Old dandruff may be related to living habits, scalp furuncle, seborrheic dermatitis, tinea capitis, psoriasis and other factors. 1. Lifestyle habits: the use of harsh shampoos, incomplete cleaning after shampooing, frequent perming and dyeing of the hair, often staying up late and other bad lifestyle habits may lead to an increase in dandruff. 2. Scalp chaffing: it may be due to scalp infection, environmental factors and other factors leading to chronic dermatitis, manifested as diffuse grayish-white chaffing-like scales, when combing the hair, dandruff will fall like snowflakes, which may be accompanied by itching. 3. seborrheic dermatitis: seborrheic dermatitis is divided into inflammatory and non-inflammatory, the former manifested as erythema on the scalp covered with greasy dandruff, yellow or brown scabs, accompanied by vesicles, itching and other symptoms; the latter scalp with rice bran-like dandruff, dry scalp, grayish-white, can be accompanied by hair loss. 4. Tinea capitis: it is a contagious skin disease caused by dermatophyte infection, manifested as single or multiple round alopecia spots with scales on the surface, and most of the patients are accompanied by scalp itching. 5. Psoriasis: the cause of the disease is not completely clear, may be related to immune factors, environmental factors, etc., manifested as thick scalp scales, often beyond the hairline, more symmetrical distribution, the hair is bundled, can be accompanied by itching. Old dandruff may also be caused by other factors, it is recommended that patients seek medical attention in a timely manner, under the guidance of the doctor to carry out the relevant examination, in order to clarify the cause of the symptomatic treatment.