Comment by nwallin

AirGas prioritizes industrial users, in the case of helium, copper welding. Argon is perfectly good enough for almost all welding purposes, but copper is different because of its heat conductivity. The heat from the weld really wants to go anywhere else. Helium has substantially higher heat conductivity than argon, which allows the heat to flow from the electric arc into the metal faster, resulting in better welds.

Obviously you can't have oxygen in welding gas; it would oxidize the shit out of everything.

A little bit of oxygen in party balloon gas is beneficial. Some kid will breathe it, and when they do, you didn't want them to asphyxiate themselves.