What is carbon monoxide (CO) and why is it dangerous to humans?

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Carbon monoxide is a colorless, odorless, and tasteless gas.  It is formed through the incomplete combustion of carbon based fuels.  Complete combustion produces only CO2 as the oxidized form of carbon, but insufficient levels of O2 or low burning temperatures produce CO as well as CO2.  Car exhaust and propane or natural gas appliances are common producers of CO.  CO is dangerous to humans not really because it is poisonous but because it causes oxygen suffocation.  Oxygen binds to iron molecules in hemoglobin to be distributed through the bloodstream.  CO can also bind to the iron in hemoglobin and actually has a much greater affinity for it that oxygen does.  So inhaled CO will bind up the hemoglobin and cause oxygen to not get into the bloodstream.  Initial symptoms of CO poisoning are nausea and vomiting but serious exposure will produce unconsciousness and possibly death.

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