I vote for nitrogen asphyxiation. It has happened accidently, and from what I recall it is particularly insidious because the victims have no sensation of discomfort whatsoever. They just pass out, and then die in short order.
The same is true for Carbon Monoxide.
The skin of the victim even looks fully oxygenated. Their skin will still have a rosy hue.
With normal asphyxiation the nail beds turn blue early then the entire skin turns blue. Not true with carbon monoxide poisoning. There are no obvious external signs of asphyxiation. The victim simply falls asleep and stops breathing. NO pain (unless it is a chronic exposure where a headache may occur), no gasping for breath and no fear.
“I vote for nitrogen asphyxiation. It has happened accidently, and from what I recall it is particularly insidious because the victims have no sensation of discomfort whatsoever. They just pass out, and then die in short order.”
Sounds very much like carbon monoxide poisoning - maybe faster.