Oxygen toxicity or oxygen toxicity syndrome (also known as the "Paul Bert effect") is severe hyperoxia caused by breathing oxygen at elevated partial pressures. The high concentration of oxygen damages cells. The precise mechanism(s) of the damage are not known, but oxygen gas is a natural free radical with a propensity to react with metals to form superoxide which may attack double bonds in many organic systems, including the unsaturated fatty acid residues in cells.