Carbon can transport toxic substances when combined with producing hemoglobin (COHb), which can not transport oxygen because COHb and O2 compete for the same functional group of hemoglobin. COHb is completely dissociable and, once the acute exposure to CO is finished, it is excreted by the lungs. This is a way to spread toxic substances to humans. Another polluting factor occurs through smoke, the greenhouse effect that occurs, industrial activities and the accumulation of CO, in the atmosphere.