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Hi! I have a student who is wondering approximately what percent of the air we breath out is carbon dioxide. He understands that we take in oxygen during cellular respiration and we give off mostly carbon dioxide along with some nitrogen and oxygen. But....he wants to understand what percent is carbon dioxide during an exhale. He wants to compare your answer to the answers he's getting for his own cellular respiration data he's taken for his science fair project.
Answer 1:

1. The air we breathe in contains about 0.04% carbon dioxide.

The air we breathe out contains about 4% carbon dioxide.

In other words, exhaled air contains about 100 times the concentration of carbon dioxide that inhaled air does.

reference

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 content.

read here

That seems to answer it.


Answer 2:

It depends on the efficiency of the breather, and different species of animals are different. Humans, to my recollection, are quite inefficient, but I can't give you a number. You could try to do an experiment to find out!



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