[wkat4242]

Huh for me it drops down to 400 within minutes when I open a window

[hgomersall]

Relying is the important point. Sure, it may be fine, but it often isn't, and probably is uncomfortable in winter. And that's before we get to energy losses.

[harvey9]

I guess you and the person before might have different circumstances in how your windows and rooms are set out, local prevailing wind and so on.

[ryan-c]

I have to open two windows in my home office, otherwise the co2 just hangs around.