What is a normal TEWL value, and why do two readings of the same skin disagree?

Healthy forearm skin reads about 8 to 10 g/m2/h, palms 25 to 70. TEWL by body site, why instruments and rooms change the number, and how to read a claim.

A normal TEWL reading on the inner forearm of a healthy adult, measured at rest in a controlled room, is about 8 to 10 grams of water per square metre per hour. In two recent studies it came out at 7.99 and 9.0 g/m2/h. Every other site gives a different "normal": a pooled analysis of 212 studies found 2.3 g/m2/h on breast skin and 44.0 on the armpit, and the palm of the hand read 25.75 in one study and 68.4 in another. That last pair is the real lesson: TEWL depends on the site, the instrument, the room, and what touched the skin in the previous few hours. Two numbers from different studies are almost never comparable. Two from the same study, site and instrument are, and that is the only way to read the measurement.

• Forearm: about 8 to 10 g/m2/h
• Pooled range: 2.3 (breast) to 44.0 (armpit)
• Palm: 25.75 or 68.4, by study
• Lips: about 3x the cheek
• 10 minutes in hot water: palm 25.75 to 58.58

What TEWL measures, and in what unit

Water moves continuously from the moist living tissue under the skin, through the dead stratum corneum, and out into the air. That diffusion is transepidermal water loss. It is not sweat: sweating adds water to the surface by a different route and inflates the reading, which is why every protocol has the subject sit still before measuring. The general picture of the barrier this water crosses is on moisturisers and the skin barrier; this page is about the number.

The modern unit is grams per square metre per hour, written g/m2/h, g/(m2 h) or g m-2 h-1. Older papers use milligrams per square centimetre per hour. The conversion is a factor of ten: 1 mg/cm2/h is 10 g/m2/h. The 1971 occlusion study by Berube, Messinger and Berdick reported base rates for its four subjects of 0.19 to 0.39 mg/cm2/hr, which is 1.9 to 3.9 g/m2/h. Anyone comparing an old figure with a new one without converting is out by a factor of ten.

Two things TEWL is not:

• It is not hydration. Stratum corneum hydration is measured by a different instrument, through electrical capacitance or conductance, and reported in arbitrary units. Mayrovitz measured both on 40 young adults in 2023 and found no correlation between them on the forearm; the two measures only moved together on the palms of the men, once hydration passed a threshold. Skin can feel dry with a normal TEWL.
• It is not a measure of how much water a balm "adds". A balm adds none. It can only lower the reading by putting a film in the way, the occlusive route described on occlusive, emollient and humectant.

Normal TEWL values by body site

There is no single normal value, because the barrier is not uniform. The largest synthesis is the 2018 meta-analysis by Akdeniz and colleagues in the British Journal of Dermatology, an update of a 2013 review by the same group. It pooled TEWL estimates for 86 skin areas from 212 studies in healthy adults. The lowest pooled value was 2.3 g/m2/h (95 percent confidence interval 1.9 to 2.7) on breast skin and the highest 44.0 (39.8 to 48.2) in the armpit. The best studied site by far was the mid volar forearm, the soft inner side, with up to 4,013 people pooled. Sample sizes for other sites went down to four.

The table gathers the figures from the studies this page draws on. Read each row against its own instrument and room, not against the others.

Three patterns stand out. The forearm sits low, in single figures or just above. Palms run far higher than the forearm: Mayrovitz's palms read on average 7.4 times their own forearms, and the spread between people was enormous, a standard deviation of 53 on a mean of 68. And the face runs high, so a forearm result says little about lips, hands or a face. Kobayashi and Tagami describe the cheek as "a site that shows far higher levels than do other body areas", and found the lip almost three times the cheek again.

Men, women and age

Mayrovitz found men's TEWL higher than women's at both sites, 10.0 against 7.9 on the forearm. The difference was statistically significant and, at about 2 g/m2/h, smaller than the gap between his forearm figure and the Tewameter study's. The 2013 workplace guidelines by du Plessis and colleagues list gender as a factor that does not affect TEWL in the studies they reviewed. Both can be right: a small difference shows up within one careful study and vanishes among the larger differences between studies.

Age is the more persistent myth. The common assumption is that older skin leaks more water. Akdeniz and colleagues found that "TEWL in elderly people seems to be generally similar or decreased compared with younger individuals", and du Plessis and colleagues summarise baseline TEWL as generally independent of age through the working years, with some studies finding slightly lower values over 60. What does change is hydration, which the guidelines describe as decreasing "slowly but steadily with age": a different quantity. Kobayashi and Tagami found the same direction on the lips: TEWL fell with age, more steeply on the lip than on the cheek.

Why two readings of the same skin disagree

The forearm figures in the table, 7.99 and 9.0, are close. The palm figures, 25.75 and 68.4, are not. What differs is everything around the skin.

The instrument

Three designs are in use, and the du Plessis guidelines list the commercial instruments by type.

