In fact, even non-recreational divers who dive with gas mixtures other than compressed air need to be wary of the possibility of oxygen toxicity. The confusing part for some is that it is safe to breath pure oxygen while at normal land pressures, as many do while hospitalized or being treated for various pulmonary problems. The difference is that at normal pressures, the hemoglobin system is able to buffer the amount of oxygen going to the tissues. At increased pressure, pure oxygen would overpower what hemoglobin is able to do, and oxygen levels would increase throughout the body. The danger of increased oxygen levels is not completely understood, but it is believed to be due to free radicals.
Free radicals are molecules that have lost one electron from a pair, and are rendered very reactive.
Because free radicals are very reactive, they are constantly looking for another molecule from which they can steal an electron. Once this occurs, that molecule is now a free radical and the process goes on. Free radicals, particularly oxygen, can damage proteins and fats by taking an electron which renders the substance incapable of functioning properly. Our cells are made up of many proteins and fats, so damage to these can cause serious cell damage. The interesting thing is that oxygen free radicals occur all the time.