Oxygen

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. 

Chemistry

Basic chemistry

Oxygen is a colourless odourless 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 ¹. It is mainly found in its diatomic form, O₂. In the high atmosphere, it is commonly found as ozone, O₃, its triatomic form.

Radiochemistry

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 ².

Toxicity

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 defences and lead to deleterious effects ³.

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

• acute lung injury (ALI)
• acute respiratory distress syndrome (ARDS)

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 ³:

• 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 synthesise radioactive nitrogen-13 for use in ammonia molecules in myocardial PET ²
• oxygen-17 has been usefully employed as a tracer to assess cerebral oxygen usage ²
• oxygen-enhanced MRI is an emerging technique to study hypoxic tumours in vivo ⁶
• In the presence of oxygen (O₂), biological tissues are more sensitive to ionizing radiation (oxygen effect) ⁷

History and etymology

Oxygen was discovered by JJoseph Priestley and CWCarl Wilhelm Scheele (1742-1786) in 1774 ^1,8. However it was not until 1786 that the word oxygen was coined by the French chemists Morveau and Lavoisier ⁴.

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 ^4,5.