Dandruff is caused when the natural cycle of skin renewal is speeded up.

This leads to patches of dead skin forming on the surface of the scalp, which come away into the hair, resulting in the flakes associated with dandruff.

What causes the skin renewal cycle to speed up is not always clear. Possible factors may include:

• seborrhoeic dermatitis – a common condition that causes oily skin; people with seborrhoeic dermatitis can also experience flaking on other parts of the body, such as the eyebrows, side of the nose and anywhere where skin folds together, such as the armpits
• malassezia – a type of fungus that normally lives harmlessly on skin, but if it grows out of control can accelerate new skin production

These are thought to be interlinked. The presence of the fungus on skin may provoke an abnormal response from the immune system (the body’s defence against infection), which can then cause skin to become oily.

In turn, oiliness of the skin may encourage further growth of the fungus, which then triggers symptoms of dandruff.

Other possible risk factors for dandruff include:

• emotional stress
• not washing your hair or, conversely, washing your hair too much – some people can irritate their scalp if they shampoo their hair too often
• using other hair products such as hairspray, hair gel and hair mousse
• very hot or cold climates
• other skin conditions such as psoriasis or eczema – two common skin conditions that can cause skin to become dry, red and flaky
• obesity
• having a weakened immune system, which can be the result of a condition such as HIV or as a side effect of a treatment such as chemotherapy

For reasons that are unclear, people with neurological conditions such as Parkinson’s disease, stroke or severe head injuries often develop both dandruff and seborrhoeic dermatitis.