The CO2 concentration in each breath exhaled is 4% whereas CO2 concentration in ambient air is 0.04%. So, about 100 times more CO2 is exhaled with each breath than is inhaled. The average human exhales about 2.3 pounds of carbon dioxide on an average day. The exact quantity depends on the activity level. A person engaged in vigorous exercise produces up to eight times more CO2. CO2 exhaled by humans is nothing compared to the CO2 emitted from burning fossil fuels.