I'm not sure if we're arguing the same point and if we agree or not, but e.g. http://piers.org/piersonline/download.php?file=MDkwMjIwMTQ0NjQwfFZvbDVObzdQYWdlNjM

I'm not sure if we're arguing the same point and if we agree or not, but e.g. http://piers.org/piersonline/download.php?file=MDkwMjIwMTQ0NjQwfFZvbDVObzdQYWdlNjM3dG82NDAucGRm http://piers.org/piersonline/download.php?file=MDkwMjIwMTQ0N... ('Ventilation Efficiency and Carbon Dioxide (CO2) Concentration', Progress in Electromagnetics Research Symposium vol 5 no 7, 2009) seems to suggest that even in workplaces with air conditioning (albeit old airco and densely populated rooms), CO2 concentrations can reach higher-than-recommended levels in everyday usage. Any modern residential building requires mechanical ventilation to keep CO2 levels in check. CO2 levels, in real-world usage, also don't rise linearly - opening the windows for 30 minutes in the morning is not ventilation. IIRC (although I don't have a reference at hand) rooms reach 70% of their pre-window-opening levels within less than an hour of closing them. (maybe, although again I don't recall details, because of CO2 in other rooms, or maybe it gets 'cached' in materials (sound unlikely though... guess I'm giving away my lack of chemistry knowledge here), or other reasons?) Point being - in building design/architecture it's assumed knowledge that CO2 does rise to higher-than-recommended levels in modern (i.e., low air leakage) buildings without mechanical ventilation.