A little known danger after surviving an airburst or ground detonation is the horrific and untimely rain-out
So you have survived the initial attack. You can see mushroom clouds on the horizon at your nearest city. You're monitoring skyshine at a mere 20 rads an hour and your shelter is rated for much more than that. As the hours go by, you're wondering what all the fuss was about. These nuclear wars are a cakewalk and you are worried you spent too much money excavating your shelter so deep. You are confident after a three week stay you should be good to start venturing out for brief periods to begin reconstruction or resupply efforts. Nothing to this nuclear war business.
Did you know if it begins to rain within the three day window of the most intense fallout, that radiation levels can rapidly climb to ten times the normal dosage rate? For many people, this would be the difference between life and death. Many ordinary PF 40 shelters would experience very intense surges of radiation during the rain-out that would leave shelter inhabitants dead or so sick with rad poisoning they would be incapacitated. The majority of people sheltering in their basements and expedient shelters could be killed by a rain-out occurring at the wrong moment.
It is funny that Sam Cohen, the inventor of the neutron bomb, predicted that rain-outs would kill more people in underground shelters than his weapon ever would. This shows you how dangerous such events were regarded by nuclear testing engineers and civil defense planners. Because of this and other unexpected effects of nuclear weapons, it is difficult to overengineer or overshield most shelters. Better to err on the side of caution in every case. If PF 40 is the minimal standard established fifty years ago for civilian structures, better for you to plan on a minimum of PF 200 for a modern shelter. What do you call PF 1000+? A good start.