Skin keeps the body’s water in far better than it keeps outside water out, and both jobs fall mostly to a layer thinner than a sheet of paper. The waterproofing isn’t a rubbery coating. It’s a layer of dead cells sealed together with fats.1Experimental & Molecular Medicine. Bricks and mortar of the epidermal barrier It is a slow leak rather than a perfect seal, which is why skin dries out, cracks, or wrinkles in the bath. The everyday choices that matter most for it, like soap and moisturizer, work on those same fats.
Bricks and Mortar in the Outer Layer
The outermost layer of skin is made of flattened cells with no nucleus.2PubMed Central. Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties They form as living skin cells collapse and their internal protein fibers are pulled into a tight network.3PubMed. Skin barrier dysfunction and filaggrin The finished cells give mechanical strength, help block ultraviolet light, and help hold water.4PubMed Central. Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties
Between these cells sit stacked sheets of fat made of ceramides, cholesterol and free fatty acids.5PubMed Central. Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties Ceramides are waxy fats, and they’re a main ingredient in many barrier creams. Because these fats repel water, they form the layer that stops water passing between cells.6PubMed Central. Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties Cells and fats together make the layer water-impermeable, like bricks and mortar.7Experimental & Molecular Medicine. Bricks and mortar of the epidermal barrier In healthy skin the three fats are present in roughly equal amounts.8PubMed Central. Clinical Perspective: Moisturizers vs. Barrier Repair in the Management of Atopic Dermatitis The structure holds up even when the mix of ceramide types shifts, as long as cholesterol and ceramides stay balanced.9Elsevier / Biochimica et Biophysica Acta (BBA) – Biomembranes. The role of ceramide composition in the lipid organisation of the skin barrier The dead cells themselves can soak up water and swell, which is why a long soak changes how skin looks and feels.
Water has to take a long, winding path around the bricks, through the fatty mortar. Remove the mortar and gaps open even if the bricks stay put.
Each brick has a history. Skin cells begin life in the deepest living layer, where they divide and are plump and active. As they move upward, they make large amounts of structural protein and packets of fat that they release into the spaces between cells. At the top they lose their nucleus and inner machinery, then flatten into tough, thin plates. That’s why the outer layer is made of cells that are no longer alive but still do real work.
The wall is never finished. New cells push up from below and flatten as they rise, while the outermost cells gradually flake away unseen.
Hidden Seals Beneath the Surface
A second seal lies under the dead layer. Just below it, living cells are joined by tight junctions, seals that close the gaps between neighbouring cells.10PubMed. Importance of Tight Junctions in Relation to Skin Barrier Function Along with other anchors that hold cells together, they work with the fats to stop dehydration.11PubMed Central. Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties
Skin’s surface is also slightly acidic, often called the acid mantle. pH is a scale where lower numbers are more acidic and 7 is neutral, and healthy skin sits below 7. Plain water and many bar soaps are neutral or alkaline, above skin’s level.
The acidity matters because the barrier fats only stack into multiple sheets in an acidic, not a neutral, environment.12PubMed. Acidic pH Is Required for the Multilamellar Assembly of Skin Barrier Lipids In Vitro Enzymes that make ceramides work best at acidic levels around pH 4.5 to 5.6, so anything that disturbs these enzymes changes ceramide levels in the outer layer.13PubMed Central. Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties
The acid mantle isn’t a separate coating you can see or wipe off. It is the slightly acidic condition of the outermost skin, and it comes from the skin’s own products, including fatty acids and the breakdown products of the skin’s natural moisturizers. Because the skin makes it, it can recover after being disturbed, though that takes time. A single wash doesn’t permanently ruin skin, but repeated alkaline washing can keep it off balance.
Other enzymes that work at this acidity cut the links between old cells so they shed in an orderly way.14PubMed. Degradation of corneodesmosome proteins by two serine proteases of the kallikrein family, SCTE/KLK5/hK5 and SCCE/KLK7/hK7 If acidity is off, shedding can turn into visible flaking.
