Carbon dioxide

Carbon dioxide is a gas, comprised of carbon and oxygen, which is formed by animal respiration and combustion of organic matter. Also, vionesium produced large quantities of carbon dioxide when exposed to oxygen. (DW: Terror of the Vervoids)

Due to frequent use of cars on Earth in the 20th and 21st centuries, carbon dioxide levels began to rise. The Atmospheric Omission System was ostensibly invented to prevent this, but it was a ruse to enable a Sontaran invasion of Earth. (DW: The Sontaran Stratagem / The Poison Sky)

The Second Ice Age of 5000 was attributed to a severe drop in carbon dioxide levels, which had been caused by reducing the amount of plants on Earth. (DW: The Ice Warriors)