Men's Rash Guards (Swim Trunks)

Worn in the water rather than beside it, a rash guard fits close from the neck to the hem. The name comes from surfing, where wax and a board rubbed a raw patch across the chest and ribs over a long session. Sun has since become the main reason people buy one, and most now carry a UPF rating, which means Ultraviolet Protection Factor: UPF 50 blocks around 98 percent of the ultraviolet hitting the fabric.

Everything about the build serves one goal, keeping the top still on your body. Nylon with 15 to 20 percent elastane stretches in all directions and returns, so the fabric clings instead of ballooning and dragging behind you. Seams are flatlocked or bonded and pushed away from the underarm, since that is where a stroke rubs hardest. Hems run long, sometimes with a silicone strip, because a top that rides up in surf exposes the exact patch you covered it for.

A loose swim tee gives similar sun cover and behaves badly in water, filling up and pulling on the shoulders with every stroke. Wetsuit tops are neoprene and exist for warmth, which a rash guard barely provides once you are out of the water in a breeze. Training tees look like the fitted version but are not built for salt or chlorine, so their elastic dies within a season. Sleeve length is the real decision, since short sleeves leave your forearms open and long sleeves cover the skin that burns first.

Full days in strong sun make the case on their own, especially near the equator where midday light burns skin in about fifteen minutes. Fair skin, thinning hair and a history of burnt shoulders all point the same way. Cold water is the wrong setting, where a wet top chills you rather than warming you. Indoor pools remove the reason entirely, since there is no ultraviolet to block.

Check the label for a UPF number rather than a general claim about sun, because tested fabric carries a figure and marketing does not. Raise your arms overhead in the fitting room and watch the hem: anything lifting above the navel there will lift further in waves. Salt and chlorine both need rinsing out the same day. Dry it in shade, since the sun this top blocks is also what breaks its elastane down.