Urban Air Pollution
Carbon Monoxide readily binds to hemoglobin. While not produced physiologically (except in minute quantities), Carbon Monoxide nonetheless is regretably common in the environment. Carbon Monoxide orms when carbon-containing molecules are burned with insufficientoxygen present for the molecules to be oxidized completely to CO2. Smoldering fires such as those of cigarettes and improperly functioningwood stoves are two sources. Most, however, comes from internal combustion engines. In cities with significant automotive pollution, Carbon Monoxide typically is one of the most important pollutants.
Because the affinity of Carbon Monoxide for hemoglobin is over 200 times greater than oxygen, Carbon Monoxide binds to hemoglobin evenwhen Carbon Monoxide is at very low partial pressures. With Carbon Monoxide attached, hemoglobin can no longer transport oxygen, rendering the hemoglobin useless until the Carbon Monoxide dissociates.The effects of Carbon Monoxide poisoning are thus identical to those of low levels of hemoglobin.
In a typical large city it is common for a nonsmoker to have roughly one percent of his or her hemoglobin inactivated by Carbon Monoxide. For smokers the average is about five percent or more. The percentages canincrease during heavy pollution, such as during a traffic jam. The people most at risk are those with heart or lung disorders in which the ability of the heart and lungs to supply oxygen to the tissues ismarginal.
Note that less oxygen is present in arterial blood in both Carbon Monoxide poisoning and anemia, because there is less functional hemoglobin. Nonetheless, the partial pressure of oxygen in the arterial blood is not directly affected; blood in the lungs stills equilibrates to the alveolar partial pressure of oxygen. Again, theamount of oxygen and the partial pressure of oxygen are related, but separate, factors.
Pure oxygen is helpful in treating Carbon Monoxide poisoning because itcompetes with the Carbon Monoxide. While the half-life of Carbon Monoxide bound to hemoglobin is 4 to 6 hours when breathing room air,it is only 40 to 80 minutes when breathing 100 percent oxygen. With hyperbaric oxygen, it is reduced to 15 to 30 minutes. The latter requires a special chamber and is used in certain more severe cases.