What she said.
AFAIK, tent CO2 or CO problems are mainly associated with being buried under snow, or using a stove or lantern. From a personal experience, I know that the seven of us in the dining tent in Nepal enjoyed the light and heat of the kerosene lamp, but I was 'paranoid' and kept my foot so that the tent door flap stayed cracked open. Trouble is, with CO poisoning, you just go to sleep and any safety-attempt like that can fail.
I searched a bit for a study on CO2 levels in sleeping campers. You would think somewhere there might be a comparison of CO2 levels with 1,2,3,4 people in 3 vs 4 season tents, and comparison of tent materials, historic and current. Would make interesting reading even if there was no problem. Surely there must be, to use a chemistry term, a 'closest packing ratio' that makes a difference.
