Hyperoxia occurs when cells, tissues and organs are exposed to an excess supply of oxygen (O2) or  higher than normal partial pressure of oxygen.In medicine, it refers to excess oxygen in the lungs or other body tissues, which can be caused by breathing air or oxygen at pressures greater than normal atmospheric pressure. This kind of hyperoxia can lead to oxygen toxicity, caused from the harmful effects of breathing molecular oxygen at elevated partial pressures. Hyperoxia is the opposite of hypoxia; hyperoxia refers to a state in which oxygen supply is excessive, and hypoxia refers to a state in which oxygen supply is insufficient. In the environment, it refers to excess oxygen in a body of water or other habitat. Signs and symptoms
Associated with hyperoxia is an increased level of reactive oxygen species (ROS), which are chemically reactive molecules containing oxygen. These oxygen containing molecules can damage lipids, proteins, and nucleic acids, and react with surrounding biological tissues. The human body has naturally occurring antioxidants to combat reactive molecules, but the protective antioxidant defenses can become depleted by abundant reactive oxygen species, resulting in oxidation of the tissues and organs.The symptoms produced from breathing high concentrations of oxygen for extended periods have been studied in a variety of animals, such as frogs, turtles, pigeons, mice, rats, guinea pigs, cats, dogs and monkeys. The majority of these studies reported the occurrence of irritation, congestion and edema of the lungs, and even death following prolonged exposures.
If supplementation of oxygen is continued, the individual will notice tightness in the chest, difficulty breathing, shortness of breath, and if exposure is continued, fatality due to lack of oxygen. Cause
Oxygen supplied at greater than atmospheric pressure has been known to damage plants, animals, and aerobic bacteria such as Escherichia coli. The damaging effects vary depending on the specimen used, its age, physiological state, and diet.The supplementation of oxygen has been a common procedure of prehospital treatment for many years. Guidelines include cautions about chronic obstructive pulmonary disease (COPD). These guidelines stress the use of 28% oxygen masks and caution the dangers of hyperoxia. Long-term use of supplemental oxygen improves survival in patients with COPD, but can lead to lung injury.An additional cause of hyperoxia is related to underwater diving with breathing apparatus. Underwater divers breath a mixture of gasses which must include oxygen, and the partial pressure of any given gas mixture will increase with depth. A mixture known as nitrox is used to reduce the risk of decompression sickness by substituting oxygen for part of the nitrogen content. Breathing nitrox can lead to hyperoxia due to the high partial pressure of oxygen if used too deep or for too long. Protocols for the safe use of raised oxygen partial pressure in diving are well established and used routinely by recreational scuba divers, military combat divers and professional saturation divers alike.