I can't give you the science, but as a student I lived in a flat with a dodgy fire. Didn't realise at the time until we had a cold snap, and the girlfriend who had previously been awake and alert on the sofa watching TV was suddenly out sparko. I felt pretty much fine, but fortunately sussed the culprit immediately and dragged her outside.
Incidentily I'm a smoker, and find the vast majority of people I fly with are affected by hypoxic symptoms way earlier than I am (even at sensible alts like 8500ft). So go figure
Have you considered getting a CO detector? They're not expensive...