Dry skin is generally treated as a shortage of moisture, addressed by applying more of it. The outermost layer of skin functions as a barrier that limits water loss, and most dryness reflects damage to that barrier rather than insufficient water.
The outer layer is a sealed structure
The uppermost layer consists of flattened dead cells embedded in a matrix of lipids, an arrangement often compared to bricks held together by mortar.
The lipid matrix, composed largely of ceramides, cholesterol and fatty acids, is what actually restricts the passage of water outwards.
Its integrity determines the rate of water loss through the skin, which is the measurement researchers use to assess barrier function directly.
Damage increases loss rather than reducing supply
Water reaches the surface layers continuously from the tissue beneath. Dryness arises when it escapes faster than it is replaced.
Surfactants in cleansers dissolve barrier lipids along with the dirt they are intended to remove, which is why frequent washing produces the effect.
Hot water, low humidity and friction all contribute through the same route, and their effects accumulate rather than resolving between exposures.
Cracking sets up a self-sustaining cycle
As the outer layer loses water it becomes less flexible, and a rigid layer cracks under the normal movement of skin.
Cracks expose deeper layers to irritants and allow further water loss, which produces the itching and sensitivity that accompany persistent dryness.
Scratching removes more of the barrier, which is why the cycle tends to worsen without intervention rather than settle.
Moisturisers work through several different mechanisms
Occlusive ingredients form a physical film that slows evaporation, which addresses the loss directly and is the fastest-acting approach.
Humectants attract water into the outer layer, though in dry air they can draw it from deeper tissue if not paired with an occlusive.
Ingredients that supply barrier lipids aim to restore the structure itself, which works more slowly and addresses the underlying cause rather than the symptom.
Why persistent dryness is worth investigating
Some barrier defects are inherited, including variants affecting the protein that structures the outer layer, which underlies a substantial share of eczema.
Dryness can also be a feature of thyroid disorders, diabetes and several medications, in which case topical treatment addresses only the surface.
Skin that remains dry, itchy or cracked despite consistent care warrants assessment rather than escalation through further products.