Oxygen (chemical symbol O) is one of the basic organic elements, and is a constituent of most of the known organic molecules - and therefore all lifeforms - on earth.


Basic chemistry

Oxygen is a colourless odorless diatomic gas with an atomic number 8 and atomic weight 15.999. It has a melting point of -219°C and a boiling point of -183°C . It is mainly found in its diatomic form, O2. In the high atmosphere, it is commonly found as ozone, O3, its triatomic form.


The stable isotope oxygen-16 accounts for more than 99.7% oxygen found on earth. Tiny percentages of oxygen-17 and oxygen-18 make up the remainder of the naturally-occurring oxygen .


Of course, oxygen is vital for the treatment of hypoxic states, however pure 100% oxygen is potentially toxic. The development of a hyperoxic state as may occur when administering high flow oxygen gas to an unwell patient can lead to the formation of reactive oxygen species (ROS), primarily the superoxide anion (O2), hydrogen peroxide (H2O2), and the hydroxyl radical (HO). The hydroxyl radical has the severest injurious effects in the body. These free radicals can overwhelm the human body's innate defenses and lead to deleterious effects .

NB: '' symbol indicates the presence of an unpaired electron and partially accounts for the highly reactive and damaging nature of these molecular species.

Clinically, oxygen toxicity may lead to:

Pulmonary toxicity

CNS toxicity

If the oxygen is being inhaled at higher than normal atmospheric pressures, then high-pressure oxygen toxicity (a.k.a. Paul Bert effect) may occur, with severe deleterious CNS effects :

  • abrupt-onset seizures, usually rapidly followed by coma (30-60 minutes)
    • usually lethal
  • also: nausea, muscular spasms, dizziness, visual impairment, delirium

Radiological importance

  • oxygen-16 may be employed to synthesize radioactive nitrogen-13 for use in ammonia molecules in myocardial PET
  • oxygen-17 has been usefully employed as a tracer to assess cerebral oxygen usage
  • oxygen-enhanced MRI is an emerging technique to study hypoxic tumors in vivo
  • In the presence of oxygen (O2), biological tissues are more sensitive to ionizing radiation (oxygen effect) 

    History and etymology

    Oxygen was discovered by Joseph Priestley and Carl Wilhelm Scheele (1742-1786) in 1774 . However it was not until 1786 that the word oxygen was coined by the French chemists Morveau and Lavoisier .

    The word oxygen is from the French word oxygène, which itself was derived from the Ancient Greek words οξυς (oxys) meaning acid and γεν (-gen) meaning producing, i.e. an acid-producing substance .