Oxygen..

[Anomalocaris]

Curiously, chickens do much better.

All birds, dinosaurs and their relatives do it better, even more distant relations of birds like alligators do it better. Most mammals do it poorly.

Another trivia, the gas exchange in a chicken happens inside its bones, abdominal cavity, as well as in its lungs. If you nick the main wing bone of a chicken, you can tape up the chicken's nose and beak, and it can still breath.

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It's also not a particularly efficient process. 75% of inhaled O2 is exhaled unused.

Also, if the air you breath has no oxygen content, your body would not alert you to the fact that you are suffocating. You just pass out peacefully and die.