What does SPF measure in relation to skin protection?

SPF, or Sun Protection Factor, is specifically designed to measure the amount of time it takes for protected skin, using a sunscreen with a specific SPF rating, to show signs of sunburn (erythema) compared to unprotected skin. For example, if your skin would normally begin to burn after 10 minutes without sunscreen, an SPF of 30 theoretically extends that time to 300 minutes (30 times longer). This metric focuses solely on the effectiveness of sunscreen in delaying the onset of sunburn induced by UVB rays.

While SPF does relate to UVB radiation, which is primarily responsible for causing sunburn and contributes to skin cancer risk, the measurement of SPF encompasses how much longer one can stay in the sun before burning when using the sunscreen. The other options do not correctly represent what SPF measures; for instance, the amount of melanin in the skin relates to natural skin pigmentation and does not pertain to the function of sunscreen. Similarly, SPF does not measure the effects of UVB radiation only, as it deals with protection against sunburn rather than radiation effects. Lastly, SPF has no relation to the temperature at which the skin burns, as that pertains to thermal burns rather than photodamage from ultraviolet light.