What is the skin barrier and why does it matter for eczema?

The skin barrier is one of those terms that gets used constantly in skincare marketing without much explanation of what it actually is, what it does, or what happens when it fails. For people with eczema, understanding the skin barrier isn't background information — it's the core of understanding the condition itself.

What the skin barrier is

The outermost layer of the skin is called the stratum corneum. It's made up of flattened, dead skin cells called corneocytes, arranged in overlapping layers and held together by a mixture of lipids — fats — that fill the spaces between them. Dermatologists often describe this structure as "bricks and mortar": the corneocytes are the bricks, and the lipid matrix is the mortar.

The lipids in the barrier are a specific mixture: ceramides make up roughly 50% of the total, with cholesterol and fatty acids making up most of the rest. When this ratio is disrupted — when ceramide levels drop, or the fatty acid composition changes — the barrier becomes permeable.

A functioning skin barrier does two things simultaneously: it keeps moisture inside the skin, and it keeps irritants, allergens, and microorganisms out. These two functions are inseparable. A barrier that lets moisture out also lets things in.

What goes wrong in eczema

In people with eczema, the skin barrier is structurally compromised. The most well-understood cause is a mutation in the FLG gene, which codes for a protein called filaggrin. Filaggrin is essential to the formation of the corneocyte layer — without enough of it, the barrier leaks.

Roughly 30% of people with eczema carry a filaggrin mutation. But filaggrin loss isn't the only mechanism. Inflammation itself reduces ceramide production, which further compromises the barrier. This creates a self-reinforcing cycle: the broken barrier allows irritants in, which cause inflammation, which further breaks down the barrier.

Why this matters for treatment

If the skin barrier is the root of the problem, then repairing it is the foundation of any effective treatment. This is why emollient therapy — regular, liberal use of moisturisers that contain barrier-repairing ingredients — is the most evidence-based intervention for eczema, ahead of any medication.

Topical steroids reduce inflammation effectively, but they don't repair the barrier. They suppress the immune response, which provides relief, but if the barrier remains compromised, the triggers keep getting through and the inflammation returns when the steroids are stopped.

What damages the barrier further

Hot water strips lipids from the barrier. Soap disrupts the barrier's pH and removes lipids. Fragrance is a direct irritant. Wool and synthetic fabrics cause friction that mechanically disrupts the surface layer. Low humidity environments accelerate transepidermal water loss — this is why eczema typically worsens in winter.

How to support barrier repair