You've seen people suck up some helium and talk like a chipmunk at birthday parties, but now you can hear the opposite. This selfless science teacher inhales sulfur hexafluoride and treats us to the Oscar Mayer song.
I've already told you why helium makes someone's voice sound high and squeaky. Different gases change the kinds of resonant frequencies inside a person's throat. Helium doesn't interact with much, and it's very light. That means that it won't glom on to molecules inside your body, and it will be easily pushed out by the lungs. The only danger is making sure you don't inhale too much and suffocate yourself.