Oxygen is essential for normal cellular respiration in aerobic organisms, and prolonged deficit of oxygen always has detrimental consequences. However, all aerobic life forms are faced with the threat of oxidation from molecular oxygen. Reactive oxygen intermediates have been suggested to play an essential role in the pathogenesis of ischaemia-reperfusion injury as well as in ischaemic preconditioning. At high fractions and prolonged exposure, hyperoxia may lead to excessive generation of reactive oxygen intermediates overwhelming cellular antioxidant defence and inducing oxidative damage. Contrary to these detrimental effects, reactive oxygen intermediates have been suggested to play a physiological role acting as signal transduction. Theoretically, increased oxygen fractions in inspired air may initiate a protective effect.