Carbon monoxide (CO) is very dangerous because it binds with the haemoglobin more efficiently than oxygen does. It "hogs the seats" so that oxygen can't get a ride. And those CO molecules keep riding around, never giving their seats up to the oxygen. This means there is no way to get oxygen to the brain, heart, or other cells and those cells start to die. And finally, it leads to suffocation and even death of a person.
