Hyperbaric oxygen therapy is when a patient is given 100 percent oxygen gas at two to three atmospheres pressure to breathe for varying lengths of time. It is used to treat carbon monoxide poisoning, decompression sickness, severe traumatic injury, infections that could lead to gas gangrene, and other conditions. Normal oxygen concentration is 0.3 milliliters oxygen per 100 milliliters of blood, but breathing 100 percent oxygen at a pressure of three atmospheres raises the plasma concentration to about 6 milliliters of oxygen per 100 milliliters of blood.