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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.

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  • $\begingroup$ Sunburn is debilitating. It is like a burn if sufficiently strong, so moving affected body parts causes huge pain. UV index of 40 will require constant and full-body protection, or nocturnal lifestyle. It will give advantage to dark-skinned people. $\endgroup$ – Bald Bear Jul 13 '18 at 3:29
  • $\begingroup$ 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. $\endgroup$ – Antarctica07 Jul 13 '18 at 3:46
  • $\begingroup$ to answer your 2 title questions: 1) eventual death. 2) use lots of sunblock, stay indoors and underground more, wear UV opaque clothing that covers the entire body, use anti cancer treatments frequently, etc. $\endgroup$ – M. A. Golding Jul 13 '18 at 16:33
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The answer to your question cannot be a sharp threshold, but rather a fuzzy indication.

As you pointed out, at the Equator it's a matter of minutes to get a sunburn. Yet many people (and other species) manage to live for more than minutes near the Equator.

The reason for this is that life is really flexible, and it can manage to adapt to certain conditions. Near the Equator and for humans this happened by an increased production of melanin, resulting in a darker skin. So, while a pale skinned man would suffer at the Equator if not adequately protected, a person with dark skin could experience no serious issues.

Also it is likely that colonists would not walk around just in their adamitic costume, but would wear some protective clothing, like the Europeans did when colonizing equatorial Africa. Or, as suggested by DrBob, they could adapt a nocturnal lifestyle, away from the dangerous light of the day.

If the poles are habitable, I am afraid that the equatorial regions can be made harsh (EUV index 40, as you stated), but not completely barren.

You can add other more impeding conditions to make conditions more and more harsh.

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  • $\begingroup$ You'd also have to explain why they didn't become nocturnal at the equator, since that's the easy way to avoid the sun. $\endgroup$ – DrBob Jul 13 '18 at 13:33
  • $\begingroup$ @DrBob, thanks for the hint. Added that in my answer $\endgroup$ – L.Dutch - Reinstate Monica Jul 13 '18 at 13:53
  • $\begingroup$ My question wasn't really about what they could do to adapt but rather, like I said, the highest UV level you could be outside before suffering severe acute effects. $\endgroup$ – Antarctica07 Jul 13 '18 at 19:59

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