What is oxygen toxicity?

Breathing O₂ at high partial pressure can have harmful effects on the body. Many organ systems can be affected by O₂ toxicity, but the most common are:

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  The central nervous system (CNS), causing seizures and unconsciousness when breathing hyperbaric O₂, which is of obvious importance for divers. CNS toxicity is known as the Paul Bert effect.

  
  
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  The lungs, causing bronchopulmonary dysplasia in premature neonates, whilst older children and adults develop absorption atelectasis and acute respiratory distress syndrome (ARDS). Lung toxicity is known as the Lorraine Smith effect.

  
  
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  The retina, leading to retrolental fibroplasia in neonates, also known as the retinopathy of prematurity.

What is the mechanism of oxygen toxicity?

Toxicity is due to the unique structure of the O₂ molecule. The O₂ molecule is made up of two oxygen atoms, each with an unpaired electron in its outer shell. It is these unpaired electrons that give O₂ its property of paramagnetism. Molecules containing unpaired electrons – or free radicals – are usually highly reactive. Fortunately, the unpaired electrons of the O₂ molecule are fairly unreactive, requiring metal ions (such as those found in metalloproteins) to help the O₂ molecule accept electrons. This is of particular importance in the electron transport chain, where electrons are transferred to O₂ molecules, mediated by the copper-containing cytochrome c oxidase (Complex IV), to form water molecules.

The reduction of O₂ to 2H₂O requires four electrons: O₂ molecules accept one electron at a time, passing through a number of oxidation states along the way. Known intermediate molecules are the superoxide anion (O₂^•−), hydrogen peroxide (H₂O₂) and hydroxyl free radicals (OH^•): these are collectively known as reactive oxygen species (ROS). ROS are also generated when cells are exposed to ionising radiation. O₂ toxicity is thought to be due to the harmful effects of ROS on nucleic acids, fatty acids, amino acids and sulphydryl-containing enzymes.

Is there a natural antioxidant system in the body?

Antioxidants can be either exogenous or endogenous. Exogenous antioxidants are obtained from the diet and include ascorbic acid (vitamin C). The body has many endogenous antioxidant systems to protect against oxidative stress. The most important are the glutathione, catalase and superoxide dismutase systems. However, at times of oxidative stress, these systems are overwhelmed by ROS, leading to cell damage.