Hydrogen (whatever isotope) and helium don't have a smell, so in one respect, the answer would be `nothing'. On the other hand, if you're in an oxygen-containing atmosphere near nuclear fusion, there's probably some ozone floating about, so the answer may be `laser printers'. I think the best answer, though, is. . .
ARGH! MY NOSE IS BURNING DUE TO THE LARGE AMOUNT OF ELECTROMAGNETIC RADIATION BEING RELEASED IN ITS VICINITY!