Carbon monoxide causes asphyxiation by attaching to hemoglobin (Hb) molecules in place of oxygen molecules. The bond between the CO and Hb is much stronger than between O2 and Hb, so once COHb is formed, that Hb molecule is forever lost to carrying oxygen. Most carboxyhemoglobin is removed from the blood (after eliminating the CO exposure) when the body filters out the old red blood cells (RBCs) and replaces them with new RBCs containing fresh, uncontaminated hemoglobin.

CO exposure effects are cumulative, since it takes time for the COHb blood concentration to build up and the COHb stays around in the blood for a while. So, it might take a few thousand ppm of CO to kill you within a few minutes, or 600 ppm over 10 hours to achieve the same effect. The COHb levels in the blood become steady over time because of the continual removal of COHb-poisoned blood cells and the formation of new COHb molecules, if the concentration of CO in the air is constant (essentially, a steady-state box model has been set up, with equal source and sink rates).