Dry Skin

• According to the Mayo Clinic, dry skin is the "most common cause of itchy, flaking skin" (see Resources).
  
  

Not Washing Hair

• If the hair is not shampooed on a regular basis, dead skin cells and excess oil accumulate on the scalp, which can lead to dandruff.
  
  

Psoriasis

• Psoriasis is a common skin condition in which dead skin cells accumulate on the scalp and form thick silvery scales that flake off.
  
  

Contact Dermatitis

• This is usually an allergic reaction or sensitivity to the ingredients in hair care products. The scalp becomes irritated, red, itchy and flaky.
  
  

Seborrheic Dermatitis

• This skin condition leaves oily and flaky scales on the scalp. The scalp becomes red and irritated.
  
  

Malassezia

• This is a fungus that grows on the scalps of most adults, and causes no harm. But in some cases the fungus continues to grow, and this causes an irritated scalp in which more skin cells grow. These flake off, causing the dandruff.