What does the SPF rating in sunscreen measure?

The SPF rating in sunscreen specifically measures the level of protection against UVB rays, which are primarily responsible for causing sunburn and can contribute to skin cancer. The SPF number indicates how much longer a person can stay in the sun without getting burned compared to if they were not wearing any sunscreen. For example, if a sunscreen has an SPF of 30, it implies that the user can theoretically stay in the sun 30 times longer without burning than they would without protection.

While SPF is a critical aspect of sun protection, it's important to understand that it does not measure protection against UVA rays, which can also harm the skin but through different mechanisms such as premature aging and some forms of skin cancer. The SPF rating does not provide information regarding how long the sunscreen lasts, nor does it indicate any skin hydration benefits. Thus, the focus on UVB protection is key to understanding the effectiveness of sunscreen products labeled with an SPF rating.