My CO2 sensor has dramatically changed my routines. My space isn't small - maybe 1200 square feet/100 sq m - but it must be pretty well sealed, because I can easily see my own breathing add to CO2. Nevermind cooking on the gas stove. Treadmill time adds 500+ ppm.
Now, I open windows every chance I get (which isn't super often, because the dewpoint is 70 oF/20 oC in Atlanta), and I've shifted a lot of my cooking to an electric tea kettle, hot plate, and toaster oven.