Ah! Oxygen - the gas of life! Without it for a few seconds
and the world disappears from view. Too much of it can be a bad thing as
Visiting a person receiving common oxygen therapy in
a hospital refutes the notion that the oxygen is pure. A heart attack victim
may be breathing oxygen through a nasal cannular. Air is also entering the
nose at the same time the oxygen is. Even "oxygen tents" were not totally
closed. Breathing pure oxygen over a period of time may lead to serious lung
irritation. And, breathing pure oxygen at pressures greater than 1 atmosphere
can lead to fatal consequences.
Years ago there was a scuba introduced that allowed the
diver to breathe pure oxygen. Because of the design, the small bottle of
oxygen, about the size of today's pony bottles, allowed the diver to remain
underwater for up to 4 hours! The unit had the appearance similar to an
accordion. The air was extracted from it and then it was filled with oxygen
from the bottle. As one breathed the bag would get smaller. As one breathed
out the exhaled air would enter the bag and it would get bigger again. However,
the amount of oxygen returning would be less due to the body's needs so more
oxygen would have to be added as it was consumed. To remove the poisonous
CO2 in the exhaled air there was a canister that contained caustic Barium
Hydroxide (similar to Drano). As the exhaled air passed through the canister
the chemical would combine with the CO2 removing it from the breathing oxygen.
So, the oxygen molecules were breathed over and over again until they were
converted to CO2. The unit was called a rebreather.
Conventional scuba is wasteful. A breath is taken, 21%
oxygen goes into the lungs, 18% comes out. Little CO2 goes in and about 4%
comes out. Even though there is plenty of oxygen left in the exhaled air,
it is discharged into the water. Just to get rid of the CO2 we waste the
oxygen. Conventional scuba is know as an "open-circuit" breathing apparatus.
Oxygen rebreathers are called, "closed-circuit." Rebreathers emit no bubbles
unless the diver is ascending and the bag gets too full. So, U.S. Navy Seals
use rebreathers to avoid detection!
If a person breathes pure oxygen at a pressure above
1 atmosphere there is a possibility of it becoming toxic. Above 2 atmospheres
almost insures it. The National Oceanographic and Atmospheric Administration
(NOAA) prohibits their divers from exceeding 1.6 atmospheres of oxygen. The
higher the pressure, the faster the poisoning. High-pressure oxygen toxicity
starts with the twitching of smaller muscles that are in use such as the
ones around the mouth holding the mouthpiece. Then the twitching can lead
to full-blown convulsions. Needless to say, convulsions and diving do not
really go together! Using a closed-circuit rebreather at 33' would allow
the diver to breathe 2 atmospheres of oxygen.
The U.S. Navy Seals are a hearty lot. They have a rather
rigorous training program. They must be oxygen tolerant. One of the parts
to that training consists of weeding out those that convulse easily when
breathing pure oxygen. Potential Seals are required to sit in an oxygen
atmosphere equivalent to 60' of seawater for 30 minutes. If convulsions do
occur they are quickly returned to breathing air and booted out of the program.
Don't think you are off the hook if you dive breathing
air. Air at depth can cause oxygen poisoning. At the surface air supplies
21% oxygen. Since the pressure at the surface is one atmosphere, the pressure
of the oxygen alone (partial pressure) is 0.21 atmospheres. Since the pressure
at 33 feet of seawater (fsw) is 2 atmospheres, the oxygen partial pressure
would be 21% of 2 or 0.42 atmospheres. If you project the partial pressure
of oxygen at 132 fsw it would be 21% of 5 atmospheres or 1.05. That would
be the samne as breathing pure oxygen at a depth of over 33'! The absolute
pressure at 297' is 10 atmospheres, so the partial pressure of the oxygen
would be 2.1 atmospheres and it could very well be toxic! Toxic air at 297'
is a no-no!!