By which route would carbon monoxide most likely enter the body?

• A. Inhalation
• B. Absorption
• C. Ingestion
• D. Injection

Answer: (A)

Carbon monoxide is a colorless, odorless gas produced by incomplete combustion, so it primarily enters the body by inhalation—breathing it in from the air. Once in the lungs, CO binds to hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin and preventing oxygen from being carried to tissues. The other routes—absorption through skin, ingestion, or injection—don’t provide a meaningful way for CO to enter the bloodstream in typical exposure, so inhalation is the correct route. In real-world EMS scenarios, suspect CO poisoning in environments with combustion sources and treat with high-flow oxygen to rapidly displace CO from hemoglobin.