CO2 is the molecular formula for carbon dioxide. Each molecule of carbon dioxide has one atom of carbon and two atoms of oxygen. Animals expel carbon dioxide when they breathe.
In humans, carbon dioxide is one of the products of cellular respiration. During this process, glucose and oxygen are used to produce energy. When the glucose is oxidized, its carbon atoms combine with oxygen atoms to form carbon dioxide. Carbon dioxide is also one of the greenhouse gases that harm the environment. Humans add carbon dioxide to the atmosphere when they burn fossil fuels or drive vehicles powered by gasoline. According to the United States Environmental Protection Agency, the most effective way to reduce carbon emissions is to reduce the consumption of fossil fuels.