Dandruff -- those dry, white flakes of skin you brush off your collar or shoulders -- is harmless. But it can be embarrassing and itchy. Dandruff really isn't about your hair, or how often you wash it. Instead, it's about the skin on your scalp.
Skin cells that grow and die off too fast are the problem. Exactly why that happens isn't clear. A very common fungus called malassezia may contribute to dandruff. This fungus lives on the scalp of most healthy adults without causing any problems. One theory is that the immune system of someone with dandruff may overreact to that fungus.
Dandruff may get worse when you're stressed or sick. Cold, dry winters can trigger dandruff or make it worse, too.
One common cause is seborrheic dermatitis, or seborrhea. Some people with dandruff may also have seborrheic dermatitis in other areas of the body, such as their ears, the center of the face, and the center of the chest.