How are the spf numbers determined?

SPF numbers are determined through laboratory testing that measures the effectiveness of sunscreen in blocking UVB rays, which are responsible for sunburn. The testing involves applying a specific amount of sunscreen to human skin and exposing it to UV radiation, then assessing the time it takes for redness to appear. The SPF rating is calculated by comparing the time it takes for skin to burn with sunscreen to the time it takes without it. For example, if skin protected by sunscreen takes 30 minutes to burn, while unprotected skin burns in 10 minutes, the SPF would be 30.