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."
UPF is not SPF, and the difference is in your favour. SPF measures UVB only, tested on human skin. UPF measures UVA and UVB, and it tests the fabric — so the number does not depend on how well you applied anything.
UPF 50 means the fabric lets through one-fiftieth of the UV hitting it — about 98% blocked.
For anyone who works outdoors, has a skin cancer history, or simply cannot be relied upon to reapply sunscreen every two hours — this is the answer.