The ultraviolet index or UV Index is an international standard measurement of the strength of sunburn-producing ultraviolet (UV) radiation at a particular place and time.
The UV Index is designed as an open-ended linear scale, directly proportional to the intensity of UV radiation that causes sunburn on human skin. For example, if a light-skinned individual (without sunscreen or a suntan) begins to sunburn in 30 minutes at UV Index 6, then that individual should expect to sunburn in about 15 minutes at UV Index 12 – twice the UV, twice as fast. Public health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat and sunglasses) if they spend substantial time outdoors when the UV Index is 3 or higher.