One quick thought on the CO2 thing. CO2 will kill, but it is not comfortable. Anyone who dives knows that the uncomfortable feeling in your chest when you hold your breath is due to CO2 build up, rather than a lack of oxygen. Try it: hold your breath, then when you start feeling really bad, exhale and you'll see what I mean. Has anyone considered car exhaust? CO is an entirely different story. It causes irreparable damage, but no discomfort. I haven't needed to do this, but if I did, it's what I would try. Some really good comments on this thread.