monoxide in the environment (NRC 1994). Another common environmental source is cigarette smoke. Indoor sources include gas stoves, furnaces, and fires. Carbon monoxide is produced inside the body by hemoglobin metabolism (NRC 1994).

Carbon monoxide is absorbed through the lungs at an approximate rate of 25.8±8 mL/min-mm Hg (Jones et al. 1982). It is then absorbed into the bloodstream from the lungs. There is competitive binding between carbon monoxide and oxygen to hemoglobin in the red blood cell, forming carboxyhemoglobin (COHb) and oxyhemoglobin, respectively (WHO 1999). The rate of formation of COHb can be predicted by a model described by Coburn et al. (1965). The affinity of hemoglobin for carbon monoxide is approximately 200–250 times its affinity for oxygen (NRC 1985). The amount of COHb depends mainly on the concentration and duration of carbon monoxide exposure, and the barometric pressure. To a lesser extent, it is also dependent on minute volume, blood volume in lung capillaries, body temperature, rate of endogenous carbon monoxide production, average partial pressure of oxygen in the lung capillaries, and the exact ratio of the affinity of blood for carbon monoxide and oxygen. Figure 3–1 shows the relationship of carbon monoxide concentration in ambient air and the COHb concentration formed at various exposure times in human volunteers