Which of the following is an effect of carbon monoxide on humans?

Carbon monoxide (CO) is a colourless, odourless gas produced by the incomplete combustion of carbon-containing fuels. Its primary danger to humans lies in its effect on the blood's ability to transport oxygen.

Haemoglobin in red blood cells normally binds with oxygen to form oxyhaemoglobin, which is then transported to body tissues. Carbon monoxide has an affinity for haemoglobin that is approximately 200-250 times greater than that of oxygen. When inhaled, CO binds tightly to haemoglobin to form carboxyhaemoglobin, a stable compound that does not release CO easily. This effectively blocks haemoglobin from binding with oxygen, thereby reducing the oxygen-carrying capacity of the blood.

Carbon monoxide does not cause respiratory tract infections (those are caused by pathogens such as bacteria and viruses), does not prevent blood clotting (that involves platelets and clotting factors), and is not known to cause gene mutations (that is associated with mutagens such as certain chemicals, UV radiation, and ionising radiation).

For examinations, remember that carbon monoxide is dangerous specifically because it competes with oxygen for binding sites on haemoglobin, and its much higher affinity means even small amounts can significantly impair oxygen delivery to tissues.