The high concentration of oxygen damages cells. The precise mechanism(s) of the damage caused by these reactive oxygen species are not known, but oxygen gas has a propensity to react with certain metals to form superoxide which may attack double bonds in many organic systems, including the unsaturated fatty acid residues in cells. High concentrations of oxygen are known to increase the formation of free-radicals which harm DNA and other structures (see nitric oxide, peroxynitrite, and trioxidane). Normally, the body has many defense systems against such damage (see glutathione, catalase, and superoxide dismutase) but at higher concentrations of free oxygen, these systems are eventually overwhelmed with time, and the rate of damage to cell membranes exceeds the capacity of systems which control or repair it. Cell damage and cell death then results.

Hyperoxia:

Hyperoxia is excess oxygen in body tissues or higher than normal partial pressure of oxygen. Hyperoxia is caused by breathing gas at pressures greater than normal atmospheric pressure or by breathing oxygen-rich gases at normal atmospheric pressure for a prolonged period of time.

Types of oxygen toxicity:

In humans, there are several types of oxygen toxicity:

• Central nervous system (CNS) oxygen toxicity
• Pulmonary oxygen toxicity
• Retinopathic oxygen toxicity