Helium

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

Arcturus's life support system included a helium regenerator, which, if deactivated, he would die. (TV: The Curse of Peladon)

When humans inhaled large quantities of helium, their voices became high-pitched. The Fourth Doctor was able to use his respiratory bypass system to avoid its effects. (TV: The Robots of Death)

Helium 2 had the potential to exist during the Leptonic Era. (TV: Time and the Rani)