As human beings, we do certain things so much by default that they become second nature. Breathing is one of them. We run on oxygen by design and simply don’t think much of it, unless it is suddenly in short supply.
About 250 million litres of air passes through the lungs of an average human in his lifetime. And unless you’re fitted with an oxygen tank, almost all of this air is sourced from your immediate environment.
This is where things get murky.