This is quite literally what a Deadman switch is for. The man closest to the wall holds the switch that sucks out the oxygen and replaces it with nitrogen and when that man lets up it kicks on high power fans in the viewing area unless you hold another switch almost out of reach that you must hold up.
But the system overall is not fail safe. Through sheer bad luck, the warning system could fail and there could be a leak into the viewing chamber.
This is quite literally what a Deadman switch is for. The man closest to the wall holds the switch that sucks out the oxygen and replaces it with nitrogen and when that man lets up it kicks on high power fans in the viewing area unless you hold another switch almost out of reach that you must hold up.