Your scalp constantly sheds old skin cells.
Usually, they are tiny enough that you never notice.
Dandruff happens when that shedding becomes more visible.
Skin cells are turning over all the time
New cells form deeper in the skin.
Older cells move outward and eventually shed.
On the scalp, oil and hair can make those cells clump together into visible flakes.
A common yeast can play a role
A yeast called Malassezia normally lives on human skin.
For some people, its interaction with scalp oils and the immune system appears to contribute to irritation and faster skin-cell turnover.
That can increase flaking.
Is dandruff caused by poor hygiene?
Not simply.
Not washing hair can allow oil and flakes to build up and become more noticeable.
But dandruff can occur in people with perfectly normal hygiene.
It is a skin condition, not a cleanliness verdict.
Why can an oily scalp still flake?
Oil and dryness are not opposites in the simple way people assume.
An oily scalp can still have irritation and increased shedding.
The oil can actually help flakes stick together and become more visible.
Why do anti-dandruff shampoos work?
Different products target different parts of the problem.
Some reduce yeast.
Some slow skin-cell turnover.
Some help remove scale.
The useful ingredient depends on the type of dandruff and how the scalp responds.
Is every flaky scalp dandruff?
No.
Psoriasis, eczema, allergic reactions, fungal infections, and other skin conditions can also cause scalp flaking.
Visible flakes are shed skin cells, but the cause is different from the damaged-cell shedding behind why skin peels after a sunburn.
Persistent severe itching, redness, sores, or thick scaling may need medical evaluation.
Dandruff is not simply a dirty scalp.
It is a skin-shedding problem with several possible contributors.
And that’s the short explanation.
Related explanations: Why do pimples form?, Why do scabs itch?, and Why does skin peel after a sunburn?