There was a guy who got CO poisoning at work one morning. Didn't realise but after lunch people thought he had a liquid lunch until someone saw the symptoms for what they were. Straight to hospital A&E and treatment. He got sent home as ok, because he seemed ok. He was not and IIRC Friday night he took a turn for the worst. Air ambulance to the police HQ and the police dive squad as the only free decompression chamber nearby (dive school was already in use I think). He spent something like 2 weeks in high pressure, high oxygen content chamber. It took probably 10 or 15 years for his brain to rewire itself such that he was close to his old self. Even so he had a lower sense of embarassment and tended to be unable to filter things he said. He ended up with a very good 6 figure payout from that and got paid by the company for 2 years on sick at full pay. After that he came back with a promotion.
Apparently CO has a 200 to 300 times the affinity to heamoglobin in red blood cells than oxygen. In other words you cannot easily remove CO from red blood cells so once attached they stop oxygen being carried around by the red blood cells. To make oxygen more likely to attach to heamoglobin you need a high O2 content (potentially toxic) and a higher air pressure environment (AIUI that lowers the concentration of O2 needed). That and some time in that environment to allow the exchange from CO to O2 in the heamoglobin.
So my advise is always be CO aware with gas appliances. Get a CO monitor. We always carried one in our tent when camping (car camping admittedly). When not car camping we cooked outside completely with no porch cooking or in tent/tarp cooking. Not worth the risk.
Oh and replace the monitors often as I do not think they last.