Airgradient's popular indoor air quality sensors are the first devices of their kind to be officially certified as "Works with Home Assistant". These open-source devices do not require an internet connection and enable local automation with air purifiers, air conditioners, or humidifiers and dehumidifiers.
Your fire alarms most likely have a CO detector, not a CO2 detector. I don’t really know the point of a CO2 detector, though, so I agree with you on everything. Just get a few more indoor plants if CO2 is a concern.
The first AirGradient I bought helped eliminate a source of headaches and poor sleep by showing how bad the CO2 level was getting in the bedroom. Did some changes to the air circulation in the room so that the CO2 levels are held below the levels that produce symptoms. Headaches disappeared and sleep improved. So CO2 monitoring isn’t critical but it could be useful.
CO2 concentration is an indicator of indoor air quality and can generally inform the quality of ventilation. Indoor plants likely won’t exchange enough to be effective.
Baseline (outdoor) is around 400 ppm, 1000 ppm is generally the ‘comfort limit’ or where some cognitive effects start to become present (drowsiness), then over 2000 ppm is generally the unhealthy limit.
Unfortunately, you can’t just put plants everywhere. CO2 conversion is also highly dependent on light levels, which sufficient light levels not being achievable everywhere.
Office buildings with a large number of people meeting rooms etc.) could also benefit from monitoring.
Heck, even at home, I have a room that quickly accumulates CO2, easily reaching 2000ppm with just a single person being inside, which makes spending prolonged time in said room a tiring affair.
https://pmc.ncbi.nlm.nih.gov/articles/PMC11253968/
https://www.pjoes.com/pdf-68875-24089?filename=The+Influence+of+House.pdf
https://www.sciencedirect.com/science/article/pii/S2666790823000502
You need a lot of houseplants to make up for a human. Humans exhale something like 1 kg CO2/day where plants consume something like 0.2 kg/hr/100 m2. Figure natural light, maybe 6-8 hours of full sun in a day, and you’d need 60-80 m2 of leaf surface.
That kg/day of CO2 is enough to raise a 200 m2 home to 3000 ppm CO2. CO2 diffuses pretty well, but my 110 m2 house equilibrates around 1000ppm when it’s sealed against the summer heat.