The Science of Your Skin Barrier
Most of what skincare can and cannot do comes down to one thin layer: the stratum corneum, the outermost part of the epidermis. Understanding how it works makes product claims much easier to judge.
The brick-and-mortar barrier
The stratum corneum is made of flattened, dead skin cells called corneocytes, surrounded by a mixture of lipids. Dermatologists often compare it to a brick wall: the cells are the bricks and the lipids are the mortar. The main lipids are ceramides, cholesterol and free fatty acids, present in roughly balanced proportions in healthy skin.
Inside the cells sits a group of water-attracting substances known as natural moisturising factor, which helps the outer layer stay supple.
What the barrier does
- It slows the loss of water from the body, measured in studies as transepidermal water loss.
- It keeps out irritants, allergens and microbes.
- It maintains a slightly acidic surface, usually somewhere around pH 4.5 to 5.5, which supports healthy shedding and the skin's microbial balance.
When the barrier is weakened
Hot water, harsh soaps, over-exfoliation, low humidity and some medicines can all deplete barrier lipids. The result is dryness, tightness and sensitivity. In atopic eczema, the barrier is weaker to begin with; in some people this is linked to changes in a protein called filaggrin. That is why emollients are a mainstay of eczema care.
How skincare ingredients fit in
| Type | What it does | Examples |
|---|---|---|
| Humectants | Draw water into the outer layer | Glycerin, urea, hyaluronic acid |
| Emollients | Soften and smooth | Plant oils, esters, squalane |
| Occlusives | Slow water loss | Petrolatum, dimethicone, shea butter |
| Barrier lipids | Replenish the "mortar" | Ceramides, cholesterol, fatty acids |
What creams cannot do
Most cosmetic ingredients act on the surface layers of the skin. Claims that a natural oil reaches "the deepest layers" or rebuilds the skin from within are not supported by evidence. Absorption into the outer layer is real and useful; transformation of the skin's structure is not something a cosmetic can promise.
When to see a dermatologist
See a doctor or dermatologist if you have a rash that keeps returning, skin that cracks or bleeds, itching that disturbs sleep, a changing mole, or acne that scars. Many skin conditions respond well to prescription treatment, and early advice saves a lot of trial and error.