Helium works so long as your helium source is pure.
Doesn't have to be helium. Can be nitrogen or argon. Or any mixture of nitrogen argon and helium.
You can get actual tanks from welding suppliers. Not cheap, and you need a bunch of pressure regulating equipment (well I say a bunch, but like a thing) so you can get a output pressure that won't be ridiculously high.
The idea with asphyxiation with inert gases like this is to breathe the gas instead of oxygen. The gas goes in your lungs and when you breathe out, it takes the CO2 with it. The urge to breathe comes from the CO2 levels in your blood. If you're breathing out CO2 then your body is tricked into thinking that you're fine. The special thing about pure inert gas is that it also takes the oxygen out of your blood too- so when this oxygenless blood reaches your brain, it just BAM, knocks you out. Your brain just shuts off immediately since there's no oxygen in the blood to sustain it.
You keep breathing nothing but the inert gas and your brain will just have no oxygen at all for a while. It's the same as if your heart just stopped- your brain isn't getting anything. But your body thinks everything is normal since you're breathing. Your breathe rate won't even increase since that's all CO2 based.
The aim is to breathe nothing put pure helium(or nitrogen, argon) for at least 4 minutes. You'll be out cold in 30 seconds and dead within 2 minutes, but aiming for 4 is a good idea.
Look up accidents in nitrogen rooms, and liquid nitrogen spill accidents.
You could just recreate a liquid nitrogen spill accident too, same principle.
What won't work is dry ice. It's carbon dioxide so your body will reject it.Post too long. Click here to view the full text.