This question is kind of hard to explain succinctly, and I apologize for that.
Obviously, humans experience acute effects from UV light on Earth in a matter of minutes near the equator (UV index of around 11).
However, these acute effects (notably sunburn) are not immediately debilitating, and do not prevent even fair skinned people from inhabiting Earth's tropics with some degree of comfort (I know UV is carcinogenic, but this does not manifest immediately after exposure to the radiation).
What would happen though if people were subject to UV index levels like 40, such as the ones which can be seen in the high Andes? Would they just burn faster or would new acute problems emerge as the UV levels increase to the point where living there without constant protection would become extremely difficult?
Worldbuilding context is that I'm trying to make it so that the equator/temperate regions of a certain planet is not worth settling due to the extreme intensity of UV light being so damaging as to preclude comfortable settlement. This is because I like my poles cold the way I made them yet wanted a logical reason for why people would be down there in the cold rather than in the much nicer climates of even the temperate region. So I needed a threshold UV number to set the tropics at so they will be effectively unlivable.