Carbon monoxide poisoning is the most common type of accidental poisoning in the United States, accounting for thousands of emergency department visits and some 800 deaths annually. Carbon monoxide, an insidious byproduct of incomplete hydrocarbon combustion, is generated in toxic amounts by internal-combustion engines, fossil-fuel furnaces, and fires. Carbon monoxide emissions from modern automobiles, though controlled by regulatory standards, are still highly toxic in poorly ventilated spaces. A stable gas at physiologic temperatures, carbon monoxide diffuses rapidly across the alveolar capillary membrane and binds tightly to iron centers in hemoglobin and other hemoproteins. Claude Bernard first proposed in 1865 that . . .