alt Hacker NewsCO2 by itself doesn't appear to be a problem until some truly absurd levels are somehow achieved.
CO2 is usually a proxy for air quality because outside is usually something consistent like 400 therefore deviation from that figure indicates staleness.
I don't have the list of relevant studies I've read on hand atm but there was at least one where they simply pumped in CO2 (or didn't) and had an effect showing pretty directly that CO2 itself is problematic at reasonably low levels - as well as potentially being an excellent proxy for other air quality issues. Problematic in a "makes you drowsy/stupid" type of way - not a "kills you" type of way. Though there's also at least one correlative studies suggesting it might have long term health effects (which really wouldn't be surprising if it has any short term effects).
Besides which, proxy or directly high CO2 means you should ventilate...