Too much oxygen can poison the brain, lungs and eyes
Oxygen keeps us alive, yet breathed at high enough pressure it becomes a poison. Divers deep underwater risk sudden seizures, while premature babies on long oxygen treatment risk damage to their eyes. The two effects once carried the names of Paul Bert and Lorrain Smith, the 19th-century researchers who described them.
The problem is hyperoxia, an excess of oxygen in the body's tissues caused by breathing it at higher partial pressure than normal. Sea-level air supplies oxygen at about 0.21 bar, and toxicity does not start below 0.3 bar, which is why simply over-breathing ordinary air cannot trigger it. The danger arises in three main settings: underwater diving, hyperbaric oxygen therapy, and supplemental oxygen given in intensive care or for chronic illness, particularly to premature infants. The risks in each are quite different.
The brain form strikes fast. Minutes to a few hours at more than about 1.6 bar, roughly eight times normal, can bring tunnel vision, ringing ears, nausea, facial twitching, irritability and dizziness, followed by a seizure with a rigid phase, then jerking convulsions, then unconsciousness. Because ordinary atmospheric pressure is only about 1 bar, this can only happen under raised pressure, which is why it concerns divers and hyperbaric patients. Carbon dioxide build-up, stress, fatigue and cold make it worse, and all are commoner underwater.
The lungs, exposed to more oxygen than any other organ, suffer from slower damage whenever the partial pressure tops 0.5 bar, equivalent to half the air being oxygen at normal pressure. After a symptom-free spell of 4 to 22 hours at over 95 percent oxygen, inflammation begins in the upper airways as a faint tickle and progresses to coughing and chest pain. Intermittent exposure lets the lungs recover and delays the damage. In premature babies, oxygen can harm the developing retina, a condition graded in four stages up to detachment.
Careful protocols in diving, hyperbaric medicine, newborn care and spaceflight have made oxygen seizures increasingly rare, leaving lung and eye damage largely a problem of caring for premature infants. The US Food and Drug Administration has warned people with heart or lung disease away from recreational oxygen bars.
Source: Oxygen toxicity
