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.
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.