The Skin’s Built-In Moisturizers
A protein called filaggrin is the main source of the natural moisturizing factor, a mix of water-holding molecules.15PubMed Central. Levels of filaggrin degradation products are influenced by both filaggrin genotype and atopic dermatitis severity In the outer layer, enzymes break filaggrin into amino acids and related molecules, including one sold on labels as PCA.16PubMed. Skin barrier dysfunction and filaggrin These molecules keep the outer layer hydrated from within.17PubMed. Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin When the enzyme that cuts filaggrin is missing, levels of these moisturizers drop sharply in animal studies.18PubMed. Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin
Filaggrin is made in an inactive, bulky form inside living cells, where it helps pack protein fibers together. It’s cut into its active pieces only after cells reach the outer layer, which keeps water-attracting molecules from building up in living tissue. Further up, the pieces are broken down into the small molecules that hold water. The timing places the moisturizers exactly where dry air meets the skin.
Glycerol, carried up from below, also holds water. Living skin cells have channels that move water and glycerol.19PubMed. Roles of aquaporin-3 in the epidermis Mice lacking these channels had about one third the glycerol and about half to one third the water in the outer layer.20PubMed Central. Glycerol replacement corrects defective skin hydration, elasticity, and barrier function in aquaporin-3-deficient mice21PubMed. Selectively reduced glycerol in skin of aquaporin-3-deficient mice may account for impaired skin hydration, elasticity, and barrier recovery Their skin was less elastic and healed more slowly.22PubMed. Roles of aquaporin-3 in the epidermis Applying glycerol to their skin restored normal hydration and elasticity.23PubMed Central. Glycerol replacement corrects defective skin hydration, elasticity, and barrier function in aquaporin-3-deficient mice
On a label, glycerin is the same glycerol skin uses. Urea, lactic acid and PCA mimic the skin’s built-in moisturizers. These draw water in rather than sealing it.
Why Skin Wrinkles in the Bath
Bath wrinkles come from blood vessels in the fingertip narrowing.24PubMed. Water-immersion wrinkling is due to vasoconstriction Water seeps into fingers and soles through their many sweat ducts and changes the salt balance in the skin.25PubMed. Water immersion wrinkling–physiology and use as an indicator of sympathetic function That sets off nearby nerves, which signal the vessels to tighten.26PubMed. Water immersion wrinkling–physiology and use as an indicator of sympathetic function The fingertip pad then loses volume and pulls the skin down into folds.27PubMed. Water immersion wrinkling–physiology and use as an indicator of sympathetic function
Swelling alone isn’t the main cause; the nerve signal is, which is why fingers with damaged nerves often don’t wrinkle. Doctors have used the wrinkle test to check nerve function in fingers.
Water needs time to move down the sweat ducts and shift the salt balance, so wrinkles don’t appear the instant skin gets wet. A short rinse leaves fingers smooth, while a longer soak is needed to start the nerve response. The wrinkles smooth out within minutes after drying.
How Much Water Skin Lets Out
Doctors measure barrier health as water loss through the skin, in grams per square metre per hour.28PubMed. Transepidermal water loss in healthy adults: a systematic review and meta-analysis update In healthy adults it ranged from about 2.3 on breast skin to about 44 in the armpit.29PubMed. Transepidermal water loss in healthy adults: a systematic review and meta-analysis update Cheek skin loses more water and has a thinner outer layer than arm skin, at all ages.30PubMed. Age-dependent changes in stratum corneum barrier function
The thickness of the outer layer varies enormously too:
- Genital skin: about 6 cell layers31PubMed. Number of cell layers of the stratum corneum in normal skin – relationship to the anatomical location on the body, age, sex and physical parameters
- Face: about 9 layers32PubMed. Number of cell layers of the stratum corneum in normal skin – relationship to the anatomical location on the body, age, sex and physical parameters
- Trunk: about 13 layers33PubMed. Number of cell layers of the stratum corneum in normal skin – relationship to the anatomical location on the body, age, sex and physical parameters
- Arms and legs: about 15 layers34PubMed. Number of cell layers of the stratum corneum in normal skin – relationship to the anatomical location on the body, age, sex and physical parameters
- Palms and soles: about 47 layers35PubMed. Number of cell layers of the stratum corneum in normal skin – relationship to the anatomical location on the body, age, sex and physical parameters
- Heel: about 86 layers36PubMed. Number of cell layers of the stratum corneum in normal skin – relationship to the anatomical location on the body, age, sex and physical parameters
Day to day, facial skin dries and reacts faster than skin on your arms. Thick heel and palm skin tolerates rough wear but can crack when dry. The armpit figure includes sweat, not only leakage.
The measurement works by detecting vapor rising from the surface in a small chamber placed on the skin. A higher number means more water is escaping, so it’s used as a stand-in for how well the barrier is working.
