SPF stands for Sun Protection Factor. It is a measure of how well a sunscreen protects the skin from ultraviolet B (UVB) rays, which are the main cause of sunburn and contribute to skin cancer

. The SPF number indicates how much longer it would take for skin to redden (burn) with the sunscreen applied compared to without it. For example, SPF 30 means it would take 30 times longer for the skin to start to burn than if no sunscreen were used

. SPF specifically measures protection against UVB rays and does not directly measure protection against UVA rays, which also damage skin. Sunscreens labeled "broad-spectrum" protect against both UVA and UVB rays

. In summary:

• SPF = Sun Protection Factor
• Measures protection from UVB rays (sunburn-causing rays)
• Indicates relative protection, not exact safe time in the sun
• Higher SPF numbers mean more UVB protection but the increase is not linear (e.g., SPF 30 blocks about 97% of UVB rays, SPF 15 about 93%)

Proper application and reapplication of sunscreen are important for effective protection regardless of SPF