The SPF on sunscreen stands for sun protection factor, a relative measurement for the amount of time the sunscreen will protect you from ultraviolet (UV) rays. UVB rays primarily affect the outer layer of the skin, the epidermis. The sun also emits UVA rays, which can penetrate the lower level of the skin, called the dermis. The higher the SPF value of your sunscreen, the more protection it offers from sunburn. So, if you use an SPF 15 product exactly as directed (applied generously and evenly and reapplied after two hours or after sweating or swimming), it will take you 15 times longer to burn than if you weren’t wearing sunscreen. Ultraviolet Protection Factor (UPF) indicates how much UV radiation (both UVB and UVA) a fabric allows to reach your skin. For example, a UPF 50 fabric blocks 98 percent of the sun's rays and allows two percent (1/50th) to penetrate, thus reducing your exposure risk significantly.