Heh. Every container of liquid nitrogen ("LN2") has a big warning about using it in a well ventilated place. Upshot is you can suffocate from it without warning because breathing it will not give any "out of breath" sensation.
They also use it to euthanize the many pets at the dog pound because the animals just "go to sleep" without any respiratory stress.
The UN monkeys should be aware that this method of euthanasia is pretty much painless, so it brings up the question of why it is being challenged.
The only thing I can think of is that the UN monkeys are confusing it with "the Bends," which is caused by breathing compressed nitrogen in deep-sea diving and coming up too fast.