Your skin barrier: the brick wall that keeps water in and irritants out

You’ll see “skin barrier” on almost every moisturizer label these days. It sounds like marketing, but it’s a real structure, and once you understand it a lot of skin care advice suddenly makes sense. Here’s what it is, what it does, and how to tell when it’s in trouble.

What is the skin barrier, exactly?

Your skin has several layers. The outermost one is the epidermis, and the very top of the epidermis is a thin sheet called the stratum corneum (Latin for “horny layer”). That sheet is your skin barrier.

It’s astonishingly thin: roughly 10 to 20 micrometres, thinner than a sheet of printer paper. Yet it’s the part of your skin that does most of the protecting.

Dermatologists describe it with a “bricks and mortar” picture, and the analogy holds up well:

• The bricks are corneocytes: flattened, dead skin cells packed with a tough protein called keratin. They started life at the bottom of the epidermis, moved upward, and hardened along the way.
• The mortar is a matrix of fats, called lipids, packed between the bricks in orderly stacked layers. Three kinds dominate: ceramides, cholesterol and free fatty acids.
• The rivets are corneodesmosomes, tiny protein connectors that hold the bricks to each other so the wall doesn’t crumble.

Inside each brick there’s one more thing worth knowing: natural moisturizing factor, or NMF. It’s a mix of small water-loving molecules, mostly amino acids, that pull water in and hold it. Much of it comes from the breakdown of a protein called filaggrin, which is why a faulty filaggrin gene is so strongly linked to dry skin and eczema.

What does it actually do?

The barrier works in two directions at once. Dermatologists call it an “inside-outside” and an “outside-inside” barrier.

Inside-outside: it keeps water in. Your body is mostly water, and the air around it is usually much drier. Without the lipid mortar, water would simply evaporate through the skin. The tiny amount that still escapes is called transepidermal water loss, or TEWL, and researchers use it as a score for how well the barrier is holding up. A higher TEWL means a leakier barrier.

Outside-inside: it keeps things out. Irritants, allergens, chemicals and microbes all have to get past this wall before they can cause trouble deeper in the skin. When the wall is intact, most of them can’t.

It also does quieter jobs. The acids released from NMF keep the surface of the skin slightly acidic, which helps the enzymes that shed old cells work properly and discourages unwanted bacteria. And one of the filaggrin breakdown products, urocanic acid, absorbs some ultraviolet light, giving the surface a little built-in sun protection (not enough to replace sunscreen).

It’s not a wall, it’s a construction site

The bricks-and-mortar image makes the barrier sound fixed and permanent. It isn’t. It’s constantly being rebuilt from underneath.

New cells form at the bottom of the epidermis and travel upward. The trip from the bottom to the surface, where the cells finally flake off, takes about four weeks. The stratum corneum itself turns over in roughly half that time. That’s why a patch of dryness that appears after a week of over-washing can also clear up within a couple of weeks once you stop.

The barrier can also respond to damage surprisingly fast. When water loss goes up, the cells just below the surface release lipids they’ve been holding in reserve, which partly reverses the leak within minutes. Within a few hours they ramp up production of new lipids to finish the repair. The catch is that repeated damage, day after day, can outpace this repair, and that’s when things start to go wrong.

Signs your barrier is struggling

A damaged barrier lets water out faster and lets irritants in more easily, and those two things produce a recognizable pattern:

• Skin that feels tight, especially after washing
• A dull, rough or scaly look, or visible flaking
• Stinging or burning when you apply products that never bothered you before
• Itch, sometimes without a visible rash
• Redness that comes and goes

Researchers describe this as a staircase. First the barrier gets stripped and water loss rises, with no visible change. Then the skin looks and feels dry. If the insults continue, the dryness can progress to eczema-like inflammation, with redness, cracking and swelling. Catching it at the “tight and flaky” step is much easier than dealing with it later.

If your skin is cracked, weeping, or not better after two weeks of gentle care, see your doctor or pharmacist. Some skin conditions, thyroid problems and medications can cause dryness that skin care alone won’t fix.

What damages it?

Most barrier damage comes from ordinary daily habits rather than anything dramatic. The most common culprits identified in dermatology reviews are:

• Over-washing and long, hot showers. Water and heat pull lipids and NMF out of the top layer.
• Harsh cleansers and true soaps. Traditional soap is alkaline, and strong cleaning agents (surfactants) strip lipids along with dirt. The better a cleanser is at removing oil, the more barrier damage it tends to cause.
• Scrubs, astringents and over-exfoliating. Anything that physically or chemically removes the top layer removes barrier along with it.
• Low humidity. Canadian winters are the classic case: cold outdoor air holds little moisture, and indoor heating dries it out further, so water leaves the skin faster.
• Irritants at work or at home. Frequent hand-washing, cleaning products, solvents and wet work all take a toll, which is why hairdressers, nurses and cleaners so often have dry, cracked hands.
• Age. Oil production drops noticeably after about 40, so the skin has less help keeping the surface supple.
• Some medications and treatments. Certain topical treatments (retinoids are a well-known example), some oral medications and cancer therapies can dry the skin as a side effect.

Genetics play a part too. People with atopic dermatitis (the most common form of eczema) often have fewer lipids in the mortar and a variant of the filaggrin gene, so their barrier starts out leakier and is more easily disturbed.

How to look after it

The good news is that the fixes are mostly about removing the insults and giving the skin what it’s short on:

• Wash with lukewarm water rather than hot, and keep showers short.
• Use a gentle, fragrance-free cleanser instead of bar soap on dry or sensitive areas.
• Moisturize while the skin is still slightly damp. Applying moisturizer right after bathing traps water in the stratum corneum instead of letting it evaporate.
• Look for moisturizers that combine humectants (ingredients such as glycerin that draw water in), emollients (lipids such as ceramides that fill gaps in the mortar) and occlusives (ingredients such as petrolatum that form a film to slow water loss). A fragrance-free moisturizer is a good starting point if your skin is reactive.
• Wear gloves for wet work and for cold, windy days.

The barrier is built to repair itself. Your job is mostly to stop knocking it down faster than it can rebuild.

Think of the stratum corneum as a thin, living brick wall: dead cells for bricks, ceramides, cholesterol and fatty acids for mortar, and a built-in humectant inside each brick. It keeps water in and the outside world out. When it’s tight, flaky or stinging, something is stripping it faster than it can repair, and the usual suspects are hot water, harsh cleansers and dry air. Ease up on those, moisturize consistently, and see your doctor if it isn’t improving.