Brief: Why Is Carbon Monoxide Poisonous?

Toxic chemicals or substances have specific properties and/or traits that make them harmful to the human body or other species. In the case of carbon monoxide, its poisonous attribute has something to do with how it reacts to hemoglobin—a protein in the red blood cells of almost all vertebrates responsible for transporting oxygen to tissues and cells.

What Makes Carbon Monoxide Poisonous?

Remember that hemoglobin functions as a carrier of oxygen. The iron atoms in this protein bind to the oxygen atoms whenever the blood passes through the lungs. As the blood flows into different areas in the body, the iron atoms in the hemoglobin unbind the oxygen. Carbon monoxide also binds to the hemoglobin via its iron atoms.

On the other hand, when introduced in the body through inhalation, carbon monoxide or CO competes with oxygen and binds to the hemoglobin to form carboxyhemoglobin or HbCO. Take note that CO has about 250 times greater binding affinity to hemoglobin than oxygen. Even at small amounts, it reduces the ability of hemoglobin to carry and transport oxygen to tissues and cells.

It is also important to note that the bond between hemoglobin and oxygen is relatively loose. However, because CO binds strongly to hemoglobin, it is difficult to release, thereby making it harmful even at low concentration. The gradual introduction of more CO in the bloodstream over a long period would render the hemoglobin unable to transport oxygen.

Essentially, breathing in CO takes up all the binding sites in the hemoglobin, thus reducing its capacity to transport oxygen. Less oxygen in the blood means restricted oxygen supply in tissues and cells across the body.

Note that aside from oxygen deprivation, carbon monoxide poisoning also involves other mechanisms. This compound also binds to an iron-binding and oxygen-binding protein called myoglobin found in the muscle tissue of vertebrates. It also binds with the cytochrome oxidase in the mitochondria, thus interfering with aerobic metabolism and triggering anaerobic metabolism.

Breathing in significant quantities of CO also damages the brain. Aside from depriving the brain of oxygen, it causes the release of nitric oxide from endothelial cells and platelets, and the formation of oxygen free radicals. Both result in lipid peroxidation that degrades the white matter in the brain that in turn, leads to brain edema and necrosis.

Why Are People Susceptible to CO Poisoning?

What makes carbon monoxide dangerous to humans is the fact that it is a colorless, odorless, and tasteless gas emitted as fumes from burning hydrocarbons or fossil fuels used in automotive vehicles, fireplaces and furnaces, gas ranges stoves, gas-fueled grills, small engines, small gas-fired generators, and lanterns, among others.

The use of these fuel-consuming machines or appliances can result in CO build up indoors, thus exposing humans placed in an enclosed space to this harmful compound without them knowing until symptoms of CO poisoning have emerged. It is for this reason that CO has also been labeled as a silent killer.

Constant exposure to carbon monoxide at only 20 to 30 parts per million for several hours can be harmful. The most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. An exposure at 2000 part per million within one hour will leave an adult human unconscious.

Several studies have also documented common incidents of CO poisoning. One notable example involves people who stay and sleep in their cars while in enclosed parking areas. Another example is during winter when individuals would use portable generators during power outages or would leave their stoves open to warm their homes.

The smoke from burning tobacco also contains CO. Tobacco smoking essentially leads to inhalation of this gas. Although the amounts of CO in the fumes are generally negligible, several studies have revealed that chain smoking or a short time between lighting cigarettes can result in substantial accumulation of carbon monoxide in the bloodstream, thus causing CO poisoning.

Undeniably, the health risk factors of carbon monoxide poisoning are prevalent in the typical human environment. Nevertheless, there are practical ways to prevent carbon monoxide poisoning. One of such is to use battery-operated CO detectors in homes or any interior spaces. Another is to use proper ventilation in areas that use gas stoves and ovens, fuel-fired portable generators and furnaces, as well as in interior parking spaces.


  • Rose, J. J., Wang, L., Xu, Q., McTiernan, C. F., Shiva, S., Tejero, J., and Gladwin, M. T. 2017. “Carbon Monoxide Poisoning: Pathogenesis, Management, and Future Directions of Therapy.” American Journal of Respiratory and Critical Care Medicine. 195(5). DOI: 10.1164/rccm.201606-1275CI
  • Sen, S., Peltz, C., Beard, J., and Zeno, B. 2010. “Recurrent Carbon Monoxide Poisoning from Cigarette Smoking.” The American Journal of the Medical Sciences. 340(5): 427-428. DOI: 10.1097/MAJ.0b013e3181ef712d
  • Wu, P. E and Juurlink, D. N. 2014. “Carbon Monoxide Poisoning.” Canadian Medical Association Journal. 186(8): 611. DOI: 10.1503/cmaj.130972
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