What is the purpose of such a room? Temporary refuge, or survival shelter? Lots of different directions to take here, depending on the parameters.
I can, however, answer the air question:
As has already been mentioned, you need to keep the room at positive pressure with respect to ambient. Also, you should have an airtight double door (like an airlock), with the intermediate chamber at higher than ambient pressure, and the room itself at higher pressure still. This will enable a person to exit the room as may be required without compromising the remaining occupants. With a large enough intermediate space, you could also keep decontamination facilities in the intermediate chamber to permit reentry in the presence of NBC agents. This is, of course, not a simple or cheap undertaking.
Air quality is maintained by circulating the air through a chemical scrubber using either Sofnolime or lithium hydroxide to remove the carbon dioxide. Scrubbers are most effective when kept dry, so you may want to dehumidify the air first, before passing it through the scrubber. Keep enough scrubber material on hand for the expected (design) duration of your confinement. (You can do an internet search for Sofnolime - it’s not that expensive). Oxygen can be kept in high pressure cylinders. With a properly functioning scrubber, you will be amazed at how long you can make it last. Deliver it to the room using a barometer activated addition valve. As carbon dioxide is scrubbed out, the pressure will decrease, and such a valve will automatically add oxygen to bring the pressure back up to the desired level.