The redness and warmth of the face when exposed to the sun may be related to physiological factors such as higher temperatures, followed by solar dermatitis, polymorphic sun rash and other diseases. 1. Physiological factors: If the temperature is high and the face is exposed to the sun, it may cause the skin to become red and hot, which is a normal physiological reaction. 2. Solar dermatitis: solar dermatitis is mostly related to excessive exposure to ultraviolet rays, and the patient’s skin will appear bright red erythema, burning sensation and tingling sensation, etc. The disease occurs most often after exposure to sunlight. The disease occurs in areas exposed to the sun, such as the face, hands and other places. 3. polymorphic sun rash: the cause of this disease may be genetic, photosensitizers and endocrine factors, commonly found on the face, limbs, etc., the patient will appear burning sensation, itching and other symptoms, lesions manifested as erythema, nodules, blisters and so on. The cause of redness and heat on the face when seeing the sun is not unique, after excluding physiological factors, it is recommended that the patient go to a regular hospital to clarify the cause of the disease.