Castelli upf 50 light arm skinscastelli can officially only rate these warmers at upf 50 but in their laboratory tests they actually blocked 99 8 of damaging uv rays castelli have used a polyester base that is not only more effective than nylon at blocking uv rays but is also much better at keeping you dry which means these skins are better in both cooler and warmer conditions the construction is very clean to eliminate seams that can rub and feature double sided silicone elastic to make