Oxygen, while essential to life, is nevertheless toxic at elevated levels. Were it not for our bodies’ chemical defense mechanisms, oxygen would cause undesirable chemical reactions resulting in cell death. At atmospheric pressure, our normal defense mechanisms offset the toxic effects of oxygen. However, at depth the increased partial pressure of oxygen in the gases divers breathe can overwhelm these defense mechanisms and result in oxygen toxicity. While oxygen can be toxic to all body tissues, its most noticeable and immediate effects take place in the lungs and central nervous system