Fresh air at sea level contains about 400 ppm of carbon dioxide.
The maximum recommended level of CO2 in offices etc is 1000 ppm. At concentrations above this people start to feel sleepy or sluggish. In a normal combustion process high levels of CO2 are formed (15% or so) provided there is sufficient free oxygen. If there is inadequate oxygen, carbon monoxide forms instead.
For a known fuel composition it is possible to measure oxygen and calculate CO2 and vice versa. Currently the most cost effective way to measure CO2 is to use infra red absorption techniques.