Why is carbon monoxide (CO) so dangerous to humans? The answer is based on the chemistry of this molecule. The CO molecule contains a triple covalent bond. The triple bond makes the molecule very stable. One unusual feature of the CO molecule is the presence of a lone pair of electrons attached to the C atom. This arrangement is uncommon because carbon does not usually have any lone pairs within stable molecular structures.

Carbon monoxide is produced when fuels burn in a limited supply of oxygen. If this reaction occurs inside a closed indoor environment, then the levels of carbon monoxide begin to increase. Carbon monoxide is a particularly dangerous pollutant because it has no odor and no color, so our senses cannot detect it. Many people who are poisoned by carbon monoxide breathe the gas while they are asleep. Exposure to low levels of CO can produce symptoms such as headaches, dizziness, nausea, and fatigue. High levels of CO can induce coma and death.

Carbon monoxide achieves its toxic effect by interfering with the transport of oxygen gas in our bloodstream. Hemoglobin proteins in red blood cells serve as the transporters of O2 molecules. Hemoglobin contains four ring-shaped heme groups, each with an iron ion (Fe2+) in its center. An O2 molecule binds to each of the Fe2+ ions; therefore, one hemoglobin transports four O2 molecules. After carrying O2 molecules through the blood, hemoglobin releases them to be used for energy-generating reactions in the body’s cells.

When we inhale carbon monoxide, the gas enters our lungs and diffuses into our bloodstream. The CO molecules enter our red blood cells and compete with oxygen to bind to the hemoglobin proteins. A CO molecule binds very tightly to the Fe2+ ion in a heme group, with a strength of attraction that is approximately 250 times greater than that of oxygen. When a CO molecule attaches to a heme group, it prevents an O2 molecule from binding there. Even if CO binds to only one or two of the four heme groups, it diminishes the ability of hemoglobin to release the O2 molecules that are bound to the other heme sites.

In effect, carbon monoxide reduces the capacity of hemoglobin to transport oxygen around the bloodstream and deliver it to cells. As a result, the body’s cells begin to die because they do not receive the oxygen they need to generate energy-sustaining reactions. The effect of CO poisoning is particularly pronounced in the brain, which requires a large supply of oxygen.

Because CO is undetectable by our senses, it is now standard practice to install a carbon monoxide detector in every house and apartment. This device alerts residents to the presence of CO molecules in the air. In many cases of reported fatalities from CO poisoning, a CO detector either was not present, or it was not functioning.