Your skin constantly loses a small amount of water to the air, even when you are not sweating. The rate of that loss, called transepidermal water loss (TEWL), is the most widely used laboratory measure of how well the skin's outer barrier is working. This article explains what TEWL is, how researchers measure it, why careful test conditions matter, and what a reading can and cannot tell you. It also shows how the measurement is used to study hygiene habits and topical materials such as silver and chitosan. The goal is to help you read claims about "barrier-friendly" products with an informed eye.
What transepidermal water loss is, and why the barrier matters
The body's water-holding barrier sits almost entirely in the stratum corneum, the outermost layer of the epidermis. It is only a fraction of a millimetre thick. Dermatologist Peter Elias and colleagues at the University of California, San Francisco, popularised a "bricks and mortar" description of this layer. Flattened, protein-filled dead cells called corneocytes are the bricks. The mortar between them is an ordered mix of ceramides, cholesterol and free fatty acids. Water escapes mainly by slow diffusion through that lipid mortar.
The design is remarkable. A layer of cells that have deliberately died keeps a living, water-filled body from drying out while still letting it breathe. It renews itself roughly every few weeks and signals the living cells beneath it to repair damage. Those who see the body as made rather than accidental will find a quiet wisdom in how finely this balance is set.
When the barrier is healthy, TEWL is low. When lipids are stripped, cells are disrupted, or the layer is thinned, water escapes faster and TEWL rises. The barrier that holds water in also helps keep irritants, allergens and microbes out. That is why TEWL is used as a general stand-in for barrier integrity.
How the laboratory measures it
TEWL is not measured directly as water leaving the skin. Instead, instruments measure the humidity gradient in the air just above the skin surface. They then calculate the flux, or flow rate, using Fick's law of diffusion. Results are reported in grams of water per square metre of skin per hour (g/m²/h). Three main designs are in use:
- Open-chamber devices. These use a cylinder open to the room. Two sensors at different heights record humidity and temperature. The gradient between them gives the rate of water loss. The method was developed by Gert Nilsson at Linköping University and published in Acta Physiologica Scandinavica in 1977. It remains the reference approach in much of the literature. Its weakness is sensitivity to drafts and room humidity.
- Closed-chamber devices. These seal a small chamber against the skin and measure how quickly humidity rises inside it. They are less affected by air currents. However, the reading is brief, because trapped water vapour soon begins to suppress further evaporation.
- Condenser-chamber devices. These keep the top of a closed chamber cold enough to condense water out of the air. This allows continuous readings without the chamber saturating.
Studies comparing devices generally find that they rank skin sites and conditions similarly. Their absolute numbers differ, however. Values from different instruments, or different laboratories, should not be treated as interchangeable. A 2018 "Research Techniques Made Simple" review in the Journal of Investigative Dermatology makes this point. Its authors were Helen Alexander, Carsten Flohr and colleagues at King's College London and collaborating centres. They note that TEWL is best used to compare groups or track change within the same person using the same method.
Getting a trustworthy reading
TEWL is easy to measure badly. Sweat, room humidity, skin temperature and even a nearby person breathing can distort the result. Researchers have therefore developed formal guidelines:
- The Standardization Group of the European Society of Contact Dermatitis issued guidance in Contact Dermatitis in 1990 (Pinnagoda, Tupker, Agner and Serup).
- The European EEMCO group published guidance for cosmetic science in 2001.
- International guidelines for non-clinical settings appeared in Skin Research and Technology in 2013 (du Plessis and colleagues).
The common recommendations are consistent:
- Participants rest in the test room for roughly 15 to 30 minutes before measurement, so skin temperature and sweating settle.
- The room is kept at a controlled temperature, typically around 20 to 22°C, with relative humidity near 40 to 60 percent and no drafts.
- No products are applied to, and no washing is done on, the test site for a set period beforehand.
- Multiple readings are taken at the same anatomical site, with the probe held still and without pressure.
- Body site is always reported, because TEWL varies greatly across the body.
A study that does not describe these conditions deserves less weight, however striking its numbers.
What the numbers mean, and what they do not
There is no single "normal" TEWL, because values depend on body site, age, climate and instrument. As a rough guide, healthy forearm skin under standard conditions often reads in the single digits of g/m²/h. The face tends to read higher. The palms and soles read much higher, largely because of sweat-gland activity rather than a weak barrier.
A 2013 systematic review and meta-analysis examined age differences. It was led by Jan Kottner and Ulrike Blume-Peytavi at Charité–Universitätsmedizin Berlin and published in Archives of Dermatological Research. It found that older adults did not have higher baseline TEWL than younger adults, and at some sites it was lower. This surprises people who assume ageing skin is "leakier." Older skin is often slower to recover after injury, however, which a single resting measurement does not capture.
