As I am in the catering industry, I can tell you that there is a difference between food grade and non-food grade. It's all about the levels of impurities. Similarly, God forbid you should ever have a lung problem, I would bet that you would not let the doctors hook you up to welder's oxygen as opposed to medical grade oxygen. However, perhaps you wouldn't object......after all, O2 is what it is, erm, O2.......isn't it?