Hollywood has always perpetuated the myth that when you are in an air tight room you'll die as you run out of oxygen. The CO2 poisoning will kill you before you run out of oxygen.
So, if you're sitting in a nuclear shelter, remember to get the CO2 scrubbers working first, then worry about the low oxygen.
I think the myth is older than Hollywood and it is ingrained in our languages - when people begin to suffocate they say „I meed more air”, not „The air is to stuffy”. And this leads to a mental model of bringing more oxygen in as helpful instead of letting co2 helpful.
So, if you're sitting in a nuclear shelter, remember to get the CO2 scrubbers working first, then worry about the low oxygen.