It takes four and a half to five hours for the amount of CO in the body to naturally reduce by half while breathing regular air, about an hour and a half with treatment with 100% oxygen, and 20 minutes with 3 atmospheres of pressure in a hyperbaric chamber.
Oxygen treatment should be continued until blood levels of carboxyhemoglobin (COHb), which is formed when CO bonds with red blood cells, drops to less than 3% and symptoms have abated.