The cellular elements of the blood—red blood cells, white blood cells, and platelets—are suspended in plasma. Bone marrow suppression is characterized by a deficiency of all or some cellular elements in peripheral blood. Hemolytic anemia is due to an increased rate of red blood cell destruction. Polycythemia is the presence of an abnormally high red blood cell count, an increased hemoglobin concentration, or increased hematocrit. Hemoglobin is the oxygen-carrying protein of the red blood cells. Carbon monoxide is perhaps the best-known example of a chemical agent that decreases the oxygen transport of the blood and produces hypoxia, a condition in which there is an inadequate supply of oxygen to the tissues. Methemoglobin is restored endogenously to functional blood pigment by the intracellular reductase system. One of the most important concerns of occupational toxicology has been the relationship of atmospheric carbon monoxide level, carboxyhemoglobin level in blood, and physiologic effects.