I live in a relatively small apartment. From everything I've read, levels between 1000ppm and 2000ppm seem to be quite normal indoors, e.g. in an office. Not he

I live in a relatively small apartment. From everything I've read, levels between 1000ppm and 2000ppm seem to be quite normal indoors, e.g. in an office. Not healthy, but normal. One of the reasons may be that houses have become more air-tight in an effort to improve insulation, while the cognitive effects of high co2 aren't quite as easy to detect as a high energy bill. That afternoon slump may be a biorhythm / digestion thing, or maybe nobody in your office bothered to open the windows since yesterday. The concentration also rises pretty fast. I think I could make a (slightly delayed) intrusion alarm just by reading CO2 values... or a window-open-detector for automatically turning off the heating...