Our set of CO2 monitors displays the airscore of a room. It's not rocket science, it's cheap and it can save lives.
https://letsair.org/airscore
https://letsair.org/airscore
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Alternative: https://AirMeter.io, which uses a Lilygo TTGO T5 instead, and very little power thanks to its e-Ink display
Thanks for the tip, I will have a look at their solution as well.