CO exposure could certainly affect pilot performance and judgement. A long time ago I was the passenger in a C150 which was later found to have a faulty muffler. I have no recollection of the pilot landing the plane, I recall riding in the right seat, and then waking up lying in the grass under the wing after we landed. The pilot said I had turned very red, and began to loose consciousness, so he landed back. Darned luck thing he did, as he was being affected too, just not as fast as I was. The certification requirement for the Piper Malibu would require that the CO concentration in the cabin air not exceed 50 PPM in flight.