Probably the hydrogen would combust with the oxygen you're bound to breathe in. then, you would get water in your lungs, and there would probably be an explosion as well... THEORETICALLY!
First of all, if you breathe in heaps of hydrogen, you would probably suffocate anyway.
Secondly, doing this is really dumb and only someone like Michael Jackson or Paris Hilton or Adolf Hitler and his grandpa Osama Bin Ladin would do something as stupid as this.
Thirdly, you body would probably protect you anyway.