What Is TEWL (Transepidermal Water Loss) in Skin?

If you moisturize and your skin still feels tight, the problem may not be too little water — it's how fast the water you already have escapes. Dermatologists call that transepidermal water loss, or TEWL. Understand this one idea, and you stop asking what else to apply and start asking what's letting the water leave.

Adding water to the surface is not always the problem to solve. What matters is the rate it leaves. Let's get this one concept right.

What is TEWL?

TEWL is the water that escapes from inside your skin, out through the surface, into the air. It's measured in g/m²/h — how many grams leave one square meter of skin in an hour. Healthy skin usually sits around 4–9 g/m²/h (Kottner et al., 2013). It varies by area, but a clear rise means there's a gap somewhere in your barrier.

Think of a cup with a hole in the bottom. However much you pour in, it drains. Moisturizing skin with high TEWL is pouring water into that cup.

What happens when TEWL rises?

When TEWL climbs, skin responds in a chain.

• Dryness and tightness: as the outer layer loses water, skin turns rough and begins to flake.
• More oil, not less: when the barrier is damaged, skin compensates by producing more sebum (Li et al., 2025). People with acne show significantly higher TEWL (13.16 vs 10.63 g/m²/h) and more sebum than those without (Sukanjanapong et al., 2024).
• More sensitivity: a weakened barrier lets irritants — UV, pollution, harsh cleansing agents — pass more easily. It's one reason a familiar product can suddenly sting.

What looks like oily skin is often water loss in disguise. More on that in why dry skin makes more oil.

Why adding water isn't enough

Moisturizing works through three mechanisms.

• Humectants draw water in — hyaluronic acid, glycerin. On their own, in dry air, they can even pull water up from deeper skin and let it evaporate (Purnamawati et al., 2017).
• Occlusives seal the surface so water can't escape — petrolatum, shea butter, squalane.
• Emollients fill the spaces between skin cells so the surface feels smooth. When all three barrier lipids were supplied together — ceramide, cholesterol, and fatty acid — the barrier came back faster, and which one should lead depended on what that skin was short of (Zettersten et al., 1997).

To actually lower TEWL, you draw water in and hold it there. One without the other is half the job.

What habits raise TEWL?

Some everyday habits quietly push TEWL up.

• Over-washing: cleansing more than three times a day, or with a stripping foam, washes away surface lipids. The further a cleanser's pH sits above skin's own (slightly acidic, pH 4.5–5.5), the more the barrier suffers.
• Hot water: it loosens the skin's lipid structure, so water loss climbs sharply.
• Indoor heating and dry air: when winter humidity drops below 30%, TEWL climbs fast. Even a humidifier helps the barrier.
• Too much exfoliation: daily AHA/BHA or frequent scrubs thin the outer layer, so it holds water in less well.

Skin exposed to hot water can more than double its TEWL (Herrero-Fernández et al., 2022), so keep water lukewarm (32–34°C).

In short: moisturizing is about keeping, not adding

Putting good ingredients on skin matters. Keeping the water you already have matters first. Changing how you cleanse can do more than any elaborate product. Your skin already knows how to hold its water — our part isn't to change that, but to help it keep the water from leaving.

One step today

• Moisturize within two minutes of cleansing. Applied while water still sits on the surface, it gets trapped before it evaporates. Try it after your evening wash this week and watch how tightness feels at bedtime.
• Bring wash and shower water down to 32–34°C, and keep indoor humidity at 40–60%. Skin exposed to hot water can more than double its TEWL. Watch this week for redness and stinging right after washing.
• Check that your moisturizer holds a humectant and an occlusive together. Hyaluronic acid, panthenol, and glycerin on their own can pull water up and let it evaporate in dry air. If one side is missing, add a ceramide layer last this week.