The acronym for the measure of sunburn protection in sunscreens is SPF, meaning sun protection factor.
SPF estimates how much protection a sunscreen provides against ultraviolet radiation that produces sunburn, chiefly UVB. In laboratory testing, the rating compares the amount of UV exposure needed to cause sunburn on protected skin with the amount needed on unprotected skin. The test assumes an application of 2 milligrams of sunscreen per square centimetre of skin.
SPF 30 does not mean a sunscreen lasts thirty times longer, and a higher rating does not remove the need to reapply. Sweating, swimming, rubbing, and insufficient application can reduce protection. Broad-spectrum products also address UVA, which contributes to premature skin aging and other skin damage but is not the main cause of sunburn.
The SPF rating system was introduced in 1974. PA is a separate labeling system associated with UVA protection, while UVA and UVB name types of ultraviolet radiation rather than the protection measure itself.