What do you mean by the "ultraviolet index" that often appears in weather forecast and the "sun protection factor" (SPF) marked on ultraviolet protection products?

The ultraviolet index refers to the degree of possible damage to human skin caused by ultraviolet radiation in the solar rays reaching the earth's surface when the sun is at the highest position in the sky (usually around noon, that is, from 10 to 3 pm). The variation range of the ultraviolet index is indicated by the number 0- 15. Usually, the ultraviolet index is 0 at night, and it is 15 in tropical, plateau and sunny days. When the ultraviolet index is high, it means that the erythema damage caused by ultraviolet radiation is more serious. Similarly, the higher the ultraviolet index, the greater the damage to the skin in a short time.

Sun protection factor, abbreviated as SPF, refers to the ratio of the energy required to produce the smallest erythema on the skin protected by sunscreen to the energy required to produce the same degree of erythema on the skin without any protection. Simply put, it is the time multiple of skin's resistance to ultraviolet rays.