6000 ppm is a normal level after 1 hour meeting in a closed conference room. Assuming non-openable windows (which is normal in modern offices).
I have so far not been in a conference room even scratching the 2000 ppm level after an hour. (Well, OK, I only leave my meter behind. At 1200ppm, I consider the level excessive and leave. But I've kept measuring to see how willing people are to burn their health to be in the office with others)