I recently bought one of the best CO_2 meters available here (and had a CO alarm since before that): the low temperature boiler (natural gas, 1991) in our kitchen contributes close to nothing to the CO_2 level despite getting its air from inside. Letting it run on automatic a whole workday contributes less than 200ppm in our flat. But an adult and two kids can bring the level from below 500ppm to above 1000ppm in around one hour -- additionally: doesn't really matter if the doors to other rooms are open or not. Ppm fluctuation is (way) less than 100ppm between rooms.

(All time low 380ppm -- 10% above the world average during my birth :-( , highest 1620ppm)