That seems reasonable. But just to clarify what I meant with airlock: some thick plastic bag with a floor-to-ceiling zipper on the "inner" and "outer" ends, and

That seems reasonable. But just to clarify what I meant with airlock: some thick plastic bag with a floor-to-ceiling zipper on the "inner" and "outer" ends, and for entry, it's first collapsed by a pump sucking the air out of it. Then you open the zipper on the outer end, step in, close the zipper, let the pump suck away the air around you, and open the inner zipper (they should probably be automatically operated, as you can't move well/much when you are "vacuum bagged"). For exit, basically just the reverse, with the pump pumping the air around the person to wherever the person came from. The general issue with unbreathable atmospheres is that a failure in their SCBA gear easily kills them. And re-breathers that are only there so you don't have to scrub the atmosphere in the room as often shouldn't be particularly expensive. You may even get away with just putting a CO2 scrubber on the exhaust path, and giving them slightly oxygen-enriched bottled air so you can keep e.g. a 50:50 oxygen:nitrogen ratio inside (so e.g. 20% O2, 20% N2, 60% Novec 1230). And it doesn't even need to be particularly effective, as you can breathe in quite a bit of the ambient air without being harmed, and the environment can tolerate some of your CO2. Like, as long as it scrubs half of your exhausted CO2 it won't even feel stuffy in there (you could handle the ambient air you'd have without re-breathers being used, as it'd be just 1.6% CO2, but you'd almost immediately get a headache). They'd have an exhaust vent for pressure equalization, which would chill the air to condense and re-cycle the Novec 1230. For pressure equalization in the other direction, they'd probably just boil off some of that recycled Novec 1230. So yeah, re-breather not needed, if you just get a mouth+nose mask to breathe bottled 50:50 oxygen:nitrogen mix. That 50% oxygen limit (actually 500 mBar) is due to long-term toxicity, btw. Prolonged exposure to higher levels causes lung scarring and myopia/retina detachment, so not really fun.