Several limits apply to any single reading:
- A high reading shows that water is escaping faster. It does not say why. Irritation, inflammation, a minor abrasion, sweating or a warm room can all raise it.
- TEWL measures one function, water retention. Other barrier roles, such as immune and microbial defence, can change without large shifts in TEWL.
- Occlusive products such as petrolatum can lower TEWL simply by forming a film. A lower reading straight after application does not necessarily mean the skin itself has repaired. Researchers often measure again after the product is removed. They may also use "challenge" tests, deliberately stressing the skin and tracking recovery.
One standard challenge is a controlled patch of the detergent sodium lauryl sulfate. Standardized protocols for this test were published in Contact Dermatitis in 1997. The detergent reliably raises TEWL, and investigators then compare how quickly skin recovers, with or without an intervention. Tape stripping, which removes layers of stratum corneum with adhesive tape, is another. Much of what is known about barrier repair comes from such designs, first in animal models and then in human volunteers.
What TEWL research has shown in people
The strongest human evidence links TEWL to eczema (atopic dermatitis). In 2006, Colin Palmer, Irwin McLean and colleagues at the University of Dundee reported in Nature Genetics that common loss-of-function mutations in the filaggrin gene are a major risk factor for eczema. Filaggrin is a protein essential to the stratum corneum.
Two infant studies followed. Flohr and colleagues reported in the British Journal of Dermatology in 2010 that infants carrying these mutations had higher TEWL at three months of age. In the BASELINE birth cohort in Cork, Ireland, Maeve Kelleher, Jonathan Hourihane and colleagues measured TEWL in newborns. They reported in the Journal of Allergy and Clinical Immunology in 2015 that higher TEWL at two days and at two months predicted eczema at one year. Notably, this was true before any rash appeared.
These findings led to a reasonable hypothesis: that strengthening the barrier from birth might prevent eczema. Two large randomised trials, both published in The Lancet in 2020, tested it directly.
- The UK BEEP trial found that daily emollient use in the first year did not prevent eczema by age two.
- The Norwegian and Swedish PreventADALL trial found that regular skin care did not prevent eczema either.
The lesson is an important one. A measurement can predict a condition reliably without an intervention aimed at that measurement changing the outcome. Good science holds both findings at once.
Occlusive moisturizers are better understood. Research from Elias's group, published in the Journal of the American Academy of Dermatology in 1992, showed that petrolatum permeates the outer stratum corneum. It also allowed normal barrier recovery to proceed after injury, rather than simply sitting on the surface.
Hygiene, materials and reading product claims
Hygiene is an act of care for oneself and one's household, and it has a cost the skin must bear. Frequent washing with soap and water removes barrier lipids. Healthcare workers, who may wash dozens of times a shift, have high rates of irritant hand dermatitis. Studies of hand hygiene, including work by John Boyce and colleagues around 2000, found that alcohol-based hand rubs with added emollients caused less dryness and irritation than repeated soap-and-water washing. The World Health Organization's 2009 hand hygiene guidelines reflect this. For families, the practical point is modest. Cleanliness and barrier care are not opposites. Moisturizing after frequent washing is part of doing hygiene well.
Topical materials can be studied the same way. Two often discussed are silver and chitosan:
- Chitosan. This is a positively charged polysaccharide made by deacetylating chitin, the structural material of crustacean shells and fungal cell walls. It forms thin films and holds water, and it is widely used in wound dressings and cosmetics. Laboratory studies describe its film-forming and moisture-retaining properties. Well-controlled human data measuring its effect on TEWL are limited, so claims about barrier benefit should be regarded as preliminary.
- Silver. Silver shows broad antimicrobial activity in laboratory studies, which is why it appears in some dressings. Cell-culture work adds a caution. A 2004 study by Poon and Burd at the Chinese University of Hong Kong, published in Burns, found silver toxic to human skin cells called keratinocytes and fibroblasts at certain concentrations. Laboratory activity does not predict effects on intact human skin. Separately, long-term ingestion of silver can cause argyria, a permanent blue-grey skin discoloration.
Products combining these materials are sold as cosmetics or skin products, not approved drugs. GermProof, a chelated-silver and chitosan formulation, is one example. For any such product, a reasonable question is whether barrier data exist. Such data would come from TEWL studies at stated body sites, under standardized conditions, with appropriate controls, and ideally in people rather than only in a dish.
You are entitled to ask for that evidence, weigh it, and make choices about your own skin with your physician. That is what informed consent looks like in everyday life. It rests on understanding rather than slogans, and on a partnership with a clinician who knows you.
Key takeaway: TEWL is a precise and useful window on skin barrier function when it is measured under controlled conditions and read as a comparison rather than a verdict, and any claim about a product's effect on the barrier should be judged by whether it rests on that kind of careful human measurement.