TEWL: What Transepidermal Water Loss Actually Measures, and What Your Number Means

A barrier product claims it cuts transepidermal water loss by 40 percent, and the number looks like proof. It is not, until you know the site it was measured on, the instrument that measured it, and the baseline it was cut from. TEWL is a real measurement, and it is also one of the easiest numbers in skincare to quote misleadingly.

What the number is, and what it is not

Transepidermal water loss is the rate at which water evaporates outward through intact skin, measured in grams per square metre per hour. It is a flux, not a quantity: how fast water leaves, not how much water the skin holds. That distinction matters, because TEWL and hydration are measured by different instruments. Hydration is measured by electrical capacitance, in arbitrary units. TEWL is measured by an evaporimeter, and the two numbers do not move together.

The instruments come in two kinds. Open-chamber devices, like the Tewameter from Courage and Khazaka, measure the vapour gradient above the skin. Closed-chamber devices, like the Vapometer, seal a small chamber over the skin and measure the rise in humidity. A 2020 consensus paper in Skin Research and Technology confirmed both as reference methods, but the numbers they produce cannot be converted into each other. A TEWL of 10 from an open chamber and a TEWL of 10 from a closed chamber are two different measurements that happen to share a unit.

And TEWL is not a direct measure of what gets into the skin. Chilcott and colleagues tested the proposition directly in 2002 and found no correlation between basal TEWL and permeability for tritiated water across human epidermal membranes. A leaky barrier loses more water, but the number does not tell you how much of an applied substance will penetrate. It is a descriptor of water flux, and the claim that it measures barrier function in full is an overstatement the data does not support.

What a normal reading looks like, site by site

There is no single normal TEWL value, because the barrier is not uniform. The largest synthesis is a 2018 meta-analysis by Akdeniz and colleagues in the British Journal of Dermatology, which pooled estimates for 86 skin areas from 212 studies in healthy adults. The lowest pooled value was 2.3 grams per square metre per hour on breast skin. The highest was 44.0 in the armpit. That is a nineteen-fold spread between two sites on the same healthy body, and any TEWL figure reported without its anatomical site is uninterpretable. Related reading: Can you use niacinamide on a damaged skin barrier?.

Healthy facial cheek skin typically reads 5 to 15 grams per square metre per hour under controlled lab conditions. The forehead runs higher, 10 to 25, because of a higher density of oil glands. The inner forearm, where most clinical studies are done, sits around 8 to 10. Palms read 25 to 70 depending on the study. Lips run about three times the cheek. The site determines the number far more than the product applied does.

Disease changes it further. Atopic dermatitis lesional skin commonly shows TEWL three to four times normal at the same site. Psoriasis lesional skin runs about one and a half times. A single reading does not establish barrier dysfunction, and the clinically actionable number is the delta, the change from your own baseline measured the same way, at the same site, with the same instrument. A drop from 28 to 19 grams per square metre per hour signals meaningful recovery even if the final number is still above the population normal. More on this: Squalane vs Ceramides: Occlusive, Emollient, or Lipid Replacement.

How to read a claim that quotes a TEWL number

Ask four questions. First, what site: a forearm number and a cheek number are not comparable. Second, what instrument: open and closed chamber values do not convert. Third, what baseline: a 40 percent reduction from a starting rate of 30 is a different achievement than 40 percent from a starting rate of 12, and a percentage without the starting rate is decoration. Fourth, what time: TEWL responds to humidity, temperature, ventilation, and whatever touched the skin in the previous few hours, so a reading taken in a controlled room after acclimation is the only kind that means anything.

The products that move TEWL are the ones that change the lipid matrix or seal the surface. Petrolatum, the most effective occlusive in common use, cuts transepidermal water loss by up to 98 percent. Ceramide-containing moisturisers restore the lipid structure itself: a 2024 trial found a ceramide lotion increased skin water content by 30.6 percent in three days, and up to 38 percent in four weeks, and a 2021 randomised trial by Kircik found TEWL and hydration restored in 28 days. The occlusive seals the leak. The ceramide rebuilds the wall. Both work, and the claim worth trusting is the one that names the site, the instrument, and the baseline.

Frequently asked questions

What is a normal TEWL value?

It depends on the site. Healthy cheek skin reads about 5 to 15 grams per square metre per hour, the inner forearm about 8 to 10, and palms 25 to 70. There is no single normal value.

Does a lower TEWL mean my skin is more hydrated?

Not directly. TEWL measures the rate water leaves the skin. Hydration is measured separately, by electrical capacitance. The two do not always move together.

Can I measure TEWL at home?

At-home devices exist, but their numbers are only comparable against your own baseline measured the same way, at the same site, with the same device. A single reading from a new device tells you almost nothing.

What actually lowers TEWL?

Occlusives like petrolatum seal the surface and cut water loss by up to 98 percent. Ceramide moisturisers rebuild the lipid matrix, with trials showing TEWL and hydration restored in about 28 days.