Dandruff appears that a microscopic fungus called Malassezia is involved in excessive dehydration of the scalp and the appearance of dandruff. This fungus, or yeast, causes irritation of the scalp and fast renewal of scalp cells. Studies show that the amount of Malassezia is multiplied by 1.5 to 2 in the case of dandruff.
In other, rarer cases, the presence of dandruff is due to psoriasis, a chronic inflammatory skin disease that often affects the scalp.
In the majority of cases, the regular use (twice a week) of anti-dandruff shampoo bar, for oily hair or dry hair, solves the problem. These shampoo bars usually contain an antifungal (which acts on the fungus), a keratolytic (an exfoliant) and soothing agents to fight against itching.