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

For safety reasons helium sold to the general public for balloons is likely to have oxygen or 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.