What Is TEWL? Why Understanding Water Loss Changes How You Moisturize

You moisturize, and your skin still feels tight. You are not alone. Adding water to the surface is not always the problem to solve. The real question is how fast that water leaves.

Dermatologists call this TEWL — transepidermal water loss. Understand this one idea, and the way you read your whole routine changes.

What is TEWL?

TEWL is the water that escapes from inside your skin, out through the surface, into the air. It is measured in g/m²/h — how many grams of water 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 is 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, your 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. This is the part many people miss. 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 (Elgamal et al., 2024). What looks like oily skin is often water loss in disguise.

More sensitivity. A weakened barrier lets irritants — UV, pollution, harsh cleansing agents — pass more easily. It is one reason a familiar product can suddenly sting.

Why adding water is not enough

Moisturizing works through three mechanisms.

Humectants draw water in. Hyaluronic acid and glycerin do this. 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 cannot escape. Petrolatum, shea butter, squalane.

Emollients fill the spaces between skin cells so the surface feels smooth. Ceramides, fatty acids, cholesterol. Healthy barrier lipids sit at roughly a 3:1:1 ratio of ceramide, cholesterol, and fatty acid — and moisturizers that mirror that ratio speed barrier recovery (Zettersten et al., 1997).

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

From the founder
For a while I kept stacking moisturizers, one after another, and my skin still felt tight. I had stripped the barrier with strong exfoliants and high-strength actives, so the water I kept adding simply left. The day I stopped asking what else to apply, and started asking what was letting the water go, my skin began to settle.

Habits that 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. Skin exposed to hot water can more than double its TEWL (Herrero-Fernández et al., 2022), as the lipid structure loosens. Lukewarm water (32–34°C) is kinder.

Indoor heating and dry air. When winter humidity drops below 30%, TEWL climbs fast. Even running a humidifier helps the barrier.

Too much exfoliation. Daily AHA/BHA or frequent scrubs thin the outer layer and weaken barrier function.

A practical way to lower TEWL

Seal the gaps first, then refill — in that order.

One — Moisturize within two minutes of cleansing. Apply it while water still sits on the surface, so you trap it before it evaporates.

Two — Check for a humectant + occlusive pairing. See whether your moisturizer holds both water-binders (hyaluronic acid, panthenol, glycerin) and sealers (ceramides, squalane, shea butter).

Three — Adjust how often, and how hot, you wash. If your skin runs dry or fragile, water alone in the morning is often enough. Save real cleansing for the evening, and keep the temperature lukewarm (32–34°C).

Four — Manage the air around you. Keeping indoor humidity at 40–60% lowers TEWL on its own.

Moisturizing is really about keeping

Putting good ingredients on skin matters. Keeping the water you already have matters first. Through the lens of TEWL, changing how you cleanse can do more than any elaborate product.

Your skin already knows how to hold its water. Our part is not to change that — it is to assist it, to keep the water from leaving, and let that ability wake up. Staying out of its way is where moisturizing begins.