The probable toxic action is CO is on the cellular respiration, in which CO competes with O2 for cytochrome a3. Our studies indicate that a relatively high concentration of COHb (50%) does not interfere with the O2-carrying capacity of the blood. The dissolved CO in plasma, which is necessary for CO to enter the tissue, probably occurs when the exchange takes place between alveolar air and the blood in the lungs. This would explain why the central nervous system could be impaired when COHb levels are below 10% and death could occur at 35%.