1. The air we breathe in contains about 0.04%
The air we breathe out contains about 4% carbon
In other words, exhaled air contains about 100
times the concentration of carbon dioxide that
inhaled air does.
2. Wikipedia's Breathing article says we
exhale this much CO2:
4% - 5.3% carbon dioxide
3. Compared to the low levels of about 0.04
percent in inspired air, exhaled air contains a
hundred times more CO2, at 4 percent
That seems to answer it.