If you usually run in a cotton shirt, you'll be amazed at the comfort of a technical shirt. Instead of absorbing moisture, technical fabrics wick away sweat and dry as you run, so you'll never have to run in a wet shirt. If it's so hot that you're thinking of going out shirtless or wearing only a sports bra, you'll actually be hotter than if you were wearing a shirt, because your sweat will stick to your skin, which will be directly exposed to the sun.
Did you know that sweating increases the risk of sunburn? Wet skin does not reflect UV rays as well. A tip if your route is very sunny: opt for a top with built-in UV protection.
Advice by Sport