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Suppose I have a planet similar to the Earth, and I want to decrease the $O_3$ levels to the minimum amount (world average, in Dobson units), so as to sustain the human civilization (bare survival, supposing the current humans on Earth went to the planet) , what would it be?

Assume this planet is also present in a Solar System-like environment.

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  • $\begingroup$ You ask this like if Ozone fell below a certain level, humans will shrivel and die. Does bare survival include humanity having access to shirts and hats to protect against the sun? $\endgroup$ – Twelfth Jan 26 '17 at 17:59
  • $\begingroup$ Every current technology available in the Earth can be used. $\endgroup$ – Siddharth Venu Jan 26 '17 at 18:02
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I believe the answer here is zero. Ozone's key protective trait is Ultra-Violet radiation and life on Earth here has evolved with this protection. Remove the protection and we are unable to handle the increased UV without protection.

That being said, life that evolved on a planet without this ozone UV shield would have to find new ways to adapt to the environment and the increased presence of UV radiation. this might include self-repairing DNA to adapt to the UV damage.

I don't believe an Ozone layer is essential to life and wouldn't prevent a human like species from coming up. It is essential to our life (or atleast we would have to adapt our behavior rapidly) as we evolved with it's presence.

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  • $\begingroup$ I am sorry but I am not talking about formation of humal like life, but rather what if the humans currently on Earth went to that planet. I'll edit the question so as to show this clearly. $\endgroup$ – Siddharth Venu Jan 26 '17 at 17:40
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    $\begingroup$ Answer is still 0. They would need access to clothes and shelter to protect them from the sun. $\endgroup$ – Twelfth Jan 26 '17 at 18:00
  • $\begingroup$ The normal $O_3$ levels are measured approximately to 200-300 dobston units. So when $O_3$ levels become lesser and lesser, it will cause mutations and eventually kill them. So I am asking for a point, like after which survival isn't possible at all (absolutely no chance of survival). $\endgroup$ – Siddharth Venu Jan 26 '17 at 18:05
  • $\begingroup$ @SiddharthVenu - there is none. UV radiation (UVB) damages skin...it doesn't harm organs or internals...it's surface radiation. As long as this human colony can put something between their skin and the sun, there is no minimum ozone required. If these humans lacked clothing and roofs, this might be an issue. It'd be a constant part of life (UV protection), but there is no minimum. $\endgroup$ – Twelfth Jan 26 '17 at 18:08
  • $\begingroup$ And what about dangerous diseases like skin cancer? $\endgroup$ – Siddharth Venu Jan 26 '17 at 18:11
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The Ozone layer protects us from UVB radiation, which is the point of this question. The big difficulty in answering this question is summed up by a 2007 study by the University of Ottawa to the US Department of Health and Human Services: there's not enough information to determine a safe level of sun exposure that imposes minimal risk of skin cancer. (Abstract

Additionally, there's no conclusive evidence that UVB is actually carcinogenic; we label the whole spectrum as "carcinogenic." We're relatively certain that they are all carcinogens, but we don't know for sure (Paper)

This makes it hard to put a number to the amount of Ozone needed.

Human skin color correlates well with the UV radiation available to a group of people over long time scales; Peoples around the equator tend to have higher pigmentation levels; in the rainforest they have middle range pigmentation, and near the poles a lack of significant pigmentation. For "human survival" it would depend on the length of time available for the colony to become adapted to the UV, perhaps over the course of many, many generations.

One thing to remember is that UV is not ionizing radiation. It doesn't chew through the length of the body, knocking bits around. UV causes damage to surfaces through chemical interactions; the skin and eyes would be vulnerable.

Another missing detail in the question: are human like species expected to evolve on this planet, or are we talking about a human colony landing?

If it's a human colony landing, the answer is "Zero Ozone." Simple fact of the matter is, because UV only damages surfaces, you can have UV filtering goggles and clothing that reflects UV covering all skin surfaces, and you are fine. This depends on humans evolving elsewhere and arriving on the planet with no ozone, but it's surviviable. This would also allow the humans to use construction materials to build domes and filters for food crops which would be damaged by high levels of UV.

If you want life to evolve on this planet into human like critters, that's a vastly different story. UV isn't ionizing, so it doesn't plow through structures; but it's powerful enough to punch right through single celled organisms, and we kinda need those for life to evolve. I've failed to find information about the UV absorption of sea water, but life finds a way, right? So it'd probably start at the deep sea vents, scaled critters could come up to higher levels in the ocean because the scales protect their skin, which would allow them to leave the oceans for the land, and evolve into feathered and hairy beings.

But then we get to a problem; it's believed that humans evolved hairlessness because our environment and new bipedal posture made wearing fur cause problems with cooling the body; we shed the fur so we could sweat more efficiently. On a high UV planet, however, it's likely that human type critters wouldn't be able to lose their fur, or would have to replace it with something else.

But this whole line of thought, while fun to follow, is moot. High powered mobile animals like us require oxygen as part of our respiration cycle. The Ozone layer is moderated by the incoming Ozone itself; higher levels of Ozone react with the Oxygen in the atmosphere to create new O3; that O3 gets knocked apart by UV which forms more O2 or more O3. It's a self-regenerating process. So any planet with enough oxygen to support animals will likely have an ozone layer adequate to control the majority of harmful UV radiation.

The concern with the ozone hole was the fear that we humans were emitting chemistry in the air which destroyed O3 faster than UV radiation could create it, disrupting the balance and allowing through more radiation. This is unlikely to be a problem on a pre-industrial world, and post-industrial societies have plenty of methods available to reduce one's exposure to high UV radiation, and to protect their pets, livestock, and crops.

TL;DR: the answer is "Zero" To have humans who look like us, we need an Ozone layer similar to the one on Earth; the lack of ozone wouldn't destroy the ability for life to evolve, but it would cause it to follow a completely different path, and therefore would not be a "human society." Humans landing on the planet will be able to shield themselves from the increased UV simply; the only technology required is the ability to make cloth and UV filtering glass, which any space-based society should be able to produce.

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  • $\begingroup$ Could you add a point on what if the human landing party does not have any UV-proof clothing and equipment? What will be the min ozone levels for survival then? $\endgroup$ – Siddharth Venu Jan 26 '17 at 18:07
  • $\begingroup$ Like a critical point below which there is no chance of survival without any UV-proof clothes $\endgroup$ – Siddharth Venu Jan 26 '17 at 18:08
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    $\begingroup$ Much more detailed answer than mine +1. I believe UVB is pretty heavily absorbed by water and doesn't make it near as far as visible light does. @SiddharthVenu - UV Proof clothes includes cloth, silk, straw, or a newspaper held above your head...if it casts a shadow, its UV proof $\endgroup$ – Twelfth Jan 26 '17 at 18:14
  • $\begingroup$ @SiddharthVenu, unless your landing party is naked, any clothing provides UV protection. Remember, UV radiation is NOT ionizing radiation, it's right there with light and infrared as "heat" spectrum non-penetrating radiation. As such, any planet you land on where you can't protect yourself with your clothing will be a planet with more problems you'll need to overcome, like extreme heat or blindingly bright light. The least of your problems will be sunburn. $\endgroup$ – Zoey Boles Jan 26 '17 at 18:23

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