Carbon monoxide is the most common of the gaseous poisons. Malfunctioning or poorly ventilated heating appliances and internal combustion engines are frequent causes of carbon monoxide poisoning. Carbon monoxide combines reversibly with hemoglobin in a manner almost identical to oxygen resulting in a decrease in the amount of oxygen carried by hemoglobin. Accidental poisoning can occur even at low levels (greater than 0.01%) of CO in the atmosphere with prolonged exposure.