It's very sad but it's not a bad way to go. I was a nuclear plant supervisor for about 20 years. One day the temp started to increase in the control room. The 3 AC units were behind the control room so I headed there to check on things. Another guy said he'd go with me. These things are as big as locomotives. Well while we were in the room a refrigerant relief valve lifted with a huge roar. Turns out, amazingly, the relief was designed to vent into the room. We made it to the card reader to get back into the control room but my card kept being rejected. I felt myself sliding down the wall and next I knew I was sitting in a chair in the control room feeling better and better. If my buddy hadn't decided to come with me I would have died in that room.

There was no panicky brain screaming "we need air" or any feeling of being O2 short. It was just normal breathing. If you're going to end it all that's the way to do it.

I wrote an incident report with the obvious suggestion to vent the reliefs outside. It was modified the next outage.

"But what if we vent the toxic gas into the place where you go to see if there's a problem with the toxic gas..."

Three died in that accident. The third lingered for a couple years before passing.