We've all done it.  Open a balloon, suck the helium out, sound like cartoon character (when I was a kid we said "sound like Mickey Mouse" or "Alvin & The Chipmunks").

Most people probably don't have the "danger" in the back of their mind.  I always did, but mainly because my mother frets a lot and worries about stuff that has a very small chance of ever happening (a little gift she handed down to me)!

But I just read a story that highlights the danger of inhaling helium.

A 14 year old girl in Eagle Point, Oregon, was at a party where people were taking turns inhaling from a tank of helium.  It came her turn.  She didn't want to, but her friends pressured her into doing it, downplaying the potential danger.

She took a "hit" off the tank.  She passed out.  And died.  It was because of an "obstruction" in a blood vessel; just like when a scuba diver surfaces too fast, a gas bubble got into her bloodstream and blocked bloodflow to her brain.

(Also interesting - and I didn't know this - helium is also commonly sold as part of suicide kits, with hoods that attach to a helium tank for people who want to kill themselves. The helium crowds out the oxygen, asphyxiating a person.)