A Simple Microcontroller that has three outputs:
-co2 Nominal co2=<7%
-co2 High 7%=<co2=<9.5%
-co2 Lethal co2=>9.5%
-(I don't see any issues with the outputs technically overlapping sometimes but if you have any let me know)
I recommend you use this setup to do stuff like activating a co2 alarm or scrubber. If you have any issues, for example, issues with the outputs technically overlapping sometimes, or questions, feel free to let me know and I will fix it to the best of my abilities.