“100% pure dry nitrogen, inhaled, is absolutely lethal. It’s also dirt cheap and non-toxic.”
Why dry? Dry air will cause irritation, coughing, etc. Makes no sense to specify “dry”. It does make sense to humidify the gas to a comfortable level.
I suppose you could use LIQUID nitrogen ...
Breathing 100% N2 causes unconsciousness within a few breaths (possibly as few as 1), and death shortly thereafter. It is not a poison, it merely deprives the body of oxygen while still allowing the expiating of carbon dioxide.