Carbon Monoxide is actually a very dangerous toxic substance. It is an especially common one you have maybe heard about. Carbon monoxide is dangerous in that it is a byproduct of combustion reactions and can be produced via woodfired stoves, gas heaters, smoking etc... When abnormal amounts of carbon monoxide are present they may begin to cause a process called carbon monoxide poisoning. Its not that carbon monoxide is especially dangerous towards affecting cells directly, but rather it interferes with the bodies ability to allow for cellular respiration.
Carbon monoxide, oxygen, and hydrogen cyanide are similar gases in that they have high affinities towards attack attaching to the hemoglobin in red blood cells. These hemoglobin are oxygen carrying proteins. However carbon monoxide has a very high affinity compared to oxygen which means they take more hemoglobin then oxygen. This eventually causes asphyxiation due to the body being unable to receive oxygen, which potentially leads to death.
While smoking doesn't exactly cause the same lethal conditions as burning wood in an enclosed room, it still does have carbon monoxide. It can mess with the amount of oxygen in the blood and greatly contributes towards poorer oxygen saturation in the blood; these make patients fatigued, discolors, and without energy, as well as contributing towards shortness of breath.
