Both the UPF (Ultraviolet Protection Factor) and the SPF (Sun Protection Factor) represent sun protection measurement standards.
SPF indicates the duration it takes for the skin to burn when exposed to sunlight. The higher the SPF number, the longer it takes. This protection measurement is a standard for cosmetics, sunscreens, and sunglasses.
UPF is a measure of the UV resistance that a piece of fabric provides. It indicates how much of the ultraviolet radiation a fabric can block and, subsequently, the amount that penetrates through it and reaches the skin. For example, a fabric with a 50+ UPF rating blocks 49 out of each 50 rays and provides 98% UV protection.
The Skin Cancer Foundation gives its seal of approval to sun-protective fabrics with a minimum UPF of 30. Fabrics with 30-49 UPF are considered as very good protection and 50+ as excellent protection.