the supply air is for combustion.. to reduce corrosion to the connector and venting.
Ventilation for humans is another issue.
The standard for combustion air is twice the amount required by engineering calculations.
My point is that the air we breath in the newer tight homes is a code deficiency that is rarely mentioned.

Commercial buildings get balence reports that show quantity not quality of air. Same with the house .very rarely do I see fresh air brought in .even if a fireplace is burning with a oven on for hours on end.
I just feel that monoxide issues are from leaking vents or nonexistent venting and not from incomplete burning . or inefficient combustion in a properly sized and maintained vent system.