Carbon monoxide is a colorless, odorless gas that has the unique ability to compete with oxygen for binding sites on the hemoglobin molecule. Carbon monoxide binds to the iron in hemoglobin in red blood cells about 200 times as readily as does oxygen, and it tends to stay bound. As a result, hemoglobin bound to carbon monoxide can no longer transport oxygen. Prolonged exposure to carbon monoxide results in carbon monoxide poisoning, with symptoms including nausea, headache, and eventually unconsciousness. If left untreated, death may result. Carbon monoxide is commonly emitted by automobiles and fuel-fired space heaters.