Since these bunkers were (fortunately) never used during an actual nuclear strike, there is no empirical track record on which to base improvements. It seems prudent that if you are going to rehab one, it makes sense to add air filtration just to mitigate particles and NBC contamination/agents finding their way into the shelter. The concrete structure under 20 feet of soil should do a good job protecting the occupants from Gamma radiation, but it seems that the unfiltered air would allow other contaminants into the structure where they could be ingested. Just my opinion.