a_unique_person
Director of Hatcheries and Conditioning
It isn't lack of oxygen that causes that panicky 'gasping for breath', but build-up of CO2. If the CO2 can be eliminated (or is so dilute it doesn't matter) the person goes woozy, unconcious, then dead. One source I read described the woozy phase as 'sleepy euphoria', or similar.
So a room full of an inert gas would be a painless way to go though, quite possibly distressing if you know what's happening.
One of those quirks of evolution. Rather than our bodies detecting a lack of oxygen, they detect an excess of CO2. It worked well enough during the stone age.