Oxygen..

[Jackalope]

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.

True. The physiological sensation of suffocation is your body's response to excess carbon dioxide, not a lack of molecular oxygen. No oxygen = no carbon dioxide.