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Sitting in an increasingly concentrated puddle of my own CO₂ when I close the door of my small home office is not a life safety issue. It just seems to affect my cognitive performance at some point. If there were CO₂ tanks or combustion in play, I'd be using a proper industrial CO₂ alarm.

This is probably to keep CO2 below 500/600 ppm, not a safety device.

are you thinking of CO?

CO2 is pretty safe because your body has a built in sensor.