Whether the instruments agree is the second confusion this page has to settle, because two respectable sources appear to contradict each other. Akdeniz and colleagues concluded that "the clinical relevance of the difference between TEWL estimates for different measurement devices seems to be minimal". Steiner and colleagues, who took 540 measurements on the backs and palms of the hands of 17 people with an open chamber Tewameter TM 300 and a closed chamber VapoMeter, concluded that "their results cannot be directly compared", and that they "were unable to find a statistical model that would allow us to transform the measurements made on one device for a comparison with the results generated by the other device".

The two findings answer different questions. Akdeniz compared pooled averages on healthy skin. Steiner compared individual readings across a wide range, including the high values of palms, and found the devices diverge at the ends: "the open TEWL meter's capacity for measuring high values of TEWL was restricted, and that the closed-chamber TEWL meter was less sensitive to differences in the lower range of measurements". So for averages on healthy skin the device matters little; for an individual reading, especially a high palm value or a very low one, it matters a great deal. The palm pair in the table, open chamber 25.75 and closed chamber 68.4, fits that pattern, though the two studies also differed in room, population and exact palm site, so the gap cannot be put down to the instrument alone.

The room

The guidelines summarised by du Plessis and colleagues list air movement, ambient temperature, relative humidity, direct light and season as environmental factors that affect TEWL. Their recommendations, partly taken from the European EEMCO group, are precise:

1. Acclimatise the person for at least 15 to 30 minutes at 20 to 22 C and 40 to 60 percent relative humidity before measuring TEWL, to eliminate sweating.
2. Expose the site to the room air for at least 10 minutes before measuring.
3. Warm the instrument: switch it on 15 to 30 minutes before use, in the room where it will be used.
4. Keep products off the site for 12 hours beforehand, and caffeinated drinks off the menu for 3 hours.
5. Record everything: room temperature and humidity, acclimatisation time, time since washing, time since any product, and how the probe was held.

Neither study in the table met all of that exactly. Mayrovitz's room was 20.4 C and 58.7 percent humidity, inside the window. Herrero-Fernandez and colleagues measured at 23 C and 35 percent humidity, a degree warmer and drier than the recommended range. Both gave at least 20 minutes of adaptation. That is normal for published work, and it is one reason that, in the guidelines' own words, there is "a lack of consensus regarding the reference values for normal and/or diseased skin". Their recommendation follows from it: report and compare TEWL "as a relative (or percent) change" at a given site, not as absolute values.

The person and the last few hours

The guidelines also list what the person brings: skin temperature, sweating, skin condition, and recent washing, wet work, surfactants, occlusion, topical products and smoking. Reports of a daily rhythm conflict, so repeat visits should be at the same time of day. Mayrovitz measured everyone between 14:00 and 16:00, with no handwashing for four hours beforehand.

What water, heat and washing do to the reading

Herrero-Fernandez and colleagues at the University of Granada measured what everyday exposures do to healthy hands and forearms, with 50 volunteers and a Tewameter. Each person put one hand in hot water (41.29 C on average) and the other in cold water (11.13 C) for 10 minutes, then had both palms measured again. Separately, each rested a forearm for five minutes on a plate with a 4 to 44 C gradient.

Hot water more than doubled the palm reading, and even cold water raised it by over a third. The authors' conclusion is that "long and continuous water exposure damages skin barrier function, with hot water being even more harmful", and they advise cold or lukewarm water for handwashing. That is the case for lukewarm water, and for the hand balm for frequent handwashing on skin that has just been washed. The du Plessis review adds that washing with anionic surfactants, antimicrobial soaps and even moisturising soaps raises TEWL, while gloves cause transient increases for about 30 minutes after they come off.

A single hot soak moved the palm reading by 33 g/m2/h. A study that does not control washing before measuring has measured the washing.

What a raised TEWL means

On the same site, measured the same way, a higher TEWL means water is crossing the stratum corneum faster, and that is usually because the barrier is impaired. du Plessis and colleagues note TEWL is higher in atopic dermatitis in general, "even when measured at uninvolved skin". It is one way the evidence on balms for eczema-prone skin can be read; the same reasoning sits behind the cheek measurements on balm for rosacea-prone skin.

What a raised TEWL does not give is a threshold. There is no agreed line in g/m2/h above which skin counts as damaged. The nearest thing in the guidelines is a study of apprentice hairdressers and nurses which found that hairdressers with hand TEWL above 15 g/m2/h had an increased risk of hand dermatitis, an increase that "was not statistically significant", and no relationship at all for nurses. Fifteen is below the normal palm figures in the table, which is the problem with any fixed cut-off: it depends on which part of the hand and which instrument.

Lips: the highest reading on the face

Kobayashi and Tagami measured the vermilion border of 303 healthy Japanese women aged 21 to 80, in winter, with a closed chamber instrument, chosen "to avoid the effect of breathing". TEWL "was significantly almost three times as high on the lips as that on the cheek", and surface hydration measured by conductance was about one third of the cheek's. They found the surface cells of the lip parakeratotic, meaning they still held nuclei, and larger than the cheek's, and conclude that "the incomplete corneocyte formation of the lip surface is responsible for the poor barrier function and water-holding capacity".

