How does dandruff occur?

A very long time ago, sebum was meant to waterproof the hair and skin and protect humans from the cold. Since humans wear clothes, this function has become superfluous. In addition, germs and microorganisms that naturally occur on the skin use sebum as a source of nutrition. Degradation products of sebum irritate the skin, make it itch and accelerate the formation of the keratinous layer. The consequence is dandruff. Due to subsequently released sebum, these skin cells stick together to form large, visible skin flakes.