A lot of black spots on arms may be caused by ultraviolet radiation, hyperpigmentation and melanosis. 1. Ultraviolet radiation, if you don’t pay attention to sunscreen when you go out in summer and usually have the habit of drinking strong tea and coffee, you may form black spots on your arms. 2. Pigmentation, if you have suffered from folliculitis and other skin diseases, the cure will leave different degrees of pigmentation, and with the growth of age, the body will produce age spots, in addition to the arm, many parts of the skin will have the performance of black spots. 3. Melanosis, if the distribution of black spots is large and symmetrical, and there is no local irritation sign and no obvious bulge, it is considered that it may be simple melanosis. It is recommended to consult the regular hospitals in time, and treat the symptoms under the guidance of the doctor after the cause of the disease is clarified.