Oxygen is a gaseous element common in the atmosphere of Class M planets, although at relatively low levels (about 20% on Earth, for example).

Oxygen is vital for the survival of most carbon-based lifeforms. Some lifeforms, however, have adapted to go without oxygen for extended periods of time, such as Fantome from the Void, who had a very large lung capacity. (VOY: "The Void")

During a scan of Aldean DNA, Doctor Beverly Crusher was able to confirm that a drop in atmospheric O2 was not responsible for the Aldeans' sterility. (TNG: "When The Bough Breaks")

One species, the Benzites, can not breathe the oxygen-nitrogen atmosphere deemed "normal" by most Federation species. (TNG: "Coming of Age")

When Kira Nerys and Doctor Julian Bashir beamed aboard the Kobliad transport ship Reyab, Bashir noted that the vessel's O2 pressure levels were dropping. (DS9: "The Passenger")

In 2369 the runabout USS Ganges returned from a routine mission in the Gamma Quadrant to Deep Space 9 because due to a power failing their life support and oxygen levels dropped dangerously low. (DS9: "Q-Less")

After the alien probe "Pup" damaged most of the station's systems in 2369, a plasma fire occured in a corridor in the habitat ring. Major Kira Nerys gave Lieutenant Jones the order to bring portable oxygen and something to treat plasma burns. (DS9: "The Forsaken")

Oxygen is also a critical component of life support functions aboard most space-going craft. Oxygen is a chemical element with symbol O and atomic number 8. Its name derives from the Greek roots ὀξύς (oxys) ("acid", literally "sharp", referring to the sour taste of acids) and -γόνος (-gοnos) ("producer", literally "begetter"), because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless, odorless, tasteless diatomic gas with the formula O2. This substance is an important part of the atmosphere, and is necessary to sustain most terrestrial life.

Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with most elements except the noble gases Helium and Neon. Oxygen is a strong oxidizing agent and only fluorine has greater electronegativity.[1] By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium[2] and the most abundant element by mass in the Earth's crust, making up almost half of the crust's mass.[3] Oxygen is too chemically reactive to remain a free element in Earth's atmosphere without being continuously replenished by the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Free elemental O2 only began to accumulate in the atmosphere about 2.5 billion years ago (see Great oxygenation event) about a billion years after the first appearance of these organisms.[4] Diatomic oxygen gas constitutes 20.8% of the volume of air.[5]

Oxygen constitutes most of the mass of living organisms, because water is their major constituent (for example, about two-thirds of human body mass[6]). Many major classes of organic molecules in living organisms, such as proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that are constituents of animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere. Another form (allotrope) of oxygen, ozone (O3), strongly absorbs UVB radiation and consequently the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraft.[7]

Oxygen was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774, but Priestley is often given priority because his work was published first. The name oxygen was coined in 1777 by Antoine Lavoisier,[8] whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water and other means. Uses of elemental oxygen include the production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy and life support systems in aircraft, submarines, spaceflight and diving.

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