Yesterday’s commuter rag carried this ‘interesting fact’:
Are the two processes so separate that one could be considered the sole cause of death?
Yes. For example, the carbon-dioxide scrubbers failing was the real problem when Apollo 13 had its accident. They had enough oxygen for the whole trip, but they couldn’t remove CO2 fast enough with three people taxing the built-for-two LEM.
ETA: Hypercapnia
Yes - and it’s worth mentioning that your body’s natural “breathe” reflex is linked to carbon dioxide - not oxygen- concentration in the blood. It’s very possible, by hyperventilating and artificially lowering your CO2 levels, to go hypoxic and pass out without any particular sense of urgency.
I imagine CO2 poisoning is a particularly bad way to go.
Apparently it is. The CO2, as you said, gives you that gasping, choking sensation. By contrast, being trapped in a space that’s flooded with nitrogen and not enough oxygen is supposed to put you out like a light with no trauma or awareness of your situation at all. (My wife works in the wine industry, and has heard stories of workers who were killed when tanks were accidentally filled with nitrogen. In these cases, the workers easily could have left the tanks had they been gasping for breath.)
I went to a couple of science club sessions when I was at school. At one they did some messing about with gases, including the classic ‘squeaky helium voice’. The volunteer kid took a hit off the He cylinder, squeaked a bit, and giggled, losing most of the gas. So he took another lungful, squeaked another few words, went alarmingly blue around the lips and faceplanted into the floor like a felled tree. Within a second or two he was sitting up and going 
.
Lessons learned:
[ul][li]Lungs full of nothing but inert gas can mess you up good with no warning[*]Think twice before volunteering[/li][/ul]
My high school chemistry class did the same thing with helium, but we also saw what happened with sulfur hexafluoride (SF6), a very heavy gas. While helium makes you talk like a chipmunk, SF6 is instant Barry White.
SF6 is far more dangerous than helium because its so heavy it won’t come out of your lungs just by breathing like helium will. You have to stand on your head for a few breaths to purge the SF6.
Are you sure about that? It’s not that your lungs are two big open containers where it could just pool at the bottom.
Actually many things can and do pool at the bottom of the lungs.
Hence the “postural percussion and drainage” approach to most pulmonary toilet procedures. This involves inverting the person enough for gravity to assist in getting fluids, mucous, (and heavier than air gases) out of the lungs while also whailing on their back with percussive devices (cupped hands, rubber clapper devices, vibrating vests).
But would CF6 pooling in the lungs be a serious hazard? I was under the impression that the act of breathing in and out will force out most of the gas, even if it’s heavier than ambient air.
I’ve also observed the low voice thing with nitrous oxide.
Saw Jay Leno take a hit of it. His high voice dissipated in the same way I’m sure we’ve all done at birthday parties. His low voice, however, stuck around for a while. To get it out, he had to basically bend over and look behind him, exhaling sharply. So yeah, it will just sit there inside you for an inordinately long amount of time. Dangerous? Depends on how much of your lungs are full of it.
Mythbusters did it, making viewers promise not to try it at home.
The same thing happens for the same reason, but I’m sure the person who inhaled the nitrous oxide had a hell of a lot more fun than the audience.
As for SF6 (or nitrous oxide/laughing gas) remaining in the lungs, I haven’t seen a person black out because of it, but it is certainly harder to spell from the lungs and standing on your head and exhaling sharply is safest. After all, as slaphead mentioned, even helium can knock you out, and helium will float out of your lungs on its own.