SPF? It stands for Sun Protection Factor and refers to how well the sunscreen protects against UV-radiation. The SPF number indicates how long it will take the skin to turn red when exposed to sunlight. If you, for instance use a SPF 15, it will prevent the skin from getting red for approximately 15 times longer than had you not used sunscreen. If you burn within 10 minutes , the sunscreen will prevent you from burning for about 150 minutes, or 2,5 hours.