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Fairly common in sci-fi literature is the scenario where the Earth is destroyed by an all out nuclear war. Typically, the protagonist has survived by remaining in underground shelters for decades until finally emerging when the radiation is gone.

But given the nature of the world's nuclear arsenals (mostly H-bombs) and the fact that it makes the most sense to detonate the bombs in the air rather than at ground level, is the above even a semi-realistic scenario considering that Hiroshima and Nagasaki are inhabitable today?

In other words: If all major cities in the world were hit, how long would it take for the radiation to present little danger to human health?

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    $\begingroup$ Note, Hiroshima and Nagasaki do have people living there, for sure - I visited! Handy link: zidbits.com/2013/11/is-nagasaki-and-hiroshima-still-radioactive "Today, Hiroshima and Nagasaki’s radiation levels match the world average background radiation of 0.87 mSv/a." $\endgroup$ – Mikey Apr 12 '15 at 21:37
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    $\begingroup$ Answer accepted. And yes, as I wrote the two cities are indeed inhabitable... Inhabitable means that people can live there ;) $\endgroup$ – Troels Larsen Apr 13 '15 at 16:47
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It depends on where you are and what you consider an acceptable level of risk.

If you're worried about an elevated risk of cancer, you'll need to stay in the shelter for about 300 years. The long-lived isotopes Strontium-90 and Caesium-137 (the two with half-lives long enough to stick around for a while, but short enough to produce high levels of radiation) have half-lives of around 30 years, and ten half-lives is a good rule of thumb for when an isotope has decayed far enough to no longer present a risk. You'll need to provide a 300-year stock of food if you plan to do this: both isotopes bioaccumulate, so food from outside your shelter will have a higher risk than the general environment.

If you're in an area with heavy fallout (50-100 km downwind of a target) and you're only worried about radiation poisoning, the Swiss civil defense organization estimates that three weeks will see enough of the radiation decay that you can safely leave the shelter for extended periods.

If you're in the middle of nowhere (say, the Canadian Arctic), you've got nothing to worry about. Almost all the fallout will leave the atmosphere before reaching you, so sit back and enjoy the fireworks.

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    $\begingroup$ Thanks for this answer. I don't know what happened, but I am the original poster (over at physics exchange). I had no idea this place existed, but I am doing some research for a PC game, so worldbuilding is a very apt place to put it. This answer satisfies my curiosity completely, thanks! $\endgroup$ – Troels Larsen Apr 11 '15 at 16:45
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You might enjoy a free pdf of Nuclear War Survival Skills by Cresson H. Kearny at www.nuclearwarsurvivalskills.com among a dozen other sites. He anticipates a rapid return, rather than waiting decades.

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