Gotta face it – If I want a shirt to block UV rays it’s not going to breathe like loosely woven cotton. This shirt compensates very well, though, with ventilation. It’s awesome. There are strips of mesh venting that run up the sides of the shirt and continue around and down the insides of the sleeves. It’s the best ventilated shirt I’ve seen yet. I was amazed when I discovered the venting because I couldn’t see it until I looked closely. Best uses: Hot & dry, or cool weather regardless of humidity. It’s my 2nd favorite outdoor shirt, but only because it’s 100% nylon instead of the 70% nylon/30% poly of my other ExOfficios. The difference is very minor, though. Plus, for looks and fit the Reef Runner wins hands down.