Why does breathing pure nitrogen kill without any warning?

Nitrogen, at about 78 per cent, and almost all of it goes straight back out again - the two atoms are bound by one of the strongest bonds in chemistry, which makes the gas nearly inert. You are swimming in it and cannot use any.

It is not entirely harmless, though. Under pressure nitrogen dissolves into the blood and tissues, and a diver who surfaces too quickly gets it coming out of solution as bubbles - the bends. At depth it also produces nitrogen narcosis, a drunkenness serious enough that deep divers breathe helium mixtures instead. And an atmosphere of pure nitrogen kills without any sensation of suffocation at all, because the urge to breathe is triggered by carbon dioxide building up rather than by oxygen running out, which makes nitrogen leaks in industrial spaces particularly lethal.

Getting nitrogen out of the air and into a usable form is the central problem of feeding people. For most of history only lightning and certain bacteria could do it; since 1913 the Haber-Bosch process has done it industrially, and roughly half the nitrogen atoms in your body have been through that process. It consumes on the order of one to two per cent of the world's energy, and without it something like half the current population could not be fed.