UPF (Ultraviolet Protection Factor)

A plain white cotton t-shirt rates around UPF 5 to 7 — barely better than no shirt at all — which surprises most people who assume any fabric between them and the sun counts as protection. UPF (Ultraviolet Protection Factor) measures how much of the sun's UV radiation actually gets through the fabric, tested under AATCC 183 and rated on a scale where 15-24 is 'good,' 25-39 is 'very good,' and 40-50+ is 'excellent,' blocking upwards of 97.5% of UV rays. Tight, dense weaves score higher than loose ones, synthetic fibers like polyester and nylon score higher than cotton at the same weight, and darker colors block more UV than pale ones.

Buyers ordering for outdoor crews, fishing charters, and summer 5Ks care about this rating specifically because the alternative — sunscreen reapplied every two hours — isn't realistic for an eight-hour shift on a job site or a boat. Columbia's PFG fishing shirts and most sun hoodies built for trail runners lean on tight synthetic knits rated UPF 30-50+ for exactly that reason.

Spec UPF 50+ for anything marketed on sun protection specifically, and don't assume a heavier fabric automatically wins — a loosely knit heavyweight cotton tee can test lower than a tightly woven lightweight polyester one. Getting the fabric wet also drops its rating, sometimes significantly, which matters for anything sold for water sports.

Treating any long sleeve as sun protection is the common slip-up here. Sleeve length keeps UV off the skin it covers, but it says nothing about how much radiation actually passes through the fabric — a loose long-sleeve cotton henley can still let more UV through than a snug UPF 50+ short sleeve.