Per Cresson Kerney, who's nuclear war survival stuff was actually tested against reality (vs. the deathtraps like at least one of those early shelter plans in the OP which was dreamed up by D.C. bureaucrats) says air tightness concerns are entirely overblown, so to speak ^_^. As I recall, the heavy stuff that falls out early is too big to be a concern of that sort, the light stuff decays to safe (enough) levels before enough of it drops and becomes an issue. We have a lot of data on this thanks to those eeeevil above ground tests.
What you need is plenty of mass between sources of radiation, i.e. fallout particles where they settle, and from "skyshine". In a sufficiently bad situation, which would be pretty common, a simple ditch where you made sure the fallout kept out and didn't accumulate on, say, the tarp you were using for that, you'd still get zapped by gamma rays that bounce off atoms in the air and come down into the ditch.
So the most basic expedient fallout shelter is a ditch with hollow core interior doors over it or the like, and foot of dirt on top, which through arching holds most of its weight.
An above-ground building is iffy, because you want, say, 3 feet of dirt all around you, and either a foot on top or if the roof is high enough that's not as much of an issue. Middle stories of a tall building could suffice.
What you need is plenty of mass between sources of radiation, i.e. fallout particles where they settle, and from "skyshine". In a sufficiently bad situation, which would be pretty common, a simple ditch where you made sure the fallout kept out and didn't accumulate on, say, the tarp you were using for that, you'd still get zapped by gamma rays that bounce off atoms in the air and come down into the ditch.
So the most basic expedient fallout shelter is a ditch with hollow core interior doors over it or the like, and foot of dirt on top, which through arching holds most of its weight.
An above-ground building is iffy, because you want, say, 3 feet of dirt all around you, and either a foot on top or if the roof is high enough that's not as much of an issue. Middle stories of a tall building could suffice.