We breathe air that is 21 percent oxygen, and we require oxygen to live. So you might think that breathing 100 percent oxygen would be good for us -- but actually it can be harmful. So, the short answer is, pure oxygen is generally bad, and sometimes toxic. To understand why, you need to go into some detail …
Your lungs are basically a long series of tubes that branch out from your nose and mouth (from trachea to bronchi to bronchioles) and end in little thin-walled air sacs called alveoli. Think of soap bubbles on the end of a straw, and you'll understand alveoli. Surrounding each alveolus are small, thin-walled blood vessels, called pulmonary capillaries. Between the capillaries and the alveolus is a thin wall (about 0.5 microns thick) through which various gases (oxygen, carbon dioxide, and nitrogen) pass.
When you inhale, the alveoli fill with this air. Because the oxygen concentration is high in the alveoli and low in the blood entering the pulmonary capillaries, oxygen diffuses from the air into the blood. Likewise, because the concentration of carbon dioxide is higher in the blood that's entering the capillaries than it is in the alveolar air, carbon dioxide passes from the blood to the alveoli. The nitrogen concentration in the blood and the alveolar air is about the same. The gases exchange across the alveolar wall and the air inside the alveoli becomes depleted of oxygen and rich in carbon dioxide. When you exhale, you breathe out this carbon dioxide enriched, oxygen-poor air.
Now what would happen if you breathed 100 percent oxygen? In guinea pigs exposed to 100 percent oxygen at normal air pressure for 48 hours, fluid accumulates in the lungs and the epithelial cells lining the alveoli. In addition, the pulmonary capillaries get damaged. A highly reactive form of the oxygen molecule, called the oxygen free radical, which destroys proteins and membranes in the epithelial cells, probably causes this damage. In humans breathing 100 percent oxygen at normal pressure, here's what happens:
- Fluid accumulates in the lungs.
- Gas flow across the alveoli slows down, meaning that the person has to breathe more to get enough oxygen.
- Chest pains occur during deep breathing.
- The total volume of exchangeable air in the lung decreases by 17 percent.
- Mucus plugs local areas of collapsed alveoli -- a condition called atelectasis. The oxygen trapped in the plugged alveoli gets absorbed into the blood, no gas is left to keep the plugged alveoli inflated, and they collapse. Mucus plugs are normal, but they are cleared by coughing. If alveoli become plugged while breathing air, the nitrogen trapped in the alveoli keeps them inflated.
The astronauts in the Gemini and Apollo programs breathed 100 percent oxygen at reduced pressure for up to two weeks with no problems. In contrast, when 100 percent oxygen is breathed under high pressure (more than four times that of atmospheric pressure), acute oxygen poisoning can occur with these symptoms:
- Muscle twitches
- Blurred vision
Such high oxygen pressures can be experienced by military SCUBA divers using rebreathing devices, divers being treated for the bends in hyperbaric chambers or patients being treated for acute carbon monoxide poisoning. These patients must be carefully monitored during treatment.