When Mythbusters uses helium to lift something, such as the one with a small child and party balloons, do they use pure helium, o "balloon" helium?

For safety reasons helium sold to the general public for balloons is likely to have oxygen o air added, to reduce the likelihood of silently suffocating to death.

Helium has in fact been suggested as an effective execution method.

The difference is small, but even a small "profit" can add up in cases like this.