What Breaks the Barrier Down
Harsh soaps can pull cholesterol, fatty acids and ceramides out of the fat layers or wedge into them.37PubMed Central. Skin Cleansing without or with Compromise: Soaps and Syndets That can cause dryness, more water loss, cracking, flaking, redness and itch.38PubMed Central. Skin Cleansing without or with Compromise: Soaps and Syndets Harsh soap removes more cholesterol than milder synthetic cleansers, likely because its alkalinity loosens the fats.39PubMed Central. Skin Cleansing without or with Compromise: Soaps and Syndets A common harsh detergent ingredient, sodium lauryl sulfate, disorders the tightly packed fats.40PubMed Central. Cleansing Formulations That Respect Skin Barrier Integrity Still, experts now think stripping fats is not even the main way soaps harm skin.41PubMed. Effect of soaps and detergents on epidermal barrier function They also disturb proteins in the outer layer, and alkaline conditions can raise the surface pH, which interferes with the enzymes that build and maintain the barrier.
For a gentler routine, look for labels saying syndet, soap-free or pH-balanced. Use less cleanser and lukewarm rather than hot water, and wash dry areas less often if they don’t get dirty.
Some people inherit a weaker barrier. They carry gene changes that reduce filaggrin, lowering their natural moisturizers.42PubMed Central. Levels of filaggrin degradation products are influenced by both filaggrin genotype and atopic dermatitis severity Babies with these changes lose more water through skin and are about twice as likely to have dry skin by 6 months.43PubMed. Filaggrin mutations in relation to skin barrier and atopic dermatitis in early infancy These changes are also linked to eczema in infancy.44PubMed. Filaggrin mutations in relation to skin barrier and atopic dermatitis in early infancy Dry skin since infancy, plus a family history of eczema, asthma or hay fever, can be one clue among several, though only genetic testing can confirm it. It’s worth discussing with a doctor if skin flares often.
Dryness itself does damage. As the outer layer loses water, it shrinks and builds tension that feels like tightness.45Extreme Mechanics Letters. Predicting hydration and moisturizer ingredient effects on mechanical behavior of human stratum corneum That tension drives chapping and cracking.46Extreme Mechanics Letters. Predicting hydration and moisturizer ingredient effects on mechanical behavior of human stratum corneum
Helping the Barrier Repair Itself
Creams and ointments improve the barrier by supplying water and fats.47PubMed. Barrier repair therapy in atopic dermatitis: an overview Skin’s own fats applied in creams don’t just sit on top. They are absorbed and built into new barrier layers.48PubMed Central. Clinical Perspective: Moisturizers vs. Barrier Repair in the Management of Atopic Dermatitis Some products contain ceramides or look-alike pseudoceramides to replace missing fats and moisturizers.49PubMed. Barrier repair therapy in atopic dermatitis: an overview
The balance of fats matters. Creams with only one or two of the three skin fats slowed barrier repair, while all three in equal amounts allowed normal repair.50PubMed Central. Clinical Perspective: Moisturizers vs. Barrier Repair in the Management of Atopic Dermatitis Tilting the mix up to 3 parts of one fat to 1 of each other sped repair.51PubMed Central. Clinical Perspective: Moisturizers vs. Barrier Repair in the Management of Atopic Dermatitis In eczema, adequate fat replacement can help reduce inflammation and improve skin function.52PubMed. Barrier repair therapy in atopic dermatitis: an overview
On a barrier cream label, look for ceramides, cholesterol and fatty acids together. Glycerin adds water-holding power. Petrolatum seals water in but doesn’t rebuild the mortar.
Timing helps too. Apply within minutes after bathing while skin is damp, use thicker ointments on cracked heels and hands, and reapply after handwashing in dry weather.
How Human Skin Compares With Frogs
Tough keratin proteins evolved to strengthen skin and hold the fats that block water loss.53PubMed. Water relations of tetrapod integument Amphibians have little keratin and a thin outer layer.54PubMed. Water relations of tetrapod integument Desert frogs that resist drying keep their fat barrier on the outside of the skin.55PubMed. Water relations of tetrapod integument Our barrier sits within the skin, so it survives washing better than a surface coating. Amphibian skin also undergoes huge rearrangement moving from water to land.56Journal of Developmental Biology. The Complex Bridge between Aquatic and Terrestrial Life: Skin Changes during Development of Amphibians
Many amphibians take in oxygen and water through their skin, so a thin, permeable skin is useful to them. A waterproof layer like ours would block those jobs, which is why most amphibians don’t have one.