Apparently, air filtration isn't as important as you may think.Here's a question. Do the bunkers draw in (and then filter) air from outside? If yes, what if the vents/ducts at the surface are blocked by debris?
The most dangerous particles have the consistency of sand, so aren't easily drawn into ventilation systems. Ventilation is a bigger issue and not letting the shelter turn into an oven!
But if completely blocked, you would probably send people out in radiation suits, for a short time, to clear the blockage by hand.