It's a bit weird to me that people get a sensor and then worry about the air. What are the symptoms of elevated CO2 levels? Why can't you feel them and need a monitor to tell you you are unwell?
https://en.wikipedia.org/wiki/Carbon_dioxide#Below_1%
The most convincing evidence is from the ISS: at 5000ppm CO2, the crew experienced "headaches, lethargy, mental slowness, emotional irritation, and sleep disruption." - so you can feel too high a CO2 level and open a window (if you know to consider that as a problem which causes these symptoms).
The studies suggesting 1000ppm may be harmful are called out in the wiki article and it seems the jury is still out as to if the effects are real/are the result of CO2 and not confounding issues.
I guess I should look at some studies, hm.