This shirt claims to be SPF one zillion and while I can't vouch for that, my experience in SW Florida sun is that it doesn't let you burn. I also imagine that the Xylitol component helps keep me cooler, but that could be the placebo effect.
One odd note is that this is the first shirt I've ever bought that has sleeves that are longer than my chimp arms. I kind of expected to find thumb holes like you have in some running shells, but they are just extra long. I get that and it doesn't bother me, but it was surprising.
