All respiring organisms need oxygen for the generation of energy, and thus their lives are supported by molecular oxygen (O2). If the concentration of oxygen is greater than that normally occurring in the atmosphere, the living organisms are exposed to oxygen toxicity. Molecular oxygen is not extremely reactive, although it contains unpaired electrons, i.e. it possesses a radical nature. The electronic structure of molecular oxygen means that it can form more reactive species by univalent reduction. Such reactive oxygen species arising play an important physiological function and can also cause damage to cell constituents.