Tree's are unique plants because they actually 'breathe' through the intake of carbon in mass amounts and then they return to the air breathable awesome oxygen!
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When a human breathes, they take into their bodies oxygen and then exhale the carbon dioxide. In trees and plants, however this process is reversed. You may notice that when you walk into a wooded area that you can breathe better. This is because the plants around you are exhaling large amounts of oxygen.