The causes of erythema on the forearms of hands include congenital vascular abnormality, dermatitis, allergic reaction and so on. 1. Congenital vascular anomalies: if the blood vessels of the patient’s forearms are congenital anomalies, such as hemangioma, congenital nevus erythematosus, etc., it will lead to erythema on the skin of the patient’s forearms. 2. Dermatitis: when bacteria, fungi and other disease-causing pathogens invade the patient’s skin, resulting in local skin infections, thereby triggering local skin erythema, itching, redness and swelling and other symptoms of lesions. 3. Allergic reaction: If the patient is allergic to certain substances, such as seafood, pollen, animal hair, etc., when contacting these allergic substances, it will trigger a series of allergic reactions, such as erythema on the skin, itchy skin, skin ulceration and other symptoms. If the patient has the symptoms of erythema on the forearms of the hands, it is recommended that the patient go to the hospital in time to improve the relevant examination, to clarify the cause of the disease, and follow the doctor’s instructions for treatment.