Itchy arms in the summer when exposed to the sun may be caused by solar dermatitis, photosensitive dermatitis, prickly heat and other causes. 1. Solar dermatitis: solar dermatitis is due to intense sunlight exposure, the skin at the exposure of acute phototoxic reaction, mainly manifested as itching, flushing, swelling, burning and other symptoms. 2. photosensitive dermatitis: photosensitive dermatitis is a kind of inflammatory skin disease that occurs in the exposed area, and ultraviolet irradiation in summer may cause allergic dermatitis, resulting in a rash with varying degrees of itchiness, tingling, redness and swelling and other symptoms. 3. Prickly heat: Prickly heat, also known as corn rash, is a superficial, inflammatory skin disease commonly seen in summer or hot environments. The symptoms of prickly heat will be aggravated after exposure to sunlight in summer, resulting in the appearance of blisters or pimples, accompanied by tingling or itching and other symptoms. It is recommended that patients seek timely medical examination to clarify the cause of the disease and then carry out targeted treatment, do a good job of sunscreen in life, avoid stimulating food, pay attention to skin care, and regular review.