science
We depend on oxygen to survive, yet it makes up only about a fifth of every breath — most of what we inhale we simply breathe straight back out.
Nitrogen dominates the air because it is chemically unreactive under everyday conditions, so once released into the atmosphere over billions of years it simply accumulated rather than being consumed the way oxygen constantly is. Our bodies have no direct use for nitrogen gas, so with every breath we draw it in and breathe it back out unchanged, while oxygen gets pulled into the bloodstream to fuel our cells.
Oxygen is essential but is only the second most common gas in air, and carbon dioxide, despite its climate importance, makes up less than half a percent of the atmosphere — far too small to be what we mostly inhale.
Because nitrogen is so abundant yet unreactive, living things evolved ways to "fix" it into usable forms, which is why nitrogen-fixing bacteria in soil are so critical to agriculture.
science
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