I own a Columbia long sleeve shirt and pants, both made with their Titanium fabric. This fabric claims a UPF 50 sun protection rating... most manufacturers that claim a UPF 50 utilize a tight-weave in order to achieve that rating. As I said before, this is the reason they utilize vents on their shirts, to aid with the fact that this fabric is not breathable. It is rare to see cotton shirts with vents because they don't need them. I realize this fabric is very thin, but it would be a mistake to assume that makes it breathable. If you hold the titanium fabric up against a light, you will notice the "pores" which the light passes through are extremely tiny. If you try to do this with a loose weave cotton (or synthetic) shirt, you will notice much bigger "pores" where the light shines through. Obviously this isn't a proper way to test for breathability, but it certainly gives you a good idea of how breathability compares between the fabrics.