What does the weather forecast mean that ultraviolet rays are unhealthy?

Unhealthy ultraviolet rays in weather forecast refer to strong ultraviolet radiation. Strong ultraviolet radiation can cause skin burns and sunburn. The ultraviolet index in weather forecast is calculated according to the intensity of ultraviolet radiation and the sensitivity of human skin to ultraviolet radiation, which is used to reflect the intensity of ultraviolet radiation and its influence on human body. The ultraviolet index is usually divided into five grades, namely, low, medium, high, very high and extremely high.