The toxic character of carbon monoxide has been well known for many centuries. At low concentrations, carbon monoxide may cause nausea, vomiting, restlessness, and euphoria. As exposure increases, a person may lose consciousness and go into convulsions. Death is a common final result. The U.S. Occupational Safety and Health Administration has established a limit of 35 ppm (parts per million) of carbon monoxide in workplaces where a person may be continually exposed to the gas.
The earliest explanation for the toxic effects of carbon monoxide was offered by the French physiologist Claude Bernard in the late 1850s. Bernard pointed out that carbon monoxide has a strong tendency to replace oxygen in the respiratory system. Someone exposed to high concentrations of carbon monoxide may actually begin to suffocate as his or her body is deprived of oxygen.
Today we have a fairly sophisticated understanding of the mechanism by which carbon monoxide poisoning occurs. Normally, oxygen is transported from the lungs to cells in red blood cells. This process occurs when oxygen atoms bond to an iron atom at the center of a complex protein molecule known as oxyhemoglobin. Oxyhemoglobin is a fairly unstable molecule that decomposes in the intercellular spaces to release free oxygen and hemoglobin. The oxygen is then available to carry out metabolic reactions in cells, reactions from which the body obtains energy.
If carbon monoxide is present in the lungs, this sequence is disrupted. Carbon monoxide bonds with iron in hemoglobin to form carbonmonoxyhemoglobin, a complex somewhat similar to oxyhemoglobin. Carbonmonoxyhemoglobin is, however, a more stable compound than is oxyhemoglobin. When it reaches cells, it has much less tendency to break down, but continues to circulate in the bloodstream in its bound form. As a result, cells are unable to obtain the oxygen they need for metabolism and energy production dramatically decreases. The clinical symptoms of carbon monoxide poisoning described above are manifestations of these changes.
Carbon monoxide poisoning—at least at moderate levels—is common in everyday life. Poorly vented charcoal fires, improperly installed gas appliances, and the exhaust from internal combustion vehicles are among the most common sources of the gas. In fact, levels of carbon monoxide in the air can become dangerously high in busy urban areas where automotive transportation is extensive. Cigarette smokers may also be exposed to dangerous levels of the gas. Studies have shown that the one to two pack-a-day smoker may have up to 7% of the hemoglobin in her or his body tied up in the form of carbonmonoxyhemoglobin.