I had a tank of helium from the welding supply store. I filled a balloon and was teasing my wienerdogs by barking at them with a high voice. I took one too many breaths of helium. Helium that is found in balloons at most carnivals and dollar stores etc. is mixed with air to save money. Helium from a welding tank has no air mixed in.
With no warning or sensation that I was short of breath I collapsed and hit my chest against the corner of our wood stove on the way down. When I regained consciousness our wienerdogs were licking my face. I was lucky that this didn’t result in any brain damage, at least none that has been proven... my wife might not agree, Their is no pain, sensation or mess from breathing an inert gas that has displaced oxygen and yes it will kill you. And you don’t need to fill a room with it. A bag filled with helium put over someone’s head will kill them quickly, and with no sensation.
I believe the reason this method is not used is because it is so easy and so painless that the authorities do not want it publicized for fear that it would increase suicide and euthanasia deaths.
Barking man on helium bump.