A white layer on the underarm sweat hair is usually caused by armpit ringworm or sweat crystals. 1. Ringworm of the armpit hair: it is caused by bacterial infection and usually occurs in summer. The clinical manifestation is that yellowish, white or red crystals appear on the patient’s armpit hair, and the hair will gradually lose its luster, become brittle and easy to fold. At this time, you need to remove the affected hair and pay attention to armpit hygiene, keep the local clean and dry. 2. Sweat crystals: often occur in the sweat gland more developed people, in a hot environment or after strenuous exercise, will lead to increased secretion of sweat. The composition of sweat contains salt, and the armpit sebaceous glands will secrete oil, in the case of cleaning up the untimely, it will be easy to form crystals attached to the armpit hair, appearing on the armpit sweat hair has a layer of white stuff. It is recommended to dry the sweat in time, take a shower regularly, change clothes regularly and pay attention to the cleanliness of the armpits. If there is a layer of white stuff on the underarm hair, it is recommended to go to the regular hospital for examination, to clarify the cause of the disease, and symptomatic treatment.