Oxygen toxicity Mechanism

Mechanism

While all the reaction mechanisms of these species within the body are not yet fully understood,[55] one of the most reactive products of oxidative stress is the hydroxyl radical (·OH), which can initiate a damaging chain reaction of lipid peroxidation in the unsaturated lipids within cell membranes.[56] High concentrations of oxygen also increase the formation of other free radicals, such as nitric oxide, peroxynitrite, and trioxidane, which harm DNA and other biomolecules.[19][57] Although the body has many antioxidant systems such as glutathione that guard against oxidative stress, these systems are eventually overwhelmed at very high concentrations of free oxygen, and the rate of cell damage exceeds the capacity of the systems that prevent or repair it.[58][59][60] Cell damage and cell death then result.[61]
http://en.wikipedia.org/wiki/Oxygen_toxicity#Mechanism