What does the SPF value on a sunscreen mean?
SPF or sunburn protection factor indicates the UVB/sunburn protection provided by sunscreen. The SPF value indicates the level of sunburn protection provided by the sunscreen product. All sunscreens must be tested according to an SPF test procedure. The test measures the amount of ultraviolet (UV) radiation exposure it takes to cause sunburn when a person is using a sunscreen in comparison to how much UV exposure it takes to cause sunburn when they do not use a sunscreen. The product is then labeled with the appropriate SPF value indicating the amount of sunburn protection provided by the product. Higher SPF values provide greater sunburn protection. Because SPF values are determined from a test that measures protection against sunburn caused by ultraviolet B (UVB) radiation, SPF values only indicate a sunscreen’s UVB protection. However, sunscreens that pass the new broad-spectrum test will have demonstrated that they also provide ultraviolet A (UVA) protection that is proportional to their UVB protection.
Sources: U.S. Food and Drug Administration