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.
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.