In theory yes, it should be painless. You’re just breathing normally and then everything goes black, that’s how it’s supposed to look like.

In practice however, body does react to loss of oxygen. I have some experience with this, I worked on an experiment involving mice. It was tumor research, mice were given an ugly tumor on the side of the body, experimental treatment was applied, stuff measured and at the end of it all, the animal was sacrificed.

The setup was such that the mouse was put under and breathed through a tube a mixture of breathable anaesthetic, nitrogen and oxygen. Electrodes were put in, results measured and when you were done you shut off the oxygen valve and wait for mouse to expire. It took a while and the animal squirmed, spasmed and made noise on occasion.

It probably didn’t hurt, the mouse was unconscious from the anaesthetic already, the lack of oxygen just added to that. The movements were involuntary, surely. Then again it wasn’t completely without symptoms either and the only way to be sure is to experience it yourself.

Coming back from an execution to tell the tale only happened once in recorded history and not everyone believe that (it’s a basis of beliefs of about 1.8 billion people though). I’d be wary of saying it certainly isn’t excruciating. It shouldn’t be, but there’s no way to tell for sure.

I don’t see why we wouldn’t add anaesthetic to people we’re executing though. It’s dirt cheap. The worst thing you can do is an overdose and that’s kind of the point anyway.

After 40 years in Industrial Safety, I can say with some authority, that after reviewing dozens of fatality reports from exposure to Nitrogen Rich or 100% Nitrogen atmospheres during work involving Confined Spaces such as pressure vessels and tanks onshore and various vessels offshore, one hallmark of these deaths was the lack of warning or signs of discomfort on the part of the victim.

To put it bluntly, they died like someone blew a candle out.

Bystanders reported the victim gave no indication that they knew they had entered a lethal atmosphere, took one or two breaths and went down like a switch had been flipped, totally unconscious….and as is “normal” in such cases of near total hypoxia, the victims were dead in a matter of no more than 5 minutes.

Would I vote for its usage in my State, Texas, as a means of execution….???

Tho I do not favor the way Capital Punishment is meted out under the current system in Texas and would support a total moratorium until every pending case is fully reviewed, and Capital Punishment becomes RARE, I also support changing to Nitrogen Hypoxia as the method for execution, for those cases that pass the enhanced review process.

There are some cases so egregious, there are acts of such craven, sociopathic brutality, that the only suitable punishment is Death.