UPF stands for Ultraviolet Protection Factor, and a UPF rating tells how effective a particular garment is at blocking out solar ultraviolet radiation. Whereas the SPF (Sun Protection Factor) only takes into account UVB rays, the UPF standard for sun protective clothing takes into account UVB and UVA rays. Another difference is that SPF measures how much time one can be exposed to the sun before getting a burn, and the UPF rate tells how much of the sun's UV radiation is absorbed by the fabric. For instance, if a shirt claims a "UPF rating of 50" this means that it only allows 1⁄50th of the sun's UV radiation to pass through it and to your skin, thus blocking 98% of the UV radiation.