 Carbon monoxide has a higher diffusion coefficient compared to oxygen and the only enzyme in the human body that produces carbon monoxide is heme oxygenase which is located in all cells and breaks down heme . Under normal conditions carbon monoxide levels in the plasma are approximately 0 mmHg because it has a higher diffusion coefficient and the body easily gets rid of any CO made . When CO is not ventilated it binds to hemoglobin , which is the principal oxygen @-@ carrying compound in blood ; this produces a compound known as carboxyhemoglobin . The traditional belief is that carbon monoxide toxicity arises from the formation of carboxyhemoglobin , which decreases the oxygen @-@ carrying capacity of the blood and inhibits the transport , delivery , and utilization of oxygen by the body . The affinity between hemoglobin and carbon monoxide is approximately 230 times stronger than the affinity between hemoglobin and oxygen so hemoglobin binds to carbon monoxide in preference to oxygen . 
