I myself bought an air quality monitor, and am happy I did (uHoo). I have several gas appliances and was concerned about NOx and other possible gases. It turns out everything is about normal, except that CO2 levels in the basement tend to get elevated. I had no idea that this was something that happened. I now make sure to try and air it out down there once in a bit and take breaks more often, since that is where my office is set up.