Seborrheic dermatitis 

Dandruff is often described as mild seborrheic dermatitis. But in seborrheic dermatitis, the flakes transform into greasy, yellowish patches of red, inflamed skin that can also appear on the ears, eyebrows, chest and other areas of the body that have oil-producing glands. (Plain ol’ dandruff appears as fine flakes but comes without the inflammation.)

Scalp psoriasis 

This autoimmune disease causes skin cells to grow too quickly and appear as patches of thick, red scaly skin, silvery white scales or dandruff-like flakes that cause itchiness, a burning sensation or soreness. Psoriasis and seborrheic dermatitis often look similar (and some individuals can have both conditions at once) making it difficult to diagnose. The scales of scalp psoriasis are thicker than those of seborrheic dermatitis and can affect the skin surface on other parts of the body.

Head lice 

These tiny wingless insects feed off blood in the scalp. Lice lay their eggs, a.k.a. nits, in the hair shaft close to the scalp. When the nits hatch and feed, this causes itching. The appearance of lice eggs, plus itching, is often mistaken for signs of dandruff.