>>13438737As such. Up to a point.
But you would either need a way to perfectly filter outside air (a roll of toiletpaper actually could be used as a filter element; that would really stop very small particles) or a way to re-oxygenate and otherwise treat your recycled air - either way, that means a lot of stuff gets used up ... and that's only so you can breathe (also needs energy obviously).
A lot of supplies means a lot of money, the space for the supplies and equipment (also costly) means a BIG shelter (bigger is more costly AND makes it harder to hide your shelter AND increases the risk of a weakspot).
And you will have to leave your shelter one day; you might not be happy with what you'll find up there. So you'd better add a garage and fuel storage to your shelter, put a high grade vehicle in that garage (something semi-military) ... and so on. Don't want to get stuck in or near your supplies-depleted shelter after all.
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