Lips lose water fastest of anything on the face and hold the least. That is why a lip product has to be reapplied, why licking makes things worse, as set out in does lip balm dry your lips, and why occlusives top the list in ingredients for chapped lips. It is also why an overnight lip mask works: hours of an undisturbed film on the site with the highest loss. Dry cabin and winter air steepen the same gradient, the subject of flying and travelling with balms and cold weather skin.

Reading a TEWL claim on a balm

Balm marketing uses TEWL in two forms: "reduces water loss by X percent" and "improves barrier function". Both can be honest, and both can be produced by the method rather than the product.

A percentage depends on the starting rate

Berube and colleagues spread white petrolatum at a fixed 0.1 mm on four people and measured how much it cut their water loss. The film was the same; the results were not.

The same film cut water loss by anything from 77.5 to 93.0 percent, and the subject with the highest base rate got the smallest percentage. The authors' explanation: "If the results are calculated as the per cent reduction of transepidermal moisture loss achieved by the application of a uniform film of product, the results vary widely from subject to subject because the subjects have different base rates." Their alternative, the occlusive thickness, is the film thickness that would in theory stop the loss entirely, extrapolated from several thicknesses. It came out at 0.38 to 0.40 mm for every subject: a property of the material, as a percentage is not.

Two lessons follow. First, a percentage reduction is only meaningful with the base rate, the site and the film thickness beside it. The figure of around 98 percent quoted for petrolatum on is petroleum jelly bad for you and do expensive balms work better is a headline for the material, and in this one study a 0.1 mm film reached 77.5 to 93.0. Second, thickness matters enormously. At the density of about 0.9 g per cubic centimetre used elsewhere on this site, a 0.1 mm film is about 9 mg per square centimetre, about four and a half times the 2 mg per square centimetre dose that the ointment base formula gives as the convention for comparing products, and far more than a normal application; how much balm to apply puts lip doses at 10 to 40 milligrams for the whole mouth. How the materials compare at equal oil content is on mineral oil vs petroleum jelly, and where each class of ingredient sits is on petrolatum and mineral oil.

A reading through a film measures the film

Measure TEWL immediately over a freshly applied balm and the probe reads the water getting through the balm. That is a legitimate occlusion measurement. It is not a barrier measurement, because the skin's own barrier may be unchanged underneath. This is why the guidelines keep topical products off the measuring site for 12 hours before a baseline, and why they ask whether the product was wiped off and how long before the reading. A claim that a balm "improves barrier function" needs readings taken after the product has gone, typically over days of use, against an untreated site. A claim of "reduced water loss" measured an hour after application is a claim about occlusion, which is what an anhydrous product does and is worth saying plainly, as the barrier repair face balm does in its claim wording.

Questions to ask of any TEWL claim

1. Which site? Forearm, face, hands and lips are different measurements. A forearm result does not transfer to lips.
2. Which instrument? Open, closed or condenser chamber, by name. Results from different types cannot be converted into each other.
3. What was the base rate, and against what control? A percentage without the starting value, or without an untreated site measured at the same time, is not interpretable.
4. How long after application? Minutes to hours means occlusion. Days of use with the product removed before measuring means something about the skin.
5. How much product, how many people, what room? A thick film on a handful of forearms is not a lip balm used normally, and without temperature, humidity and acclimatisation time the number cannot be checked.

Where the claim sits legally is another matter. A claim to treat or repair damaged skin can move a product from cosmetic into drug territory, the line drawn on cosmetic vs drug claims. For buyers, the same questions apply to whatever the jar says, using the approach on reading a balm label.

Can you measure TEWL yourself?

Not usefully: the meters need calibration, warm-up time and a controlled room. A maker can borrow the logic, though. TEWL studies compare a treated site with an untreated one on the same person at the same time, which cancels out the room, the season and the person. The bench version is a split test, one product on one hand and the old formula on the other, judged at the same time of day over several days, using the protocols on sensory testing.

Where these numbers stop applying

The values on this page are for healthy adults at rest in controlled rooms. The forearm and palm figures come from two studies of adults in Florida and Granada, one of 40 and one of 50 people, on two different instruments. The pooled extremes come from an abstract of a meta-analysis whose underlying studies used many devices and conditions. The lip data are from Japanese women measured in winter and are given as a ratio to the cheek, because the abstract gives no absolute values. None of the sources measured children, infants or diseased skin directly, and none tested a balm. The Berube figures come from four people in 1971, measured with the methods of the time, and are used here only for what they show about percentages.

What the evidence does settle is how to read the number. Healthy forearm skin reads about 8 to 10 g/m2/h, other sites far more or less, and the reading moves with the instrument, the room and the last hour of the person's day. Against a baseline on the same site, device and conditions, TEWL is a good measure of the barrier. As a standalone number it says almost nothing.

Reviewed and updated 20 October 2026. Spotted an error? Tell us and we will fix and log it.