SURPRISINGLY USEFUL. We can officially only rate these warmers at UPF 50 , but in our laboratory tests they actually blocked 99.8 percent of damaging UV rays. We 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 arm sleeves are better in both cooler and warmer conditions. We’ve kept the construction very clean to eliminate seams that can rub, and we used our double-sided silicone elastic to make sure they stay in place. They fold up extremely small, so they’re great to stick in your pockets for your next big mountain day.