The human body depends on oxygen for the burning of fuel (food) to provide the energy that allows cells to live and function. Oxygen makes up approximately 21% of the atmosphere, and enters the lungs during breathing. In the lungs it combines with a blood component called hemoglobin.
When saturated with oxygen, it is called oxyhemoglobin.
After being carried by the bloodstream to the cells of the body, oxyhemoglobin releases oxygen to the body tissues. Carbon monoxide is dangerous because it bonds much more tightly to the hemoglobin than does oxygen. Once hemoglobin combines with carbon monoxide to form carboxyhemoglobin, its ability to combine with oxygen is competely lost.
As more carboxyhemoglobin is formed, the amount of oxygen carried to the cells and organs in the body decreases. Carbon monoxide starves the blood of oxygen, literally causing the body to suffocate from the inside out. When the carboxyhemoglobin concentration reaches a certain level, people get nauseous, become unconscious, and ultimately die. How quickly symptoms appear depends upon the concentration, or parts per million (ppm) of carbon monoxide in the air and the duration of exposure. A person's size, age and general health are also factors in how quickly effects of the gas will become evident.
Since oxygen and carbon monoxide are approximately the same density, they mix equally well in air. Therefore most alarms measuring carbon monoxide can be placed anywhere in a room.