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.
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.
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.
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.
