Aussie Enviro

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.

In the conditions in which you're jumping into one of these you don't really want much ventilation - too much heat/smoke, and potentially even the fire burning through some of the oxygen from the contained air. Underground might not be an ideal place to be but IMO it's likely the best of a set of bad choices if you can't get out ahead of time - dirt is a pretty good thermal insulator.

I reckon you would give it a go if it was ‘try it’ vs ‘burn to death’

Also, what would a person do if something fell on top of the entrance? Running out of oxygen and trapped inside, sounds like a really warm coffin.

Probably better than inhaling smoke, but yeah. Some provide oxygen supplies, though.