Oxygen is one of the essential ingredients of life. So you may tend to think that breathing pure oxygen would be safe. But you would be wrong. It has been found out that breathing pure oxygen for long periods of time could be harmful. The culprit is believed to be a highly reactive form of oxygen (not the oxygen molecule itself) which destroys proteins and certain cells of our body. But pure oxygen under low pressure has less effect on the body and is sometimes used to treat some patients.
