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. (TV: The Robots of Death)

Helium 2 also existed. It was created in a supernova. (TV: Time and the Rani)