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The air that makes up Earth's atmosphere contains 20.9 percent oxygen. This is true up to about 70,000 feet above sea level. However, above 10,000 feet, the atmosphere becomes too thin to support human life, even though its proportions do not change much.


Exhaled air is comprised of approximately 4 to 5 percent less oxygen than was inhaled. The exchange of gases takes place in the alveoli within the lungs in a process called diffusion.


Green plants produce the oxygen that humans and other animals require for life. Plants produce this oxygen via photosynthesis. Photosynthesis is the process by which plants create their own food. Oxygen is a by-product of the process of photosynthesis.


Since the atmosphere has a mass of approximately 5.15 x 10^18 kilograms, the total mass of oxygen in the atmosphere is approximately 1.08 x 10^18 kilograms. Oxygen makes up approximately 20.9 percent of the Earth's atmosphere.


The air humans breathe is made up of about 78 percent nitrogen, 21 percent oxygen, and 1 percent carbon dioxide and argon. Trace gases include krypton, methane, neon, helium and hydrogen. Air generally contains trace amounts of water vapor, as well, with greater concent...


In dry air that does not contain pollution, the predominant gas is nitrogen, which makes up 78 percent of the atmosphere. The next two most voluminous gases are oxygen (21 percent) and argon (1 percent). Other gases appearing in trace amounts include carbon dioxide.


According to HowStuffWorks, humans who breathe 100 percent pure oxygen can experience several negative affects, including accumulation of fluid in the lungs, chest pain and slowed gas flow across the alveoli, which causes the person to breathe more to retain enough oxyg...