Solar urticaria (SU) is an abnormal reaction to sunlight or artificial light. When exposed to light, the skin cells of someone with solar urticaria release potent chemicals (including histamine), causing their blood vessels to open and fluid to collect within the skin. Their skin feels itchy and has red patches, which may be swollen. These may look like weals or a nettle rash, and can take up to an hour to appear after exposure to light, then coming on quickly and settling within a similar period. There is no permanent change to the skin.