Well, helium is a noble gas with the lowest melting point of any substance in existance. If one were to ingest solid helium, they would probably cease to exist due to the severe cold they were experiencing. Helium does not form any compounds, so the only way that someone would ingest helium would be in the form of a gas. The gas is much less dense than air, so their vocal cords vibrate a heck of a lot faster as the He is expelled causing their voice to be much, much higher. The dangers with ingestion of helium is simply asphyxiation caused by the displacement of oxygen. There are no chemical reactions which occur in the body, and as soon as the helium has escaped normal respiration begins once more. So in all actuality, inhaling helium gas really isn't a danger. It's just when people forget that they need oxygen to live that the problems occur.