This is the ultraviolet protection factor (from the English Ultraviolet Protection Factor). The textile industry uses it as a standard to determine the level of sun protection of a garment's fabric, rated from 15 to 50+. Our UPF 50+ rashguards have the maximum UV protection , i.e. 98% of the sun's rays are blocked, even when wet. This protection is obtained without chemical treatment , thanks to the very tight mesh of the micro fibers. It is therefore permanent and will remain wash after wash.