Which of the following is the most common inhaled poison?

• A. Chlorine
• B. Cyanide
• C. Propane
• D. Carbon monoxide

Answer: (D)

Carbon monoxide is the most common inhaled poison because it is colorless, odorless, and produced in many everyday situations (furnaces, space heaters, car exhaust in attached garages, fires). That stealthy presence allows exposure to go unnoticed, making it a frequent cause of poisoning.

Once inhaled, carbon monoxide binds to hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin. This severely impairs the blood’s ability to carry oxygen and also shifts the oxygen dissociation curve, making it harder for the remaining healthy hemoglobin to release oxygen to tissues. The result is tissue hypoxia, especially affecting the brain and heart, which explains why early symptoms are often headaches, dizziness, and confusion and can progress to fainting or coma.

In practice, this is why you should think of carbon monoxide poisoning in anyone with headache or malaise who has been in an enclosed space with potential combustion sources, and you should administer high-flow oxygen as soon as possible. In severe cases, hyperbaric oxygen may be considered.

Chlorine is a strong irritant gas, cyanide poisoning is more linked to certain industrial or fire-related exposures and acts by inhibiting cellular respiration, and propane concerns are more about asphyxiation or flammability hazards than the classic toxic effects of CO. The unique combination of stealthy exposure and widespread occurrence makes carbon monoxide the most common inhaled poison.