Hemoglobin binds with carbon monoxide 210 times more readily than with oxygen. Because of this higher affinity of hemoglobin for carbon monoxide than for oxygen, carbon monoxide is a highly successful competitor that will displace oxygen even at minuscule partial pressures. The reaction HbO + CO → HbCO + O almost irreversibly displaces the oxygen molecules forming carboxyhemoglobin; the binding of the carbon monoxide to the iron centre of hemoglobin is much stronger than that of oxygen, and the binding site remains blocked for the remainder of the life cycle of that affected red blood cell.  With an increased level of carbon monoxide, a person can suffer from severe tissue hypoxia while maintaining a normal pO because carboxyhemoglobin does not carry oxygen to the tissues.