We breathe oxygen. Is more oxygen always better? Not exactly. The percent of oxygen you breathe has considerable effects on your health and well-being.
Clearing the Air
The Earth's atmosphere is 21% oxygen. Humans breathe best when they breathe air that is a mix of nitrogen, oxygen, carbon dioxide, and other gases, in proportions similar to the normal composition of the atmosphere. Our lungs are designed to remove oxygen from the air and exhale the other gases. When people breathe 100% oxygen at normal pressures, strange things begin to happen. Their lungs fill with fluid. They can no longer efficiently transfer oxygen to the blood, and breathing becomes difficult. Eventually, their lungs can collapse. On the other hand, astronauts in extremely low-pressure environments can breathe pure oxygen without major problems.
Too little oxygen can also harm people. The condition of not having enough oxygen is known as hypoxia. People suffering from hypoxia become dizzy and disoriented. Eventually, they pass out. When oxygen falls to below 6% of the breathable air, death can occur within minutes. Doctors sometimes give oxygen to patients who are struggling to breathe. However, most of these patients receive a supply of air mixed to have between 22% and 40% oxygen. This is enough to help them get more oxygen into their blood, but still close enough to the normal composition of room air to avoid lung damage.
See for yourself: http://kidshealth.org/teen/your_body/body_basics/lungs.html
Watch the following videos to find out how oxygen levels in the blood are measured, witness one man's near-death experience with hypoxia, and learn about a young girl's creative way of coping with a lung disease.