The big problem with all underground shelter is humidity. Not only when occupied, but especially when unoccupied.
My goal before this summer is to have an automated system that runs the air conditioning unit/dehumidifier at a threshold set to keep the shelter cool and dry inside at all times. It needs intelligence to know when the shelter is occupied - through passive infrared detectors (I have 12 now I bought at $1.50 apiece that put out a 3 volt signal when a heat source is anywhere in their detector range out to six meters) and when it is unoccupied.
I had an idea that it would be great to constantly flush the entire shelter with ozone when it was unoccupied and then turn on the fans and import clean air when occupants arrive. This would suppress the growth of all mold and mildew in combination with the dehumidifier. There might even be a timed release of vaporized tea tree oil or another fungal antigen, say every thirty minutes a small spray in all areas, maybe staggered at intervals.
Of course, the other advantage of passive IR is that the shelter can turn on the lights and then the fans, greet the shelterees with a prerecorded WAV file and immediately give status reports on every major shelter system ... food, water, temperature, air quality. This for me is the real result I am working towards, to make the shelter self-diagnosing in all regards. For example, using a magnetic reed switch to know if the positive pressure valve in the storage drum is open and that someone should close it if the shelter is put into pre-attack check mode.
I bought an absolute truckload of bracket mounted reed switches of all kinds but I have not mounted them yet because I want to improve the air quality so they don't rust internally.
Herein has been my dilemma - first continuous power from solar and wind available 24/7 charged automatically, then automated AC, then intelligent climate control. I still have not made it past step one. I am reluctant to mount any more electronic components until I solve the humidity problem. My Rabbit 2000 monitor has accumulated chunks of crusty white fungus inside the casing that look like alien spores, although it has not affected the board's functionality as of yet. I cannot install any more computer hardware until I purify the air down there and all surfaces.