to 77%), TEWL dropped back down because the smaller difference in moisture between skin and air meant less evaporation pressure. Skin temperature also matters: higher temperatures increase the rate of evaporation.
TEWL in Skin Conditions
Elevated TEWL is a hallmark of inflammatory skin conditions. In atopic dermatitis (eczema), the skin barrier is structurally compromised, often due to a genetic shortage of key barrier proteins. TEWL readings at eczema lesions are consistently higher than both uninvolved skin on the same person and skin on healthy individuals. More severe disease correlates with higher TEWL values.
Psoriasis shows a similar pattern. Psoriatic plaques have significantly higher TEWL than surrounding unaffected skin, reflecting the rapid, disorganized cell turnover that prevents a proper barrier from forming. In both conditions, tracking TEWL over time gives clinicians an objective way to gauge whether a treatment is actually restoring barrier function, beyond what’s visible to the eye.
How TEWL Is Measured
In research and clinical settings, TEWL is measured with specialized devices placed directly on the skin. These instruments detect the rate of water vapor leaving the skin surface and calculate the evaporation rate per unit area. Common devices include the AquaFlux, DermaLab Combo, and portable analyzers like GPSkin. Measurements are taken in controlled conditions, with sweating eliminated as a variable, to isolate the passive water loss through the skin barrier from active perspiration.
Consumer-grade skin analyzers have started incorporating simplified TEWL readings, though they’re less precise than lab instruments. For most people, the practical takeaway doesn’t require a device: if your skin feels persistently dry, tight, or reactive despite moisturizing, your TEWL is likely elevated, and your barrier needs repair rather than just surface hydration.