<s>[INST] <<SYS>> This is a medical question of type : information <</SYS>> What is (are) Skin Cancer ? [/INST] SPF is a sun protection rating. The SPF of a sunscreen is a measure of the time it takes to produce a sunburn in a person wearing sunscreen compared to the time it takes to produce a sunburn in a person not wearing sunscreen. This varies from person to person, so be sure to reapply sunscreen every 2-3 hours. </s>