If the surface air was contaminated, as in fallout, wouldn't you need a CO2 scrubber (bunker air) or a NBC filter (surface air)for prolonged existance. It seems that pumping radioactive surface air into the shelter, or croaking of CO2 poisoning, would negate the entire idea of a fallout shelter.
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Hope for the best and prepare for the worst.

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane