UPF stands for Ultraviolet Protection Factor. It indicates what fraction of the sun’s ultraviolet rays can penetrate the fabric. A shirt with a UPF of 50, for example, allows just 1/50th of the sun’s UV radiation to reach the skin.
Below are five examples of real fabrics magnified, all with different amounts of fiber or yarn per unit of surface area and providing different amounts of sun protection. The higher the UPF (ultraviolet protection factor), the greater the protection.
Clothing is our first line of defense against the sun’s harmful ultraviolet (UV) rays and protects us by absorbing or blocking much of this radiation. The more skin you cover, the better. A long-sleeved shirt covers more skin than a t-shirt, especially if it has a high neckline or collar that shields the back of the neck. Likewise, long pants protect more skin than shorts.