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?