Irritated, oily skin (seborrheic dermatitis). This condition, one of the most frequent causes of dandruff, is marked by red, greasy skin covered with flaky white or yellow scales. Seborrheic dermatitis may affect your scalp and other areas rich in oil glands, such as your eyebrows, the sides of your nose and the backs of your ears, your breastbone, your groin area, and sometimes your armpits.