just partial asphyxia......... like holding your breath, fills your lungs, then bathes your vocal chords on the way out, you sound squeeky cos your vocal chords vibrate faster in helium than air.
High feeling is from oxygen starvation.just like holding your breath for too long. No toxic effect, but because your still breathing you don't get the panic "choking " reaction.
If you filled a room with helium, you'd die, but not because of it being poisonous or anything.