What is dandruff caused by?

Depends: Dandruff is flaking of the scalp. It can be simple shedding of skin. Can be due to sun burn. Can be due to infection, inflammation or other acute causes. Mostly, however, it is due to seborrheic dermatitis. This is an inflammation of the skin often due to a low grade fungal infection or a reaction to oils in the scalp. Best treated with tar shampoos or antifungal shampoos. Maybe topicals.

Dry Skin: Most commonly dandruff is dry scalp skin. It can accumulate in the hair, and "flake" onto dark clothing. Dandruff shampoo can help control the problem. If if persists, it may be something that requires professional treatment, and you family doctor, internist or dermatologist can help.