Sunscreen for your skin is rated according to its SPF, or sun protection factor. Fabrics are rated based on their UPF, or ultraviolet protection factor. It’s an indicator of how much UV radiation can penetrate the fabric. A UPF 50 fabric blocks 98 percent of the sun’s rays, which means only 1/50 gets through the cloth to your skin. Lots of things affect a fabric’s UPF rating, including density of cloth, how it is woven, and how much stretch it has. And some fabrics are treated with UV-inhibiting chemicals to boost their sun-blocking powers.