Helium

Helium was a light element found on Earth. It was a gas at room temperature.

When humans inhaled it, their voices became high-pitched. Gallifreyans may also have been affected, but had a respiratory bypass system that allowed them to avoid inhaling it. (DW: The Robots of Death)

Helium 2 also existed, being created in a supernova. (DW: Time and the Rani)