UPF stands for Ultraviolet Protection Factor. The UPF refers to the amount of protection a piece of clothing offers from the sun UV rays. The higher the protection number, the more protection.
Clothing with a UPF of less that 15 is not considered UV protective, such as T shirts.
UPF 15-20-good-4 to 6.6% UV radiation will penetrate the fabric.
UPF 25-35-better-4 to 2.6% UV radiation will penetrate the fabric
UPF 40-50 -Very good- protect from all but about 2.5% of UV radiation
UPF 50+ - best- less that 2% of UV radiation will pentrate the fabric
Keep in mind as UPF clothing ages, it may lose some it effect. Clothing that is faded or stretch out will let more UV radiation pass through. UPF clothing that has been treated with a chemical will loss some of its effect as the chemical is washed out over time.
For best results, machine wash in cold water on gentle cycle with a mild detergent. Do not wring out your garment, but let it air dry.
Keep in mind people who work in Florida, still need sunscreen, the UPF clothing will only protect the part of the body it covers.