Don't fully airtight structures need a lot of redundant safety systems to protect against just leaving a candle on killing everyone from carbon dioxide poisoning?
A little bit of research indicates that candles are not all that different in oxygen consumption and CO2 production than a human and that a single candle (or person) wouldn't use up the oxygen quickly if the ventilation ceased.
As long as a failure of the ventilation system is obvious (so a window can be opened or whatever), it doesn't seem like it would be a major concern.