UPF stands for Ultraviolet Protection Factor and indicates what level of UV radiation is absorbed by the fabric (and is therefore blocked before it reaches your skin).
The Australian Government tests and rates the UPF rating of fabrics.
A fabric with a UPF rating of 5 indicates that 1/5th of the sun’s UV rays pass through the fabric. That is, 20% of all sunlight passes through a fabric with a UPF rating of 5. A regular cotton t-shirt may only have a UPF rating of 5.
Your skin is your largest organ and has a long memory. Sun exposure and ultraviolet (UV) damage is cumulative throughout your life. Research shows that sun damage contributes to more than 90% of wrinkles, brown spots, premature skin ageing as well as precancerous and cancerous skin lesions.