Gas

Gas was a state of matter.

Earth at an early stage of its development was an enormous mass of superhot gases surrounding a core of molten rock. Over millions of years, the sun "eroded away" the gases, causing the planet to solidify. (AUDIO: Exploration Earth)

The Twelfth Doctor claimed to know of a race made of sentient gas who threw fireballs as a "friendly wave". (TV: Flatline)

The Doctor was susceptible to gases in the praxis range. (TV: The Caves of Androzani, et al.)

The Empty Child plague caused gas masks to be fused to the skin of people's faces. (TV: The Empty Child)

The Zircon planned to wipe out the population of Earth using a galactic cyclone created by the destruction of a dying planet. By the time it approached Earth, it was a cloud of poison gas. (COMIC: Doomcloud) By 2007, Deffry Vale High School had a sign in a classroom reading "GAS CONTROL". (TV: School Reunion )

In the early 21st century, caesofine gas was released into the Earth's atmosphere by ATMOS. (TV: The Sontaran Stratagem / The Poison Sky)

The Gelth were made of gas. When a human body decomposed, they produced gas, and the Gelth used this fact to use the corpses of Charles Dickens' driver and his coach as vehicles. (TV: The Unquiet Dead)

Craig Owens suggested that the Eleventh Doctor excreted some sort of gas which made people love him. The Eleventh Doctor denied his, claiming that he never excreted "any weird alien gases" at Craig yet he loved him. (TV: Closing Time)