@titania
To give an idea of just how nitrogen can be toxic
80% of the air we breathe is Nitrogen. It's only poisonous at great pressure such as deep-sea diving.
At sea-level pressure or lower it can only harm by displacing normal air completely and so eliminating the oxygen.
Gases that can kill or render unconscious are rare in aircraft. As you point out CO - Carbon Monoxide is one.
Another one very relevant to aircraft is HCN gas - Hydrogen Cyanide. It's a by-product of heating certain types of plastics and was the primary cause of death in at least one otherwise survivable accident - where a plane crashed and a small fire started that generated smoke and HCN from seat cushions that killed most passengers before they could exit.
However HCN would be very unlikely to be generated without large amounts of smoke.