Why Helium
Lots of near space weather baloons going up, but they all seem to be full of Helium, God's second favourite element, (you didn't think he had a favourite? Haven't you read your Bible ?)
Helium is expensive and getting rarer and Hydrogen (HIS favourite) is cheap *and* H2 has half the molecular weight of He.
It seems to me that not only can Hydrogen get you more lift, but you can play the trick of not inflating the balloon so much meaning that it pops at a higher altitude.
Yes, I know it burns, but I just can't being myself to care, if somehow you managed to make it go bag several thousand metres up, you might not even see it, let alone hurt anyone, also it is slightly harder to make it explode than you might think.
So why isn't it used ?