Sun-protective clothing is the most effective sun protection there is, and the reason is not the fabric — it is you.
SPF is measured in a lab at a thick, even application. People apply about a quarter of that in real life. And protection does not fall gently when you under-apply — it collapses.
Tested at the amount people actually use, an SPF 70 sunscreen delivered an actual SPF of 19. An SPF 100 delivered 27.
A UPF 50 shirt delivers UPF 50. You cannot under-apply a shirt. You cannot miss a spot. It does not sweat off at hour two, and you never have to remember to reapply it.
The Skin Cancer Foundation says it outright: "Clothing is the most effective form of sun protection."
Five things determine how protective a fabric is, and they are worth knowing when you shop:
Colour — dark and bright colours absorb UV; pale colours let it through. Weave — dense beats sheer. Hold it up to the light; if you see through it, so does UV. Fibre — polyester and silk reflect UV and do surprisingly well. Unbleached cotton has natural compounds that absorb it. Fit — loose is better. Tight clothing stretches the weave open. Condition — wet or stretched means less protection. Always.