Is it harmful to breathe 100-percent oxygen?
We breathe air that is 21-percent oxygen, and we require oxygen to breathe. So you might think that breathing 100-percent oxygen would be good for us, but actually it can be harmful. How much trouble it causes for you, though, depends upon the pressure of the oxygen (normal air pressure is 760 torr or 1 atmosphere) and the time of exposure. Before we discuss the effects of breathing 100-percent oxygen, let's look briefly at how your lungs work:
Your lungs are basically a long series of branching tubes from your nose and mouth (trachea to bronchi to bronchioles) that end in little thin-walled air sacs called alveoli (singular - alveolus), much like soap bubbles on the end of a straw. Each alveolus is lined with a single layer of epithelial cells. Surrounding each alveolus are small, thin-walled blood vessels lined with a single layer of epithelial cells called pulmonary capillaries (pulmonary because they are found in the lung). Between the capillaries and the alveolus is a thin wall (about 0.5 microns thick) through which various gases (oxygen, carbon dioxide, nitrogen) can exchange between the air space of the alveolus and the blood in the capillary.
When you breathe in air (high concentrations of nitrogen and oxygen, but low carbon dioxide concentration), the alveolus fills with this air. Because the oxygen concentration
is high in the air and low in the blood entering the pulmonary capillary, oxygen flows or diffuses from the air into the blood. Likewise, because the concentration of carbon dioxide is higher in the blood entering the capillary than it is in the alveolar air, carbon dioxide diffuses from the blood to the alveolus. 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 alveolus becomes depleted of oxygen and rich in carbon dioxide. When you breathe out, the alveolus empties this carbon-dioxide enriched, oxygen-poor air.
Now let's look at the effects of breathing 100-percent oxygen. In guinea pigs exposed to 100-percent oxygen at normal air pressure for 48 hours, fluid accumulated in the lungs (pulmonary edema) and the epithelial cells lining the alveolus and pulmonary capillaries were damaged. This damage was probably caused by a highly reactive form of the oxygen molecule called the oxygen free radical, which destroyed proteins and membranes in the epithelial cells. In humans breathing 100-percent oxygen at normal pressure, the following effects were observed:
- Pulmonary edema (intensive-care patients on breathing machines at 30 hours or more exposure)
- Decreases in the rate of gas exchange across the alveoli (intensive-care patients on breathing machines at 30 hours of exposure)
- Chest pains that were worse during deep breathing (volunteers with 24 hours of exposure)
- Decrease in the total volume of exchangeable air in the lung (vital capacity) by 17 percent (volunteers with 24 hours of exposure)
- Local areas of collapsed alveoli when plugged by mucus, a condition called atelectasis (patients, volunteers). The oxygen entrapped in the plugged alveolus gets absorbed into the blood, no gas is left to keep the plugged alveolus inflated and it collapses. Mucus plugs happen normally but are cleared by coughing. Also, if alveoli become plugged during air breathing, the nitrogen entrapped in the alveoli keeps them inflated.
- blindness caused by inadequate development of the capillaries in the lens and retina of the eye (premature infants). Reducing the oxygen to 40 percent can prevent this blindness.
However, 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 (above 3000 torr), acute oxygen poisoning can occur with these symptoms:
- muscles 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.