CO gets in your blood 250 times better than oxygen, a small concentration is all that it will take, like 5% and curtains.
I will assume that you have posted this as a courtesy to other, less-knowledgeable FReepers. (During my career, I worked with trace-gas detectors.)
You are, of course, referring to the blood concentration, not the ambient air concentration (which can be deadly even around only 800 parts per million).
Regards,