system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When Performing CPR, aren't we just putting carbon dioxide into the person's lungs during mouth to mouth?
gpt
Yes. But the percentage of O2 in the atmosphere is about 21% and the percentage of CO2 less than 1%. But when we breath out, the resulting air mix is about 16% O2 and 4-5% CO2. 

Is it as good as breathing in "room air"? No, probably not. But it's still got plenty of oxygen and it's a far sight better than not breathing in at all.