A colorless, odorless, poisonous gas, produced by incomplete burning of carbon-based fuels, including gasoline, oil, and wood. Carbon monoxide is also produced from incomplete combustion of many natural and synthetic products, including cigarette smoke. When carbon monoxide enters the body, it reacts with chemicals in the blood and prevents the blood from bringing oxygen to cells, tissues, and organs. The body needs oxygen for energy, so high-level exposures to carbon monoxide can cause serious health effects and even death. Symptoms of exposure to carbon monoxide can include vision problems, reduced alertness, and general reduction in mental and physical functions.