I bought a CO2 monitor, and did the same experiment at my office, few weeks ago. I work in a very crowded room, with only one window.<p>At first the CO2 measurement was "out of range" (>=3000 ppm), but it was not a surprise because I already suspected something was wrong with the ventilation system. It has been fixed now, but CO2 levels hardly fall under 1500 ppm.<p>My advice is to open one or more windows whenever possible, at least one hour per day. If possible, one window should be opened the whole day. CO2 tends to accumulate in ambient air and it takes times to dissipate, far more than I imagined.