Redness of the skin when exposed to the sun may be caused by capillary dilatation, solar urticaria or solar dermatitis. 1. Capillary dilatation: Long-term ultraviolet radiation penetrates deep into the skin and causes capillary dilatation, which can lead to redness of the skin when exposed to the sun. 2. Solar urticaria: usually refers to the skin for a short period of time exposed to the sun occurred in the skin, often manifested as a large area of itching, erythema and other symptoms. 3. Solar dermatitis: after strong ultraviolet radiation to the skin, especially in the exposed parts of the redness, pimples, accompanied by itching, and even a lot of peeling phenomenon. Skin red when exposed to the sun may also have other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.