Ultraviolet Protection Factor (UPF) 30 - Ultraviolet (UV) radiation is the invisible part of sunlight associated with suntans, sunburns, premature aging and skin cancer. The Ultraviolet Protection Factor (UPF) rating system measures the UV protection provided by the fabric. A garment with a UPF of 30 only allows one thirtieth of the UV radiation falling on the surface of the garment to pass through it. In other words, it blocks twenty-nine thirtieths or 96.6 percent of the UV radiation. Fiber selection, construction and finishes determine the level of UPF protection. Only covered areas are protected, and protection may be reduced if the fabric is wet or stretched, the garment has seen heavy use or after repeated launderings.