Gas

Gas was a state of matter.

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

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

In the early 21st century, sezerfine 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. According to the Ninth Doctor, the Gelth could "slip inside [the body and] use it as a vehicle, just like [ Charles Dickens'] driver and his coach". (TV: The Unquiet Dead)