Broadly two sets of clinical settings exist one where the patient is exposed to very high concentrations of oxygen for brief periods of time (eg. The re-emergence of hyperbaric oxygen therapy (HBOT), coupled with more aggressive oxygen therapy for pulmonary insufficiency, has caused large number of patients to be exposed to high oxygen pressures. This value can be increased by either breathing 100% oxygen, thus theoretically increasing the PO 2 of inspired air to 760 mm Hg, or by increasing the pressure of the breathing mixture by which means there is, theoretically, no upper limit. The partial pressure of oxygen, in inspired air, at sea level is about 160 mm Hg. This marked the discovery of pulmonary toxicity of oxygen, the ‘Smith Effect’. In 1899, J Lorain Smith, while trying to reproduce ‘Bert effect’, noticed fatal pneumonia in rats after 4 days of exposure to 73% oxygen at 1 ATA. The CNS toxic effects of oxygen are hence called ‘Bert effect’. The first important contribution in field of oxygen toxicity was by Paul Bert who, in 1878, demonstrated convulsions in larks exposed to 15–20 ATA (atmosphere absolute) air. Priestley, who discovered oxygen, was himself amongst the first to suggest that there may be adverse affects of this ‘pure air’, when, in 1775, he observed a candle burn out faster in oxygen than in air and wondered if ‘the animal powers be too soon exhausted in this pure kind of air’. Those forms survived that had better protection against increasing oxidative stress of the changing atmosphere. The gradual increment in oxygen concentration in atmosphere allowed further evolution of animal species depending on aerobic metabolism. It is believed that its concentration in atmosphere was infinitesimal until 2.5 billion years ago when the first photosynthetic organisms made their appearance. Oxygen has been present in atmosphere for 5 billion years.
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