• I reckon two people could spend five hours in that bunker with the ventilation shafts fully closed.

    But often you’d only be in there for an hour or so. It doesn’t take that long for the worst of a fire front to pass by… also it’s an absolute last resort. What you are supposed to do is evacuate and not be anywhere near the fire.

    Your body will naturally start breathing faster as CO2 builds up in the air. I wouldn’t expect that to start to be noticeable until long long after the fire has passed. But yeah, don’t sleep in it overnight (you won’t be able to, you’ll go into full on panic attack mode eventually).

    The real risk would be doing something stupid like starting a generator in the bunker. Those burn oxygen but don’t convert it to CO2, which means your body won’t recognise the danger and instead of panicking you’ll feel perfectly fine until you drift off to sleep, then die.

    Our body can’t detect a lack of oxygen, instead we detect too much CO2. Definitely want to understand that if you’re going to use an underground bunker.