Using several Raspberry Pis to monitor CO2 levels in my house. Each Raspberry Pi has a CO2 sensor - wrote a little Python script to retrieve data from sensor and upload it to a server Which is also a Raspberry Pi.
You can get cheap NDIR based sensors from aliexpress (around $20) such as MH-Z14A, which I've used with a Pi, via the UART interface. I just hooked it up to the 5V line on the Pi pin header and then used the Pis UART lines for the data, as that runs at 3.3V.
This would be a good application for a Pi Zero W (with WiFi) - if you want to collect the data in a central location. I've linked the CO2 sensor I used in another comment.
I had a suspicion that winter-time headaches we were having might be triggered by high CO2 levels so I simply wanted to record the CO2 levels in different rooms. Thanks to the sensors, I did, in fact, find that CO2 levels were very high in our bedroom. Once I improved ventilation, things seem to have gotten better.
CH4 is also dangerous. It's a different problem. CH4 blows up your house and is very rare. CO kills you and is very rare. CO2 buildup causes mild issues (like headaches, sleepiness) but is very common.
