I don't live in an apartment but I have a basement and I experience the same thing. My CO2 levels constantly hover around 500ppm. Opening windows will clear out CO2 if it gets to critical levels but has diminishing returns at the 500ppm level. I suspect this is because CO2 is a heavy gas that often sits near the floor. My next experiment is to install a small fan near the floor and run it 24/7 to see if I can "pump out" the CO2 while the split units I have are running.
Technically, any level of CO2 causes detriment. Even our ambient level is higher than some decades ago and apparently it is possible to measure the effect on us (mostly related to nervous system).
Personally, I can't discern any difference below 600 any way I tried to measure it.
I carry my Aranet4 with me and in some apartments of my friends and family it is normal for CO2 to not go below 2000ppm in winter. So you are doing pretty well.
I think this is incorrect. You may be thinking about PM2.5 levels of which there are no safe level, any amount is harmful. Similar for CO. But a certain non-zero amount of CO2 is actually required.
a) Good ones are expensive, and the consumer grade ones are valuable mostly for trends rather than precise readings.
b) They do occasionally require re-calibration. Meaning, leave it outside for 24 hours. The ones I have experience with expect an occasional return to atmospheric baseline, but if that never happens naturally - which never does in my apartment - they tend to drift.
c) Your basement might have amazing circulation. I have an air pump (it's a big radon fan) pumping in fresh air to my apartment any time CO2 rises about 700ppm, and the exhaust fan pulling air out as well. In theory, my circulation is quite good. However, even with all that the CO2 is quite high (700-1000ppm) while we're home.
