Carbon dioxide CO2 and carbon monoxide CO are sometimes confused. They are quite different in their effects on humans.
Carbon dioxide CO2 is present in the atmosphere today at 385 parts per million (by volume) or 0.039 percent. 50 years ago it was 280 parts per million. The increase is caused by people burning things, especially fuels - coal, oil, and natural gas, especially coal. Burning wood also puts CO2 in the atmosphere.
Almost all life is dependent on plants getting CO2 from the atmosphere. That's where the carbon in wood and leaves comes from.
Human and animals breathe in air, take some of the oxygen from it (air is 20 percent oxygen), and breathe out a mixture that includes CO2 obtained by combining part of ones food with oxygen.
Air can be about 5 percent CO2, from a stuffy room, before people become uncomfortable. According to the Wikipedia article, amounts above 800 ppm are considered unhealthy, amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm are considered dangerous to animal life.
Carbon monoxide CO, unlike CO2, is a bad poison. CO is produced by incomplete combustion, i.e. when there isn't enough oxygen to make CO2, you get CO.
Carbon monoxide binds very strongly to the iron in the hemoglobin in the blood. Once carbon monoxide attaches, it is very difficult to release. So if you breath in carbon monoxide, it sticks to your hemoglobin and takes up all of the oxygen binding sites Your blood loses all of its ability to transport oxygen, and you suffocate.
Because carbon monoxide binds to hemoglobin so strongly, you can be poisoned by carbon monoxide even at very low concentrations if you are exposed for a long period of time. Concentrations as low as 20 or 30 parts per million (PPM) can be harmful if you are exposed for several hours. Exposure at 2,000 PPM for one hour will cause unconsciousness.
Many common devices produce carbon monoxide, including cars, gas appliances, wood stoves and cigarettes, especially if used in a confined space.
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