Carbon dioxide

As was evident from the name, carbon dioxide (or CO₂) was a compound of carbon and oxygen. (TV: The Sontaran Stratagem)

It was most often considered in its gaseous state, particularly on the planet Earth, where it was a vital part, as the Second Doctor once remarked, of the Earth's "lower atmosphere". (TV: The Ice Warriors) On other planets, it was less important, as was obvious from the Sontaran attempt to change Earth's atmosphere to something more hospitable to their species. (TV: The Sontaran Stratagem)

The Dregs required carbon dioxide to breathe and exhaled oxygen as waste. (TV: Orphan 55)

Importance to Earth
Carbon dioxide was particularly important to Earth's atmosphere. According to the Second Doctor, "The carbon dioxide in the Earth's atmosphere help[ed] to retain the Sun's heat." The removal of that gas resulted in "a sudden freeze up". Carbon dioxide levels began to drop precipitously around the year 4900. As Clent made clear, that was when Earth conquered hunger through artificial means. Since it was no longer necessary to grow food, people encroached on farmland all over the world. A dramatic reduction of vegetation ensued, resulting in the elimination of much of the atmosphere's carbon dioxide. What followed was the Second Ice Age of 5000. (TV: The Ice Warriors)

There was also danger to Earth's atmosphere if the carbon dioxide levels rose. People in the 20th and 21st centuries were concerned about this phenomenon, largely brought on by the increased use of cars. They were initially relieved by the introduction of ATMOS, a bit of technology retrofitted onto cars worldwide. For a brief time in the early 21st century, it seemed to be the answer to the atmospheric worry. However, it was revealed by UNIT and the Tenth Doctor to be a ruse which changed the Earth's atmosphere into one that enabled a Sontaran invasion of Earth. (TV: The Sontaran Stratagem / The Poison Sky)

Elsewhere in the universe
Though its terrestrial origin was usually organic life, it was created by other things elsewhere in the universe. For instance, the mineral vionesium produced large quantities of carbon dioxide when exposed to oxygen. (TV: Terror of the Vervoids)

The Scabus came from a planet whose atmosphere was once rich in carbon dioxide before a more advanced species colonized the planet and altered the atmosphere to do so, drastically reducing the carbon dioxide present. (AUDIO: Terror of the Master)