helium is used to fill balloons and airships because its density is lighter than air. it does not burn, so is safe for that kind of use. it is also used in some kinds of light bulbs. people can breathe in helium: it makes their voices sound higher than it normally does. this is a joke, but is dangerous as if they breathe in too much, hypoxia can injure or kill them as they are not breathing normal air. breathing too much helium can also cause long-term effects to vocal cords.