UPF is the fabric equivalent of SPF. A UPF 50 rating means the weave lets through about one fiftieth of the ultraviolet that hits it. It applies to the area the fabric covers and to nothing else, which is the part people forget on a long-sleeve rash guard with a low neck.
What the rating is measured on
Dry, unstretched, new fabric. Stretch a swim shirt over a larger chest and the weave opens; the effective protection drops. Wet fabric usually performs slightly worse than dry for the same reason. Neither effect turns UPF 50 into nothing, but it is an argument for taking the size that fits rather than the one that skims.
Colour matters too. A black or navy version of the same shirt blocks more UV than the white one, which is the opposite of what most people assume about staying cool.
Swim trunks, rash guards and the sun shirts in between
The swim and sun section of this catalogue runs to 39 cards. Trunks here come with 7, 9 and 11 inch inseams behind the same listings, mostly in 100% polyester with a mesh liner and a drawstring waist. The long-sleeve pieces are where the UPF claim earns its keep, because they cover the forearms and the back of the neck, the two places sunscreen gets missed.
Chlorine
Rinse in cold fresh water the moment you get out. Chlorine attacks elastane, and a pair of trunks with 8% elastane left in a wet bag overnight will come back with a slack waistband. Polyester itself does not care.
