The toxicology of molecular oxygen

Molecular oxygen, itself not very reactive, can be converted by photosensitization to electronically excited singlet states, and by partial reduction to the superoxide and hydroxyl free radicals and to hydrogen peroxide. The very considerable toxicity of oxygen, which is due primarily to the properties of these derivatives, is ordinarily overlooked because aerobes have evolved an elaborate system of defenses which is reasonably adequate under ambient conditions. This toxicity becomes all too apparent when these defenses are overwhelmed at elevated pO2 or through the action of compounds which increase the conversion of oxygen to its more reactive derivatives. We will here consider the threat posed by oxygen and the defenses which make aerobic life possible.