system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What is the importance of nitrogen in air? Almost 80% of the atmosphere is N. If this amount drops to e.g. 50-60%, what would happen?
gpt
Nitrogen in the air is, to a large degree, just filler. It doesn't do much, but is there so that there isn't too much of anything else.

So, if you dropped the amount of nitrogen in the air, what would replace it? If it was some other largely inert gas like argon or helium, then nothing. It would work perfectly well as air. Indeed, deep sea divers do replace some or all of the nitrogen with helium, because at high pressure nitrogen is toxic.

If you replaced it with oxygen, which is what the other 20% of the air is, then fires would be much easier to start and much harder to put out. Metal things would oxidize (rust or corrode) faster.