Homonym: SPF (Sunblock)
SPF, or Sun Protection Factor, is a measure of how well a sunscreen protects the skin from harmful ultraviolet (UV) rays. The number associated with SPF indicates the level of protection; for example, SPF 30 blocks about 97% of UVB rays, while SPF 50 blocks about 98%. Higher SPF values offer slightly more protection, but no sunscreen can block 100% of UV rays.
It's important to apply sunscreen generously and reapply it every two hours, especially after swimming or sweating. Using a broad-spectrum sunscreen, which protects against both UVA and UVB rays, is recommended for comprehensive skin protection.