What Is TEWL (Transepidermal Water Loss) in Skin?

TEWL, or transepidermal water loss, is the water that leaves your skin through its surface and into the air. If your skin still feels tight after moisturizing, slow that loss before you add more.

The problem may not be too little water — it's how fast the water you already have escapes. Understand this one idea, and you stop asking what else to apply and start asking what's letting the water leave.

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. Even in healthy adults it varies widely by area — from 2.3 g/m²/h on breast skin to 44.0 on the underarm (Kottner et al., 2013). A clear rise on the same spot 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.
• Acne: 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.

Skin that looks oily can be losing water at the same time. More on that in why oily skin can still feel dehydrated.

Why adding water isn't enough

Moisturizing works through three mechanisms.

• Humectants draw water in — hyaluronic acid, glycerin. On their own, 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. Applying all three barrier lipids together — ceramide, cholesterol, and fatty acid — has been tested as a way to help the barrier come back faster. In the skin of people around 80, a cholesterol-heavy mix helped the barrier come back fastest, while a fatty-acid-heavy mix slowed it (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 many times a day, or with a stripping foam, washes away surface lipids. Cleansers with a pH well above skin's slightly acidic level are thought to put more strain on the barrier.
• Hot water: it is thought to loosen the skin's lipid structure, so water leaves faster.
• Indoor heating and dry air: dry heated rooms are thought to push TEWL up.
• Too much exfoliation: daily glycolic or salicylic acid or frequent scrubs can thin the outer layer, so it may hold water in less well.

Hot water sharply raises water loss, so wash and shower in lukewarm water. A palm soaked in 41°C water more than doubled its TEWL (Herrero-Fernández et al., 2022). That was measured on hands, not faces.

In short: moisturizing is about keeping, not adding

Before adding another moisturizer, change your wash temperature and when you moisturize. TEWL is water leaking through gaps in the barrier, and humectants alone don't close those gaps. A palm soaked in hot water more than doubled its TEWL (Herrero-Fernández et al., 2022). So keeping the water you already have comes before adding more. Look at how you cleanse before buying more products.

One step today

• Moisturize right after cleansing, while your skin is still damp. The idea is that water still on the surface gets trapped before it evaporates; this hasn't been tested in a trial. Try it after your evening wash this week and watch how tightness feels at bedtime. If skin cracks red or the itch won't settle, see a dermatologist.
• Switch wash and shower water to lukewarm, not hot. A palm soaked in 41°C water more than doubled its TEWL (Herrero-Fernández et al., 2022). Watch this week whether redness and stinging right after washing ease. If stinging after washing continues even with cooler water, see a dermatologist.
• 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 (Purnamawati et al., 2017). Look on the ingredient list for an occlusive such as petrolatum, shea butter, or squalane. If there is none, add a ceramide layer last this week, and if tightness and flaking continue, see a dermatologist.