system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : When I'm doing CPR on a person and I'm breathing into his mouth, am I not breathing CO2 into his mouth because i exhale CO2?
gpt
The air we breathe in is about 21 % oxygen, and contains virtually no (0.04 %) carbon dioxide.

The air we breathe out is still about 15 % oxygen, and only 5 % carbon dioxide. There’s still plenty of oxygen available, and a ‘second-hand’ breath is much better than no breath at all.