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Some of the chemical constituents of the atmosphere are involved in maintaining reasonable temperatures on our planet but, if the amounts of these greenhouse gases are altered, Earth's temperature may change. The air that you breathe has the same chemical composition as the atmosphere. However, the chemical composition of inhaled air is different from that of exhaled air. Why is this?
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Air that is inhaled reflects the concentrations of gases in the atmosphere. This means that inhaled air is composed of approximately 78% nitrogen, 21% oxygen and 0.04% carbon dioxide. However, humans use inhaled oxygen as a reactant in cellular respiration. As a result, exhaled air is only 16% oxygen. Since carbon dioxide product of cellular respiration, which is carried to the lungs to be expelled, its concentration in exhaled breath is usually 4.0%. Also, exhaled breath has picked up water vapour from the warm, moist linings of the respiratory system and as a result will contain 4.0% water vapour.