Here is something most skincare brands would rather you did not think about too carefully: your skin is losing water right now. Not because anything is wrong with it, but because that is simply what skin does. Water moves continuously from the deeper layers of your body, through your skin, and out into the air around you. The question is never whether this happens, but how fast.
That rate of water loss has a name: transepidermal water loss, almost always shortened to TEWL (pronounced “tule”). It is one of the most studied measurements in dermatology and cosmetic science, and it sits at the very center of what skin flooding is designed to address. Understanding TEWL explains why some people seem perpetually dehydrated no matter how much moisturizer they apply, and why the sequence and timing of products matters far more than most people realize.
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The Basic Mechanics of TEWL
Water does not stay put inside the body. It follows concentration gradients, moving from areas of higher water content toward areas of lower water content. The deeper layers of the skin are relatively water-rich, while the outer surface of the stratum corneum is comparatively dry, and the air beyond it is drier still. This creates a constant outward pressure, a gentle but relentless pull that draws water toward the surface and eventually into the atmosphere.
Under healthy conditions, the skin barrier manages this pressure beautifully. The lipid matrix of the stratum corneum, that carefully organized mixture of ceramides, cholesterol, and free fatty acids, acts like a series of watertight membranes stacked together. Water molecules can still move through this system, but they must take a winding, tortuous path that slows their progress considerably. Healthy skin loses roughly 3 to 5 grams of water per square meter per hour through this passive process. It sounds like a lot until you compare it to what happens when the barrier breaks down.
TEWL vs. Sweating: An Important Distinction
It is worth clarifying that TEWL is entirely separate from sweating. Sweat is an active, regulated process driven by the eccrine glands in response to heat, exercise, or emotional stress. TEWL is passive and constant, requiring no glandular activity at all. It happens while you sleep, while you sit still, and in every climate. Researchers measure it with specialized devices called tewameters or vapometers that detect the moisture gradient just above the skin’s surface, making it possible to assess barrier function precisely without any invasive testing.
When TEWL Becomes a Problem
A certain baseline level of TEWL is not just normal; it is unavoidable. The problems begin when that baseline rises significantly. In skin affected by eczema, for instance, TEWL rates can be three to five times higher than in healthy skin, even in areas that appear visually unaffected. Psoriasis, rosacea, and contact dermatitis all show similarly elevated TEWL rates. The barrier is not just cosmetically compromised in these conditions; it is functionally failing at its primary job.
But you do not need a diagnosed skin condition to experience problematic TEWL. Everyday habits and environmental exposures can raise TEWL meaningfully in otherwise healthy skin, quietly draining moisture faster than any moisturizer can replace it.
Common Causes of Elevated TEWL
Harsh cleansing is one of the most significant and most overlooked drivers of elevated TEWL. Surfactants that strip the skin’s natural oils also disturb the lipid matrix of the stratum corneum, disrupting the organized lamellar structure that slows water loss. Hot water compounds this effect by softening and temporarily loosening the tight junctions between corneocytes, which allows water to move more freely. Long, hot showers followed by vigorous towel-drying are, from a TEWL perspective, a fairly efficient way to dehydrate your skin.
Over-exfoliation is another common culprit. Physical scrubs and high-concentration chemical exfoliants accelerate the shedding of corneocytes. Remove too many too fast and the barrier thins, lipid content drops, and TEWL climbs. Low-humidity environments, whether from cold outdoor air or dry indoor heating, pull water from the skin’s surface more aggressively, effectively increasing the concentration gradient that drives TEWL outward. UV exposure damages the structural proteins and lipids in the stratum corneum over time, leading to chronically elevated water loss in sun-damaged skin.
How Skincare Products Influence TEWL
Every product category in a well-designed skincare routine has a different relationship with TEWL, and understanding those relationships is what turns a collection of moisturizers into a coherent strategy.
Humectants like hyaluronic acid, glycerin, and sodium PCA do not directly reduce TEWL. What they do is attract water into the upper layers of the stratum corneum, raising the water content of the tissue itself. This matters because well-hydrated corneocytes are more pliable, better organized, and more effective at slowing passive water movement. A humectant-saturated stratum corneum is simply a better barrier than a dehydrated one.
The Role of Occlusives in Controlling TEWL
Occlusives are the ingredient category with the most direct impact on TEWL. They work by forming a physical film on the skin’s surface that water molecules cannot easily pass through, essentially adding a supplementary barrier on top of the skin’s own. Petrolatum is the most studied and most effective occlusive available, capable of reducing TEWL by more than 98 percent under controlled conditions. Dimethicone, beeswax, lanolin, and certain plant-based waxes offer varying degrees of the same effect.
This is why the final sealing step in a skin flooding routine is not optional window dressing. It is the step that actually controls TEWL long enough for the humectant and emollient layers beneath it to do their work. Without an occlusive layer, the water drawn in by humectants has a straightforward exit route. With one, it stays put while the skin gradually absorbs and benefits from everything that was applied before it.
Emollients and Barrier Restoration
Emollients occupy a middle position in the TEWL story. Ingredients like squalane, shea butter, and linoleic acid-rich plant oils do not form an occlusive film the way petrolatum does, but they integrate into the spaces between corneocytes and help restore the lipid mortar’s organization. Over time, consistent use of the right emollients helps the skin’s own barrier function more effectively, gradually reducing baseline TEWL rather than just managing it temporarily from the outside.
TEWL as a Measure of Routine Success
Dermatologists and cosmetic researchers use TEWL measurement as an objective way to evaluate whether a skincare routine is actually strengthening the barrier or merely creating the temporary sensation of comfort. A routine that lowers TEWL consistently over weeks is genuinely improving barrier integrity. One that feels nice in the moment but does not affect TEWL meaningfully may be offering sensory satisfaction without structural benefit.
For the rest of us without access to a tewameter, the practical proxy for TEWL is simple enough: does your skin feel comfortable and look plump several hours after you have applied your products, without needing to reapply? If the answer is yes, your routine is managing TEWL adequately. If your skin feels tight again within an hour or two, TEWL is winning the argument, and it is probably time to reconsider either the products you are using or the order in which you are applying them.