TEWL: the water your skin loses every day, and why researchers measure it

Right now, a little water is evaporating through your skin. Not sweat, just a quiet, constant trickle passing through the outer layer. Scientists call it transepidermal water loss, or TEWL, and if you’ve ever read a study about a moisturizer, you’ve seen the term. Here’s what it means and why researchers care about it so much.

What is transepidermal water loss?

Your skin’s outer layer, the stratum corneum, is a wall of flattened dead cells held together by fats (lipids). Its main job is to keep the water inside your body from evaporating into the drier air outside. It does that job very well, but not perfectly. Some water always diffuses through.

That steady leak is TEWL. It’s measured as the weight of water vapour leaving a patch of skin over time: grams per square metre per hour, written g/m²/h.

The key idea is simple: the better the barrier, the lower the TEWL. When the wall is intact, only a little water escapes. When the lipid “mortar” is stripped away, or the wall is broken by a scratch or a wound, water pours out faster and the number climbs. That’s why researchers treat TEWL as a barrier score.

How is it measured?

You can’t see water vapour, so TEWL is measured with a small probe pressed against the skin. The probe contains sensors that detect how quickly the humidity right above the skin surface changes. There are three main designs:

• Open-chamber probes let room air flow freely over the skin. They’re fast, but sensitive to drafts, humidity and temperature, so they need a controlled room.
• Closed-chamber probes seal a tiny pocket of air over the skin and track how humidity rises inside it. They’re less bothered by air movement but slower, and leaving them on too long traps moisture and skews the reading.
• Condenser-chamber probes use a cooled surface to pull vapour out of the air. Most precise, but slower and costlier.

A single reading takes about 40 to 60 seconds for the number to settle, with the probe held straight (perpendicular) against the skin. Because readings vary, good studies take three measurements per site and average them, and they measure TEWL before anything else, since touching the skin with other instruments can change the result.

Why the measuring conditions matter so much

TEWL is a fussy measurement. Sweat, warm skin or a breeze from an air vent can all push the number around. So research guidelines are strict:

• The person rests in the measurement room for at least 15 to 30 minutes first, ideally at 20 to 22 °C and 40 to 60 percent relative humidity, so the skin settles and stops sweating.
• The test site is uncovered for at least 10 minutes before the probe touches it.
• No creams or washing on the site for a good while beforehand.
• The device is calibrated regularly and its type reported, because the three designs don’t give identical numbers.

That’s why a “barrier score” from a hand-held consumer gadget, taken in your bathroom after a shower, doesn’t mean much. The number is only meaningful under controlled conditions and compared with another number taken the same way.

There’s no single “normal” number

What’s a healthy TEWL? Honestly, it depends where you measure. A recent review pulled together baseline readings from healthy adults and found big differences between body sites. The sole of the foot, with its thick, lipid-rich skin, can sit under 10 g/m²/h. The top of the foot, which is thinner, lets through two to three times more (roughly 15 to 25). Cheek readings in recent studies were around 20. The palm tends to give the highest values of all. Even on one forearm, readings are higher near the wrist and elbow than in the middle.

Age matters less than you’d think: baseline TEWL is fairly stable through the working years, and some studies find it slightly lower after 60, even though skin hydration slowly declines with age. Whether sex or ethnicity change it is unclear; studies disagree.

So a TEWL number on its own tells you little. What researchers actually use is the change: before versus after a treatment, or treated skin versus an untreated patch on the same person.

Why TEWL rises in dry skin and eczema

Dry skin usually starts with an insult from outside: over-washing, harsh cleansers, hot water, low humidity or an irritant at work. Each one strips some lipids and some of the skin’s built-in humectant (natural moisturizing factor) out of the outer layer. Water then escapes faster, TEWL rises, and the layer dries out. Interestingly, TEWL goes up before you can see or feel anything, which is one reason researchers like it: it catches damage early.

Skin isn’t passive about this. When TEWL rises, cells just under the surface release stored lipids within minutes and partly plug the leak, then ramp up lipid production over the next few hours. Problems come when insults outpace repair.

In atopic dermatitis (eczema), the barrier is leaky from the start. The outer layer contains fewer lipids and a different mix, and many people carry a change in the gene for filaggrin, a protein that helps build the cell “bricks” and produces the humectant inside them. Increased TEWL corresponds with increased skin permeability, and readings are highest on active eczema patches.

What the weather and the air do to it

Canadians will be curious here, and the research is more mixed than you’d expect. In a climate chamber, where everything is controlled, TEWL rises as the temperature goes up and falls as humidity goes up. In real life, though, a systematic review of 11 studies found contradictory results on season and climate: some found higher TEWL in summer, others higher readings in winter on parts of the face. No consensus was reached, partly because a summer measurement also captures more sweating, and the body site matters a lot.

One finding was consistent: all four studies that looked at air pollution found higher TEWL in people exposed to fine particulate matter or nitrogen dioxide, probably through free-radical damage to the barrier.

So the “winter dries your skin” experience is real, but TEWL research alone can’t yet put a clean number on it.

What it means when a moisturizer “reduces TEWL”

This is the claim you’ll see most often, and it needs unpacking. Moisturizers contain humectants (such as glycerin) that pull water in, occlusives (such as petrolatum) that form a film to slow evaporation, and emollients (such as ceramides and fatty acids) that fill gaps between the cells. An occlusive film lowers TEWL right away, which is genuinely useful for dry skin. But a lower reading an hour after applying an ointment isn’t proof that the barrier underneath has healed.

Researchers themselves point this out: whether a moisturizer’s effect comes from acting as a physical barrier, from improving the skin’s own barrier, or both, still needs rigorous mechanistic studies. The more convincing studies measure TEWL after the product has worn off, track skin hydration too, and follow people for weeks. That’s the kind of evidence to look for.

TEWL is the everyday trickle of water through your skin’s outer wall, measured in grams per square metre per hour with a probe under carefully controlled conditions. It rises when the barrier is stripped, wounded or, as in eczema, built leaky from the start, and it rises before you can see the damage. There’s no single normal value, so what counts is the change. And when a product claims to lower it, the question to ask is whether the effect lasts once the product is gone. If your skin stays dry, cracked or itchy despite gentle care for a couple of weeks, see your doctor or pharmacist.