Don't go too low either. There are CO2 sensors on AliExpress for less than $10, they are scams. What they do is that they take the cheapest sensor available th

Don't go too low either. There are CO2 sensors on AliExpress for less than $10, they are scams. What they do is that they take the cheapest sensor available that reacts to something in the air, and extrapolate a reading. It may give you an indication that the air is "stuffy", but the numbers are completely made up.