We know which caves (on Mendip) routinely suffer poor air - the main ones are, in no particular order: White Pit, Maesbury Swallet, Little Crapnell Swallet, Cuckoo Cleeves, Tynings Barrows Swallet and GB Cavern. What we do not know are the reason(s) causing this; however, for the first time we have access to accurate (!) testing devices and may be in a better position to begin pinpointing the cause(s) of poor air. The present debate, however, seems to focus on what to do once it is known that a site has poor air - should a sign simply inform people who then make up their own mind or should access control bodies/landowners be more proscriptive in their actions? - assuming the latter, how often do you test and what is the cut off line/time for reappraising any control(s) placed on the site.
Why limit yourself to a 100 year time frame, though? - surely it's just as easy to say that over the last couple of million years since people have been using caves we haven't figured out the definitive cause of poor air in any individual site and, presumably by extending the line, we therefore won't be able to do so for another x years?
Going off on a fantasy tangent might include high fencing, guard dogs, CCTV cameras, filling sites in with concrete, setting up machine gun nests etc..
