It takes very little CO in air to be deadly. Just one molecule of CO for every 1999 air molecules (oxygen, nitrogen, water, argon, etc.) can make a lethal combination (ie. 500 ppm). This is because of the blood's enormous avidity for CO. It produces asphyxia.
Hemoglobin (Hb) in the blood picks up oxygen molecules in the lungs and drops them off to the cells. The CO molecule can assume a shape so similar to oxygen that the hemoglobin can carry it as well (COHb). To make things worse, hemoglobin has an affinity for CO that is about 250 times stronger than that for oxygen. What this means is that not only would hemoglobin rather carry CO, it won't let go of it!
The binding of CO to Hb also increases Hb's affinity for oxygen, so it won't let go of any oxygen it may be carrying either. This makes the situation far worse than a comparable non-CO type of hypoxia, or a serious case of anemia.
Serious CO poisoning occurs when so much of the blood's hemoglobin is carrying CO that the cells don't get enough oxygen to survive.
There also is evidence that the hypoxic effects of CO may involve a histotoxic component. CO binds to myoglobin and cytochromes inside cells which may critically limit cell metabolism by stopping the processes inside the cell that produce energy for cellular processes. Current research is examining this avenue of CO's action.
Most frequently CO poisoning occurs in the home as a result of faulty heating or cooking appliances. Automobile exhaust can cause CO poisoning in enclosed spaces such as garages, tunnels and parking garages. Smoking cigarettes raises the amount of CO in a smoker's blood stream, and may make him or her even more susceptible to environmental CO. Firemen are also at risk for CO poisoning from smoke inhalation.
Back to COHQ Main Index